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A

PROJECT REPORT

ON

“SATELLITE GROUND STATION”

Submitted By

RAVI SHANKAR MANTOO KUMAR SINHA

VAIBHAV MISKEEN PIYUSH ERANDE RITESH ANAND

Under the Guidance of

Prof. S. D. JADHAV

DEPARTMENT OF ELECTRONICS AND TELECOMMUNICATION ENGINEERING BHARATI VIDYAPEETH COLLEGE OF ENGINEERING

CBD, SEC – 7, BELPADA, NAVI-MUMBAI UNIVERSITY OF MUMBAI

2009 - 2010

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BHARATI VIDYAPEETH’S COLLEGE OF ENGINEERING SECTOR – 7, C.B.D., NAVI - MUMBAI - 400614

Affiliated to University of Mumbai

DEPARTMENT OF ELECTRONICS AND TELECOMMUNICATION

Certificate

This is to certify that the Project Work entitled

“SATELLITE GROUND STATION”

Submitted by

RAVI SHANKAR

MANTOO KUMAR SINHA VAIBHAV MISKEEN

PIYUSH ERANDE

Students of B E. Electronics and Telecommunication Engineering during the academic year

2009 - 2010.

This Project embodies the work carried out by the candidate, Towards the partial fulfilment of Bachelor degree

Electronics and Telecommunication Engineering conferred by the University of Mumbai

.................................... ...................................... .................................. Prof. S. D. Jadhav Prof. P. A. Kharade Dr. D. P. Mishra Project Guide H.O.D. EXTC Principal B.V.C.O.E ................................... ........................... ....................................

Internal Examiner College Seal External Examiner

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ACKNOWLEDGEMENT

While preparing this Report we received endless help from number of people. This report would be incomplete if we don’t convey our sincere thanks to all those who were involved. First and foremost we would like to take this opportunity to thank Aerospace Engineering,IIT,Bombay for involving us in to their PRATHAM SATELLITE project as ground station team and rendering all whenever we needed it. We are grateful beyond words to our H.O.D, Prof. P.A.Kharade Department of Electronics and Telecommunication Engineering for being so generous to us with his support and wise words in spite of his busy schedule.

We are indebted to the project guide Prof.S.D. Jadhav for his invaluable guidance and appreciation for giving form and substance to this report. It is due to his enduring efforts; patience and enthusiasm, which has given a sense of direction and purposefulness to this project and ultimately made it a success.

We also thank Mr.Jhonny Santosh Jha and Saptarshi Bandopadhyay (project managers, Pratham IIT-B satellite) for necessary guidance and assistance whenever we needed and associates in spite of their busy schedule for us to carry out the project. Their help and cooperation are gratefully acknowledged.

We would like to tender our sincere thanks Prof. S.N. Kadam (Mechanical department) for helping us in antenna mounting design and rotation mechanism. We also very grateful to Mr. D.M. Wankhede (workshop dept.) and Mr.Pujari (Lab assistance) for their support during our antenna fabrication work.

Last but not the least we owe our thanks to all the staff of our Electronics & Telecommunication department, who have helped us directly or indirectly. We would like to express our heartfelt words for all these people without them the concept of this project was a far-fetched dream.

Ravi Shankar Mantoo Kumar Sinha Vaibhav Miskeen Piyush Erande

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INTRODUCTION

The IIT Bombay Student Satellite Project is a landmark project taken up by IIT Bombay

students. The objective of this project is to make IIT Bombay a respected centre for

advancement in Satellite and Space Technology in the world. The project aims at launching

at least 5 satellites within the next few years. These Satellites could be test-beds for new

technology that is being developed in the institute and need space qualification.

'Pratham' is the first satellite under this project. The plan is to build a fully functional

microsatellite in less than two years which would then be launched by ISRO. This is entirely

a student initiative with mentorship provided by ISRO scientists and IIT Bombay Faculty.

The satellite will fit in a 30*30*30 cm cube and will weigh less than 15 kg. The satellite will

be launched probably in mid 2010 .The project will be involving students from other

universities in the Satellite mission by building ground stations in the respected universities

and enabling students and faculty to gain knowledge and experience in the field of Satellite

and Space technology and also to empowering the Satellite Team with the skills to develop

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the Satellite through various phases of Design, Analysis, Fabrication and Testing until the

Flight Model is made.

'Pratham' has a four-fold mission statement:

Enabling students and faculty to gain knowledge and experience in the field of

Satellite and Space technology.

Empowering the Satellite Team with the skills to develop the Satellite through various

phases of Design, Analysis, Fabrication and Testing until the Flight Model is made.

Launching the satellite into orbit and measuring Total Electron Count of the

Ionosphere.

Involving students from other universities in our Satellite mission by building ground

stations in their universities.

For the relevance of the satellite to the student community, they will be transmitting

satellite data when the satellite passes over India so that any interested university with

a small ground station will not only be able to detect the beacon signal from the

satellite but will also be able to measure TEC above their ground station. This is

proposed to spread awareness among the student community about this exciting field.

The various contributors behind this project are.

• Indian Space Research Organisation (ISRO)

• ISRO Satellite Centre (ISAC)

• Vikram Sarabhai Space Centre (VSSC)

• Industrial Research And Consulting Centre (IRCC) IITB.

• Centre for Distance Engineering Education Programme (CDEEP) IITB

• Aerospace Engineering Association (AEA) IITB

• Society for Applied Microwave Electronics Engineering and Research

(SAMEER)

• Tata Institute of Fundamental Research (TIFR)

• Boeing India Ltd.\

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BACKGROUND

The challenge to build a Student Satellite was taken up by two Aerospace Department

students namely Saptarshi Bandyopadhyay and Shashank Tamaskar, around July 2007. The

feasibility of the concept was proved to the Aerospace Department, which agreed to support

the team in this initiative. During September – October 2007, a team was chosen from

amongst the students spanning various departments and batches. During December 2008, the

team worked together and understood some of the essentials that were required for a project

of this magnitude. Work on the Satellite began in full earnest from January 2008. In

February, the project-managers and team leaders of all Sub-Systems went to ISAC (ISRO

Satellite Center) Bangalore, where they interacted with Raghava Murthy (Project Director,

Small Satellite Project) and his team of engineers who reviewed our progress. Finally after

months of debating and reviewing, the Payload for our first Satellite was chosen to be Total

Electron Count of the Ionosphere, around May 2008.

During the Summer Vacations (May-July, 2008), the team worked on the Conceptual Design

of the Satellite which culminated in the Conceptual Design Review (CDR) held within IIT

Bombay on 31st July 2008, which was attended by a number of faculty members. Then the

entire team of 33 students went to ISAC in August for the project CDR. It was attended by Dr

Raghava Murthy and other Project Directors and Engineers. The team was lauded for the

work done so far and suggestions were givn for improvement. The next milestone for the

Satellite project is the Preliminary Design Review.

The project is under the aegis of the Aerospace Department, but is truly an institute level

project as the work involves many other departments like Electrical, Mechanical, etc. About

20 faculty members belonging to Aerospace, Electrical, Mechanical and Computer Science

Departments are involved with the project. Although 'Pratham' is a student initiative, the team

leader for Pratham is Prof. Sudhakar from Aerospace Engineering Department.

All through the project, a lot of emphasis has been given to documentation of the work.

Detailed descriptions of the methodology and the results of the work done by the team have

been included in the documentation. The Requirements Capture Report was written during

the months of March – May 2008. The Conceptual Design Report was written after the

Conceptual Design was finalised in July 2008.

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TIMELINE OF THE PROJECT AT IIT-B

The project has been divided into 4 phases:

Phase I: Conceptual Designing (Dec 07-Aug 08)

The phase began with a 1 month project on functional decomposition, defining interfaces,

building components and sythesising the system in December 2007. This was a training

ground where we learnt that satellite making is a tricky business and importance of working

as a team.

In Feburary the team went on a trip to ISAC Banglore where we interacted with Dr.

Raghavmurthy and his team of system engineers who reviewed our progress. This trip made

us realise the importance of system integration and documentation for our project.

Over the next two months we spe nt many hours debating over the feasibility of our payload

At the same time the team continued to learn the intricacies of the individual subsystems.

In the month of April and May we had extensive reviews of our work and we finally

converged onto a payload.

Phase II: Preliminary Design (Sep 08-Dec 09)

This is the next step in any design process after the payload has been identified and its

requirements from various subsystems known.

This is where we fix what components go into making individual subsystems.

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Phase III: Detailed Design (Dec 09-June 10)

Once we procure various components and have a hands on experience with them we will

enter into the detailed design phase. In this phase we will be dealing with various nitty-gritty

details on each of the components and how to join them together to make a prototype model.

Phase IV: Testing Phase (July 10-Launch)

This is when each of the components will be tested many times and in all possible ways until

we are sure that the satellite is capable of handling conditions found in space. The satellite

will undergo various tests in this phase - ThermoVac tests, Radiation tests, Vibration tests etc

to ensure the robustness of various components.

Fig: representation of a student satellite.

Finally when the satellite is fully tested in all ranges & conditions, it will be handed over to

ISRO which launch it successfully in its orbit

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PARTICIPATIONS ACROSS INDIA

Team Pratham feels that the entire country should benefit by the knowledge and experience

that the team has gained while going to the satellite building process. They propose to share

their knowledge by making other universities to participate in the field of satellite

technology. For this purpose different universities across india are participating in this

project. They can establish their own ground station that will track the satellite and receive

the telemetry data from the IITB satellite named PRATHAM .

So, for that purpose around 8 universities have participated in the purpose for making their

individual ground station in their respective universities, & tracking & receiving signals from

the satellite. This will motivate the participating students and get them interested in satellite

technology.

The 8 participating universities are:

• Thapar University, Patiala

• Saveetha Engineering College, Chennai

• Faculty of Engineering Technology, Saveetha University, Chennai

• Atharva Engineering College, Mumbai

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• BIT Mesra, Extension centre Jaipur

BVCOE, Navi Mumbai

• GGTIM, Bhopal

• Jiwaji University, Gwalior

IIT-B kept all the participating teams updated by organising workshops from time to time. In

these workshops all the teams were described about their tasks & details regarding the tasks.

Here everybody got an opportunity to interact with each other and share their experiences.

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WORKSHOP DETAILS

The First Ground Station workshop, conducted on 12th October, 2008 at IITB, which was

to enable students to learn the technology of making a ground station. Through these

workshops, student teams will be able to make a working low cost ground station (<25,000

Rs) capable of receiving signals and data from the satellite. Around 8 universities participated

in this workshop.

The basic agenda of the workshop is given below:

Communication Work and Requirements

Beacon

Telemetry

Link Budget

General Decision

Ground Station

The Second Ground Station workshop was conducted on 22nd August, 2009 at IITB. It

dealt with simulation and design of Ground station crossed Yagi- Uda antennae using 4NEC2

software.

• There was a brief overview of the satellite project in the workshop which consisted of:

• Brief introduction by Ms. Haripriya.

• Followed by an introduction to crossed Yagis by Mr. Jhonny Jha in the morning

session.

• Mr. Saptarshi Bandyopadhyaya gave a brief talk on "failure modes" in the afternoon

session.

• Followed by simulation and optimization of crossed Yagis which was taken up by Ms.

Deepika Thakur of the Communication and Ground station subsystem.

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BVCOE GROUND STATION

Various universities across India are making student ground station .All ground station teams will be

establishing one of the ground station in their campus which will receive the telemetry data from IIT-

B PRATHAM satellite.

For the relevance of the satellite to the student community, it will be transmitting satellite data

when the satellite passes over India so that any respected university with a small ground station will

not only be able to detect the beacon signal from the satellite but will also be able to measure TEC

above their ground station. Here comes the role of our ground station. BVCOE ground station team

will be establishing one of the ground station in the campus which will receive the telemetry data

from IIT-B PRATHAM satellite .We are establishing our ground station at the roof of the Bharati

Vidyapeeth College of engineering exactly above our ADC (Advance Communication) lab.

It all started when some of our team members visited IIT-B for electrical department technical fest

“AAGOMANI”. There they attended a seminar which gave them the idea of the project. We liked the

whole concept of the project so, decided to go for it .After that we used to attend workshops ,

where we were told about the various tasks according to which we had to do the work step by step.

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The main objectives of our project are:

• To establish two independent crossed yagi antenna at 437 & 145 Mhz in BVCOE

campus.

• The data collected from ground station will be used in measuring TEC of ionosphere

& further in making the TEC map of India.

• Data can also be used in radio astronomy.

• Demonstrate the setup and principles of satellite communication & antennas to the present as well as future batches.

• Create a background for students wherein in future networking with other universities

is also possible.

• Involving juniors from our department in the project of building the ground station.

• Sharing knowledge within our institute in the field of satellite communication &

antenna designing.

• Gaining knowledge & experience through this project in the field of antenna

designing & satellite communication.

• This ground station will be extra equipment in EXTC Dept. lab.

• To make the BVCOE, Navi Mumbai a respected centre in field of student satellite

communication technology.

With this goal in mind we four students from EXTC department are working on this project.

The BVCOE Ground station project is a dream of the students of our department. We hope to

see the working ground station that is which can receive the data from the PRATHRAM IITB

satellite by the end of this year.

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BASIC CONCEPTS OF OUR GROUND STATION

Ground station technology

Fig 2: Relation between Space segment, Ground station and Data User

Above is shown the basic relationship diagram between the three segments involved

in the project. The data user is the IIT Bombay team, ground station team is ours & various

other teams across India & space segment is the “Pratham” satellite.

As the ground station team our main objective will be to receive the mission signal

that is the telemetry and the mission data from the IIT-B satellite “Pratham” which will be

launched by end of 2010. The objective of data user will be receiving the mission data from

the ground station teams & processing the data from further examination.

A ground station is basically an earth based point of communication with the space

segment. They are our source of interaction with the satellite; hence play an important part

for any satellite related operation and it is very important to have a very good communication

link between the ground station and the satellite/space segment. Usually a ground

segment/ground system involves following tasks.

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• Tracking and determine the position of satellite orbit

• Telemetry operation to acquire and record satellite data and status

• Controlling operation to determine orbital parameters,to schedule all satellite

passes and to monitor and load the on-board computer

• Data processing operations to present all the engineering and the scientific

data in formats required for the successful progress of the mission.

• Voice and data links to the other worldwide ground station and processing

centres

Normally a ground segment can be divided into 4 main components.

• Hardware

• Software

• People

• Operations

a) Software:

There are mainly three different software used in the different ground station

operations .

• Pre-pass software

• Real time software

• Post pass software

Pre pass software:

These are the software which are required in advance to the pass of the spacecraft.

• Determination and prediction of the orbit of the satellite.

• Observation planning and scheduling.

• Command list generation and simulation.

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Nova for windows software , win orbit software and “virtual ground station”

are used as pre pass software to determine the satellite AOS,LOS, Azimuth and

Elevation angles.

Real time software:

These are the software which are required when the satellite is visible to the ground

station. This includes the antenna tracking software, computer control software, command

and data control software, nova for windows, win orbit software and “virtual ground station”

to determine the satellite crossing angles according to satellite position.

GMS (ground station management software) is used to control the radio & antenna

autonomously according to satellite position. “Pre packet pro” to decode packet from satellite

downlink. The “ham scope” & “cwGET” to decode the cw beacon from downlink.

Fig : satellite tracking software ineterface.

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Post pass software:

These are the software that are required for housekeeping , quality control, health

assesement, data processing and orbit determination for data analysis.

b) Hardware:

Basically the hardware consists of antennas, rotors, transceivers, computer, power supplies,

peripherals, data recorders etc.

Fig 4: A basic Ground station Configuration.

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People:

In a commercial ground station people are involved in many different areas of

responsibilities however in a student ground station like ours all the operations are carried out

by few staff members and the group of students.

Operations:

This is the part that brings the hardware, software & the people unit of the project

together. It is the fundamental human unit that integrates the mission software and hardware

into an effective routine process.

Fig : Block diagram of operations

Finally all the units come together under one roof and work in sync with each other to make the

ground station work properly.

Fig : A pictorial representation of a basic groundstation lab.

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TIMELINE

Our project work is based on the various tasks which we have performed according to our

projects time line. The description of timeline and various tasks performed is given as

follows:

TASK SUBTASK DATED

1ST TASK

Started the project with

permission of our HOD.

20-03-09

a) Studied basic antenna

parameters.

23-03-09 to 30-03-09

b) Studied basic YAGI-

UDA & its design

parameters.

01-04-09 to 07-04-09

c) Studied details of NEC

(numerical

electromagnetic code)

software.

09-04-09 to 15-04-09

d) Designed YAGI–UDA

antenna with specified

parameters using NEC &

sending the obtained

output to IIT-B.

18-04-09 to 27-04-09

e) Studying about TEC in

Ionosphere.

30-04-09 to 3-05-09

2nd

TASK Antenna Fabrication from

the verified parameters.

25-08-09 to 25-02-2010.

3rd

TASK Characterization of

fabricated antenna.

10-03-2010 to 5-04-2010.

4th

TASK Designing of mounting

interface for antennae's

using stepper motor.

10-04-2010 to present

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5th

TASK Studying of analog

polarization measurement

set-up & its installation.

10-04-2010 to present

FINAL TASK Installation of the full

ground station & showing

its working.

October-November 2010.

Now the various tasks will be explained one by one which will give a brief idea of the work

which we have done till now and how it has been done, in accordance with the timeline.

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1st TASK

The first task which involved the understanding the basic concepts is given as follows.

Design a Yagi antenna having following characteristics:-

1) Frequency = 437 MHz

2) Gain = 10 dB approx.

Steps to complete this task:-

• Read general concepts of antenna and Yagi antenna from “Antenna Theory:

Analysis

And Design" by Constantine A. Balanis. Chapter #1, #2 and a chapter on Yagi

antenna.

• Download software nec2 (open source) and learn to simulate a Yagi antenna.

• Validate the parameters obtained from simulation with us or some of your

professors

Construct a Yagi antenna with the help of your parameters.

We started with the first task by getting a basic idea about the basic antenna parameters

which we may come to use during the process of our project. The details about all the

parameters are given as follows.

Basic Antenna Parameters

• Polarization

• SWR(Standing wave ratio)

• Antenna Bandwidth

• Impedance

• Directivity Of Antenna and Beamwidth

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• Radiation pattern

• Gain Of The Antenna

One by one we will explain the parameters and their importance in the designing.

Polarization

Polarization is a property of certain types of waves that describes the orientation of their

oscillations. By convention, the polarization of light is described by specifying the orientation

of the wave's electric field at a point in space over one period of the oscillation. It can be

basically defined as the orientation of the electric field vector with respect to the earths

magnetic field.

Mainly there are two kinds of polarization:

1. Linear polarization:

When the direction of propagation of electric field vector is perpendicular to the

earth’s magnetic field then it is linearly polarised.

Here E-field vector exist in a single plane(vertical or horizontal).

2. Circular polarization

Signal having two plane waves of equal amplitude but differ in phase by 90 degree.

In our case we will be dealing with linearly polarized electromagnetic waves.

SWR (Standing Wave Ratio):

• Measurement of efficiency of antenna system regarding power radiation.

• Mathematically:

• VSWR = Vmax / Vmin

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• VSWR = Zo / ZL ( Zo > ZL )

• VSWR = ZL / Zo ( ZL > Zo)

• VSWR = ( √PF + √PR )/ (√PF - √PR )

• In our antenna we need SWR as close to one as possible.

Antenna Bandwidth

Pass band bandwidth is the difference between the upper and lower cutoff frequencies of,

for example, an electronic filter, a communication channel, or a signal spectrum

• Range of frequency over which antenna can be used to obtain a specified level of

performance with good efficiency.

• Our ground station is working at center frequencies of 145 MHz & 437 MHz .

Antenna Impedance:

• Ratio of the voltage to current at any given point of antenna.

• It may be either resistive or complex depending on operating frequency.

• In our antenna impedance should be near to (200+0j) Ω.

Directivity and Beam width:

Directivity is defined by direction to the radiation intensity averaged over all directions.

A more general expression of directivity includes sources with radiation patterns as functions

of spherical coordinate angles θ and φ.

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Where ΩA is the beam solid angle and is defined as the solid angle in which if the antenna

radiation intensity is constant, all power would flow through it. In the case of antennas with

one narrow major lobe and very negligible minor lobes, the beam solid angle can be

approximated as the product of the half-power beam widths in 2 perpendicular planes.

• Property to exhibit directive effect (i.e. radiate more power in some particular

direction).

• Directivity is defined as a measure that takes into account only the directional

properties of the antenna and therefore it is only influenced by the antenna pattern.

• Figure of merit of power radiated.

Fig: beam width of antenna.

Radiation Pattern:

• Graphical Representation of the intensity of the radiation plotted against angle.

• Major part of signals radiated and received are through main lobe.

• Side lobes usually attract spurious noise.

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Fig: radiation pattern of a yagi.

Gain of the Antenna:

Antenna gain relates the intensity of an antenna in a given direction to the intensity that

would be produced by a hypothetical ideal antenna that radiates equally in all directions

(isotropically) and has no losses. Since the radiation intensity from a lossless isotropic

antenna equals the power into the antenna divided by a solid angle of 4π steradians, we can

write the following equation:

Although the gain of an antenna is directly related to its directivity, the antenna gain is a

measure that takes into account the efficiency of the antenna as well as its directional

capabilities.

• Combination of directivity and efficiency w.r.t a reference antenna (an isotropic one).

• We are designing antenna for 10dB gain.

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Fig: an example showing antenna gain pattern.

This was all about the details of the 1st subtask of the 1st task. Now we go into details of the

next subtask that is studying the basics of Yagi uda antenna and its various design

parameters.

YAGI-UDA ANTENNA

GENERAL DESCRIPTION OF A YAGI

The word "Yagi" is used to describe a type of antenna and is credited to very famous

Japanese antenna experts by the names of Yagi and Uda! Most hams refer to this type of

antenna as the "Yagi" rather than use both men's names.

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They discovered (1954) that by adding "elements “of various lengths and spacing’s in front

of and behind a dipole antenna that the performance and effectiveness of the dipole could be

greatly increased and the pattern of the dipole rf energy could be "beamed" or focused in one

direction, with the resulting "effect" of making it appear that the transmitter was running lots

more power than it actually was, yielding much stronger signals both on receive and transmit!

. It is usually used at frequencies between about 30MHz and 3GHz, or a wavelength range of

10 metres to 10 cm.

A Yagi-Uda Antenna, commonly known simply as a Yagi antenna or Yagi, is a directional

antenna system consisting of an array of a dipole and additional closely coupled parasitic

elements (usually a reflector and one or more directors). The dipole in the array is driven, and

another element, typically 5% longer, effectively operates as a reflector. Other parasitic

elements shorter than the dipole may be added in front of the dipole and are referred to as

directors. This arrangement increases antenna directionality and gain in the preferred

direction over a single dipole. Directional antennas such as the Yagi-Uda are commonly

referred to as beam antennas or high-gain antennas. Yagi-Uda antennas are widely used by

amateur radio operators for communication on frequencies from short wave, through

VHF/UHF, and into microwave bands. The bandwidth of a Yagi-Uda antenna, which is

usually defined as the frequency range for which the antenna provides a good match to the

transmission line to which it is attached, is determined by the length, diameter and spacing of

the elements. For most designs bandwidth is typically only a few percent of the design

frequency.

THE ELEMENTS OF A YAGI

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THE DRIVEN ELEMENT

The driven element of a Yagi is the feed point where the feed line is attached from the

transmitter to the Yagi to perform the transfer of power from the transmitter to the antenna.

A dipole driven element will be "resonant" when its electrical length is 1/2 of the wavelength

of the frequency applied to its feed point. Feed point is on the centre of the driven element.

THE DIRECTOR

The director/s is the shortest of the parasitic elements and this end of the Yagi is aimed at the

receiving station. It is resonant slightly higher in frequency than the driven element, and its

length will be about 5% shorter, progressively than the driven element.

The director/s length/s can vary, depending upon the director spacing, the number of directors

used in the antenna, the desired pattern, pattern bandwidth and element diameter. The number

of directors that can be used are determined by the physical size (length) of the supporting

boom needed by your design.

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The director/s are used to provide the antenna with directional pattern and gain.

The amount of gain is directly proportional to the length of the antenna array and not by the

number of directors used. The spacing of the directors can range from .1 wavelength to .5

wavelength or more and will depend largely upon the design specifications of the antenna.

THE REFLECTOR

The reflector is the element that is placed at the rear of the driven element (The dipole). Its

resonant frequency is lower, and its length is approximately 5% longer than the driven

element. Its length will vary depending on the spacing and the element diameter. The spacing

of the reflector will be between .1 wavelengths and .25 wavelengths. Its spacing will depend

upon the gain, bandwidth, F/B ratio, and side lobe pattern requirements of the final antenna

design.

BANDWIDTH AND IMPEDANCE

The impedance of an element is its value of pure resistance at the feed point plus any

reactance (capacitive or inductive) that is present at that feed point. Of primary importance

here is the impedance of the driven element, the point on the antenna where the transfer of rf

from the feed line takes place.Maximum energy transfer of rf at the design frequency occurs

when the impedance of the feed point is equal to the impedance of the feed line. In most

antenna designs, the feed line impedance will be 50 ohms, but usually the feed point

impedance of the Yagi is rarely 50 ohms. In most cases it can vary from approximately 40

ohms to around 10 ohms, depending upon the number of elements, their spacing and the

antenna's pattern bandwidth. If the feed line impedance does not equal the feed point

impedance, the driven element cannot transfer the rf energy effectively from the transmitter,

thus reflecting it back to the feed line resulting in a Standing Wave Ratio. Because of this,

impedance matching devices are highly recommended for getting the best antenna

performance.

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The impedance bandwidth of the driven element is the range of frequencies above and below

the centre design frequency of the antenna that the driven elements feed point will accept

maximum power (rf), from the feed line. The design goal is to have the reactance at the

centre design frequency of the Yagi = (0)…. (j + 0).

The impedance matching device will now operate at its optimum bandwidth. Wide element

spacing, large element diameter, wide pattern bandwidth, and low "Q" matching systems will

all add to a wider impedance bandwidth.

Yagi-Uda Antenna Design

Theoretical limits for a Yagi-Uda antenna are generally specified in most antenna theory

books. However to get the desired radiation pattern for the specific purpose of a radio

telescope is somewhat more of a trial and error process through the use of a simulation

package. This is due to the fact that antennas have various applications and an extensive

difference in the radiation pattern that needs to be produced.

The design parameters that affect the results of the simulation are:

• the length of the driven element,

• the length of the reflector and directors,

• the spacing between all elements,

• the diameter of all elements, and

• the number of extra parasitic elements.

Table I. Optimal Lengths for Yagi-Uda Elements, for Distinct Boom Lengths

0.4 0.8 1.2 2.2 3.2 4.2

d=0.0085

SR=0.2

Boom Length of Yagi-Uda Array (in )

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R 0.482 0.482 0.482 0.482 0.482 0.475

D1 0.442 0.428 0.428 0.432 0.428 0.424

D2

0.424 0.420 0.415 0.420 0.424

D3

0.428 0.420 0.407 0.407 0.420

D4

0.428 0.398 0.398 0.407

D5

0.390 0.394 0.403

D6

0.390 0.390 0.398

D7

0.390 0.386 0.394

D8

0.390 0.386 0.390

D9

0.398 0.386 0.390

D10

0.407 0.386 0.390

D11

0.386 0.390

D12

0.386 0.390

D13

0.386 0.390

D14

0.386

D15

0.386

Spacing

between

directors,

(SD/ )

0.20 0.20 0.25 0.20 0.20 0.308

Gain (dB) 9.25 11.35 12.35 14.40 15.55 16.35

Consider the table published in "Yagi Antenna Design" by P Viezbicke from the National

Bureau of Standards, 1968, given in Table I. Note that the "boom" is the long element that the

directors, reflectors and feed elements are physically attached to, and dictates the length of

the antenna.

We took the help of the above table for taking the geometrical parameters of antenna. We

cross checked the result by using the “antenna maker” software which is a dos based software

for designing the parameters of an Yagi. This is the next step of the task which is up next.

28

Now next subtask was to go for the designing of the antenna. For the designing purpose we

used various simulation software for obtaining the geometrical parameters of the antennas.

The rest is explained as follows.

29

NEC2 (Numerical Electromagnetic code)

Simulation of Yagi Antenna through NEC2 software

And

Antenna maker software

30

1. Center frequency of antenna on which we want to design.

DOS-Antenna maker Software

Before implementing the NEC2(Numerical Electromagnetic code) to design our antenna we

first implemented DOS-Antenna maker software because this software was very useful for us

in using NEC2 software, since we got a rough idea about the geometrical structure of

yagi antenna by this DOS Software for required parameter(Gain=10dB at center frequency

=437MHz) In NEC2 software ,we inputted all Geometrical parameter(like Length of

elements , Spacing between elements) of yagi antenna.

Since there is no any options for optimization so this is the main reason of not using the result

of this software for simulating our antenna. Because there will be problem in getting

optimized values. we only used this software for getting a rough idea about geometrical

parameter so that we can input these values in NEC2 software that’s it .

This software is very easy to implement, here we have to mainly input three parameters-

2. No. of elements according to our required gain.

3. Diameter of the element.

After entering these all above three value we can get the result. The main input interface and

output results of this software is given below as specified step wise.

31

DESIGN:

32

33

OUTPUT:

Length of elements:

34

Spacing between elements:

Using this we got all the required geometrical parameters which we simulated using the NEC

2 simulation software which is described as next.

35

NEC (Numerical Electromagnetic Code)

Introduction:-

The Numerical Electromagnetic code (NEC-2) is a computer code for analyzing the

electromagnetic response of an arbitrary structure consisting of wires and surfaces in free

space or over a ground plane. The analysis is accomplished by the numerical solution of

integral equations for induced currents. The excitation may be an incident plane wave or a

voltage source on a wire, while the output may include current and charge density, electric or

magnetic field in the vicinity of the structure, and radiated fields.

The Numerical Electromagnetic Code (NEC-2) is a user-oriented computer code for

analysis of the electromagnetic response of antennas and other metal structures. It is built

around the numerical solution of integral equations for the currents induced on the structure

by sources or incident fields. This approach avoids many of the simplifying assumptions

required by other solution methods and provides a highly accurate and versatile tool for

electromagnetic analysis. The code combines an integral equation for smooth surfaces with

one specialized for wires to provide for convenient and accurate modeling of a wide range of

structures.

The excitation may be either voltage sources on the structure or an incident plane

wave of linear or elliptic polarization. The output may include induced currents and charges,

near electric or magnetic fields, and radiated fields. Hence, the program is suited to either

antenna analysis or scattering and EMP studies. The integral equation approach is best suited

to structures with dimensions up to several wavelengths. Although there is no theoretical size

limit, the numerical solution requires a matrix equation of increasing order as the structure

size is increased relative to wavelength. Hence, modeling very practical on a particular

machine. In such cases standard high-frequency approximations such as geometrical optics,

physical optics, or geometrical theory of diffraction may be more suitable than the integral

equation approach used in NEC-2.

36

Simulation of any antenna through NEC2 software mainly consists of

following steps:-

1. Creating geometrical structure of an antenna

2. Creating Electrical environment for antenna

3. Run the program

4. Optimization

• Using Inch and Feet as the basic length unit....

1. Creating geometrical structure of an antenna

As we know that yagi antenna only consist of wire geometry, there is no any surface or solid

geometrical structure in it.so we are only concentrated about wire geometry.

Wire modeling :-

A wire segment is defined by the co-ordinates of its two end points and its radius.

Create an antenna model using 'Geometry

• specify 'Geometry Edit' as the preferred Edit method using the 'Settings'

Menu on the 'Main' window

• Furthermore select 'Feet' as the length-unit and 'Inch/Awg'

• As the 'radius-unit' using this same 'Settings' menu, as follows.

• When done, select 'Edit -> Input-file' on the 'Main' window or use the <F6> key to

start 'Geometry-edit'. A picture of the selected example file is displayed.

• If not already set, select 'Options -> Set Segmentation -> Medium' on the Edit

window to set medium segmentation density

37

Fig: settings for geometry edit.

To create a new model, Select 'File -> New' on the Edit-window. As shown in fig below.

38

• Setting design frequency

2. Creating Electrical environment for antenna

For creating electrical environment we should go through following steps

• Add new wire(s)

• Add feed/transmission-line(s)

• Add voltage source

• Add wire-conductivity

• Specify ground parameters

• Run NEC-engine and create far-field pattern

Fig: Options available in geometry edit window to create the electrical environment.

Now one by one each step is explained below in details with examples.

Setting center frequency

39

Setting design frequency

When starting a new model, initially on the lower right, frequency 'data' is displayed. This

because one of the first things we will have to do is specifying the antenna design-frequency.

Enter frequency (MHz) in the 'frequency' text-box on the right part of the window. When

entered, click the 'wire' button on top of the window (the one with the single line in it). Notice

the grid-size changing from .025 to .5 feet, corresponding to about half a wavelength for the

window-width. Furthermore the default 3D-display view is now set to 2 dimensional XZ

plane. (The Y-axis is pointing backwards)

Add new wire(s)

To start adding a new wire, click the 'Add' button. The mouse-pointer changes to across-

hair, indicating 'Add-mode' is activated. The Y-position text-box on the right is now

highlighted. If required you can specify a certain 'depth' position, but for now we will stay in

the XZ-plane for an Y-position equal zero. When you will try to locate a mouse-pointer

position for eg. the height Z equals 70 feet, you shouldn't succeed, because the grid-size is too

small to cover a Z position of 70 feet.

First increase grid-size to 1 feet by clicking on the left arrow for the 'Zoom' scroll-bar. When

done, point somewhere inside the picture-box (that part of the window where the antenna

structure is displayed), hold down the right mouse-buttonand move the X-axis to almost at

the bottom of the picture-box. Now you should be able to locate a position for which Z equals

70 feet somewhere in the upper region of the picture-box.

Because we want to create a line at Z=70 feet with a length of 33.7 feet we will have to locate

a point for which Z = 70 and X = -33.7/2 = 16.85 feet. However, because the 'Snap to grid'

box is checked you won't succeed in this. For now we will locate a position for which X

equals -17 feet. To start drawing the wire, click and hold down the left mouse-button and

drag the mouse-pointer to the second position for which Z=70 and X=17 feet, then release the

mouse-button. Because this is the first wire added to the model, a pop-up window is

displayed asking for the initial/default wire diameter. Use the value which you want to. On

the right of the picture-box all data belonging to this wire is listed. You can edit the end-1 or

end-2 coordinates text-boxes to further refine the end positions. You will also notice that the

number of segments is set to 25, corresponding to 'medium segmentation'.

40

Note:-Next we will have to add the feedline. But, before doing so we need some knowledge about how wires are identified in Nec-2/4. All wires are assigned a unique tag-nr. Mostly the tag-number equals the wire-number. Ater delete, copy or paste operations however this sequence may have changed. Tag-numbers should still be unique. You may use 'Resequence tag-numbers' in the 'Option' menu to make tag numbers equal to the corresponding wire-numbers again.

Each voltage/current-source, transmission-line or RLC-load (see below) is 'assigned 'to a wire using this unique tag-number. To specify the position of the source, TR-line or load on the specified wire a segment-number between 1 and the nr-of-segments for the wire is used. Using Geometry-edit these tag- and segment-numbers are automatically assigned. It is allowed to change these number manually. When doing so please note how these tag- and segment-numbers are used within Nec-2/4.

Add feed/transmission-line(s)

Adding/creating a transmission-line is done by clicking the 'TR-line' button (the one with

the ladder picture). If not in 'Add-mode', click the 'Add' button to start adding a new Tr-line.

Locate the mouse-pointer on the middle of the first wire and click and hold down your left

mouse-button and move the mouse-pointer to the middle of the second wire. When reached

release the mouse-button. When positioning was not too rude a new transmission-line is now

added. If not, try again. Note also that we did not take the velocity-factor into account, we

just used an electrical length.

Add voltage source

To prevent loosing the changes, backup the model using 'File->Save as' and choose a folder-

and file-name for your new model.The next thing to do is add a voltage-source. While still in

Add-mode, click the 'Source button' (right of the 'Wire button'). Next click and hold down

your left mouse-button somewhere in the picture-box. At the current mouse-pointer position a

new source-object is displayed. Drag the source-object to the middle of the second wire, just

between the two lower wires-ends of the feedline and release the mouse-button. When

properly positioned a new source is now added. If not try again. For now we will set a default

voltage-source of 1+j0 volt (1V @ 0 deg.) Select the 'pointer' button to switch back to select-

mode mode. The mouse-pointer changes to the default arrow-pointer indicating 'Select-

mode' is active.

41

Add wire-conductivity

We use aluminum wire for our antenna, so we will have to include this in our model

(the default is perfect wire with zero losses). To do this, click the 'Loading button' (the one

with the RLC symbols), click somewhere in the picture-box and drag the new load-object to

any place on the first (upper) wire and release the mouse button. The new load-object is now

'connected' to the first wire.

The default load however is a lumped load. To change this to distributed/wire-load,

change the 'Par-RLC' selection for the Load-data on the right of the screen to 'Wire-ld'. The

box shape on the first wire should now have changed to a red line segment. The initial

conductivity is set to 10000 mho/m. Change this to 'Aluminum' by using the 'G (mho/m)'

selection-box on the right of the picture. To specify wire-conductivity for the whole structure,

first change from 'spot load' to 'single-wire' (see lower right) . Notice how the whole wire

becomes 'wire-loaded'.

Next change to ‘Whole struct'. The 'Wire-conductivity' is not visible any more. This

would not deliver us additional information because the whole structure is now loaded (both

wires). To enable wire-loading display for the complete structure use 'Option > Show wire

loading' .For now we have added all required objects. Switch back to 'Select-mode'.

3.Run NEC-engine and create far-field pattern.

To run the NEC-engine and evaluate your model, click the 'Run Nec-engine' button

(the one with the calculator picture) or push <F7>. A new pop-up window is displayed asking

you for additional settings. To create a full 3D far-field pattern, select the second option 'far-

field pattern', specify 'Full' and a 5 degree resolution. Then click <Generate>.If the DirectX

based version of 4nec2 is installed, push <F9> to visualize the new antenna-structure Select

'Pattern' or push the 'R' key to see the 3D far-field pattern. You can return back to your model

by pushing <F6> or clicking the 'Geometry-edit' window. You may alter your model (e.g. set

to 'Free-space') and recalculate to see the results of your changes.

42

After running NEC-engine we will get un-optimize result which is shown below.

Fig: unoptimized result and radiation pattern.

As the above results are un-optimize(not up to desired level),desired result was as follows –

Impedence=( 200+0j) Ohm

SWR=4

Gain=approx 10dB

Which we did not get, so we can not start fabrication of antenna with this results, we need to

go through a further process called optimization to get the optimized values (above mention

desired result).

4.

“Optimization is the process in which we convert geometrical parameters of antenna in to

variables ,and we keep changing the variable by the help of this(NEC) software till we not get

our desired result.”

Its consist of the following steps :-

Optimization

Create variable

43

Open optimizer window & select variables

Set priority

Start optimization

Create variable

After running the NEC,we can open Nec-output file in notepad as shown below,it is special

4nec2 "SY" cards are included. With this card it is possible to specify

symbols(VARIABLES), constants or mathematical expressions (equations)..

Here the first column is for wire tag no., second is for no of segment in corresponding wire

tag , next six columns are for the two end co-ordinates(x1,y1,z1 and x2,y2,z2) and last

column is for radius of each wire. As we can not set wire tag no and the corresponding

segment no as variable so we need to create only 7 variables.

We have given symbols as fl, rl, dl, rs , ds , fs and rad for length of folded dipole, length of

reflector, director length, reflector spacing, director spacing and the spacing between the arms

of folded dipole respectively.

Geometrical Parameters without creating variables :-

CE

GW 1 19 0.10976 0 0.15092 0.10976 0 -0.15092 3.e-3

GW 2 21 -0.15092 0 0.1646 -0.15092 0 -0.165 3.e-3

GW 3 19 -0.01 0 0.15778 -0.01 0 -0.15778 3.e-3

GW 4 19 0.01 0 0.15778 0.01 0 -0.15778 3.e-3

GW 5 3 -0.01 0 0.15778 0.01 0 0.15778 3.e-3

GW 6 3 0.01 0 -0.15788 -0.01 0 -0.15788 3.e-3

GW 7 19 0.21952 0 0.15092 0.21952 0 -0.15092 3.e-3

GW 8 19 0.32928 0 0.15092 0.32928 0 -0.15092 3.e-3

GE 0

EK

EX 0 3 10 0 1 0

GN -1

FR 0 1 0 0 437 0

44

Geometrical variables for optimization (after creating variables):

CE

sy fl=0.31556 : length of folded dipole

sy rl=0.32928 : length of reflector

sy dl=0.30184 : length of director

sy rs=0.1509 : reflector spacing

sy ds=0.10976 : director spacing

sy fs=0.02 : spacing between arm of folded dipole

sy rad=3.0e-3 : radius of elements

GW 1 19 fs/2 0 fl/2 fs/2 0 -fl/2 rad

GW 2 19 -fs/2 0 fl/2 -fs/2 0 -fl/2 rad

GW 3 3 fs/2 0 fl/2 -fs/2 0 fl/2 rad

GW 4 3 fs/2 0 -fl/2 -fs/2 0 -fl/2 rad

GW 5 21 -rs 0 rl/2 -rs 0 -rl/2 rad

GW 6 19 ds 0 dl/2 ds 0 -dl/2 rad

GW 7 19 2*ds 0 dl/2 2*ds 0 -dl/2 rad

GW 8 19 3*ds 0 dl/2 3*ds 0 -dl/2 rad

GE 0

EK

EX 0 2 10 0 1 0

GN -1

FR 0 1 0 0 437 0

45

Open optimizer window & select variables

Start the Optimizer by entering the F12 key. A new window appears with a number of

selection- and input-boxes as shown below-

Fig: optimizer window.

First we set the traditional optimizer by selecting 'Optimize' in the Function-box and

'Default' in the Option-box. After this we select the variable(s) we want to optimize, by

clicking on the variable in the list-box(fl, rl, dl, rs, ds, fs) with the 'variables' heading. The

selected variable(s) will show-up in the right list-box as in above fig.

Set priority(weighting factor) & start optimization

Furthermore we must select one or more antenna properties to optimize, together with

their "Importance" (weighting factor, contributing in the total result).

46

First we set target value by Clicking with the right mouse key on one of the property-

boxes to change the value of antenna parameter which we want to get and then after we give

the corresponding priority by setting the weighting factor.

Here we have given highest weighing(200%) to X-in, the reactive part of the

impedance because our intention is to get its value as minimum as possible so that there will

be no losses in received signal power , and have fixed target value for impedance.

R-in+X-in= 200+0j ohm(R-in=200,R=0)

The target value for gain and SWR have given as 10dB and 4 respectively and equal

weighting factor 100 % for both.

After clicking the 'Start' button the optimizing process starts and the button text changes to

'stop'.In the upper right box, the selected variables together with the direction and relative

amount in which they are changed are displayed. In the lower left box the calculated property

values are displayed for each new optimization step, together with the calculated overall

result (Res%) and the step-size used. In the lower right box the corresponding variable

value(s) is/are listed, so it is possible to follow the optimizing process.

Fig: optimizer process after completion.

47

After some time the process should stop with the message 'Optimized in xx steps',as

shown in above fig. indicating the optimization is ready. To premature abort the process, you

may click the 'Stop' button. It is possible the process is not immediately halted. If so, please

wait till the active calculation step is ready. Sometimes it may be necessary to click the

button once more. After the process is ready/aborted, you may change the variables or

properties and continue optimization by clicking the 'Resume' button.If the optimization

results are OK, you may use the 'Update NEC-file' button to up-date your NEC-file with the

new variable value(s). Use 'Exit' to quit the optimizer without saving.

Simulation results for two antennae at center frequency 437MHz &

145MHz

After going through the successful optimization process for our both yagi antenna one at 437

MHz and another at 145MHz ,we got our desired antenna parameters and various

patterns(optimized values and patterns). Now with the help of these obtained results we can

start our fabrication work. Here we have only shown the various pattern of 437 MHz.

Fig: Output results and radiation patterns after optimization.

48

Geometrical model of 437MHzYagi obtained:

Output 3-D far field radiation pattern and smith chart:

DESIGNED ANTENNA PARAMETERS AT CENTRE FREQUENCY 437MHz

Driven length =0.3076mtr

Spacing Between Directors =0.1101 mtr

Director Length =0.2883 mtr

Spacing Between Folded Arms =0.02mtr

49

Reflector Spacing =0.1532mtr

F/B =18.79

Gain =9.85

Z(impedance) =199.59-j0.39

SWR =4.01

The same process was repeated for the 145 MHz antenna also. The variables created before

optimizations are as follows:

SY

SY RL=1.04

SY DL=0.938

SY FS=0.042

SY RS=0.45

SY DS=0.38

SY RAD=0.003

SY LD=0.88

GW 1 25 -RS RL/2 0 -RS -RL/2 0 RAD

GW 2 25 -FS/2 DL/2 0 -FS/2 -DL/2 0 RAD

GW 3 25 FS/2 DL/2 0 FS/2 -DL/2 0 RAD

GW 4 1 -FS/2 DL/2 0 FS/2 DL/2 0 RAD

GW 5 1 -FS/2 -DL/2 0 FS/2 -DL/2 0 RAD

GW 6 23 DS LD/2 0 DS -LD/2 0 RAD

GE 0

EK

EX 0 2 13 0 1 0

GN -1

FR 0 1 0 0 145 0

50

Results and all patterns of antenna working at 145 MHz

DESIGNED ANTENNA PARAMETERS AT CENTRE FREQUENCY 145MHz

Reflector length - 1.0472m

Driver length - 0.9219m

Folded dipole spacing - 0.042m

Reflector spacing - 0.4528m

Director spacing - 0.378m

Director lengthg - 0.8586m

F/B ratio = 17.9

Gain = 7.3

51

FABRICATION

OF

ANTENNA

52

FABRICATION

After we simulated both the antennae (one at 437 Mhz and another at 145 Mhz) on

NEC2(Numerical electromagnetic Code ver2) software. So our next task was to fabricate the

crossed yagi antenna. Since we are doing student project we were keen to fabricate with the

minimum number of elements keeping cost in mind. For the 437 Mhz antenna we have 5

elements (3 directors, 1 driven element and 1 reflector) and for the 145 Mhz antenna we have

3 elements (1 director, 1 driven element, 1 reflector).We used solid Aluminum material to

fabricate the crossed yagi antenna and also for the boom just to make yagi more rigid and

robust. There were two major concerns

1. Design of coupler

2. Making of folded dipole

Design of coupler

Since the design of the coupler was little bit tough work as there were three elements that

should pass through, the boom and the two elements of crossed Yagi. And proper gap should

be maintained to avoid any kind of interferences. The size of the coupler is 40*40*25 mm so

as to make it robust and size would be less. We also fit some screws onto the coupler to tight

all the elements.

A=40mm

B=40mm

C=25mm

D=6mm

E=6mm

F=20mm

Fig: design of coupler

53

Although there were some errors during the design of coupler as the gap between the holes that were slotted for the two crossed yagi feeding elements was less as compared to the distance between the two arms of the folded dipole. However with slight change in the distance of the elements we managed to get the acceptable figures.

Making of folded dipole

As we all know the distance between the folded arms plays the very important role in

determining gain of the antenna. It was very important for us to maintain the fix required

distance. First we had to fold from one side of the rod and to keep the driven length accurate

and then to fold from the other side. Usually while doing this length of the driven element

does not remain same. For this we made several attempts to get accurate figures.

Fig :Fully fabricated antenna at centre frequency 437Mhz

Besides this there were some small works like while cutting the elements the edge got rough

so we filed them to make it smooth so as to minimize the unwanted radiations and we need

to arrange all the elements in parallel.

Fig: fully fabricated antenna at 145 MHz frequency.

54

Coaxial Cable:

As there are various cables available in the market and their specification are different first of

all we searched for the 50 Ohm cable and the list was quite large then according to the

velocity factor and the losses in dB the most suitable cable was RG8A/U. But due to in

availability of the cable in the market we have used RG58C/U cable.. We have used RG58

cable having 50ohm impedance for the balun and for the connection.

Fig: Specifications of RG series cables.

55

Balun:

A balun is a device that joins a balanced line (one that has two conductors, with equal

currents in opposite directions, such as a twisted pair cable) to an unbalanced line (one that

has just one conductor and a ground, such as a coaxial cable). A balun is a type of

transformer: it's used to convert an unbalanced signal to a balanced one or vice versa. Baluns

isolate a transmission line and provide a balanced output. A typical use for a balun is in a

television antenna. The term is derived by combining balanced and unbalanced.

In a balun, one pair of terminals is balanced, that is, the currents are equal in magnitude and

opposite in phase. The other pair of terminals is unbalanced; one side is connected to

electrical ground and the other carries the signal.

Since in the simulation of our antenna we have kept the desired antenna gain 200 Ohm

and we are connecting it to 50 Ohm cable. To match the load of 200 Ohm with the cable we

have used balun. For the purposes we have used 4:1 balun to match the 50ohm cable with the

200ohm antenna impedance We have used RG58C/U for the balun as well as for the cabling.

Now next is the characterization of the antennae, which is described next.

56

CHARACTERIZATION

OF

FABRICATED

ANTENNAE

57

A network analyzer is an instrument used to analyze the properties of

Characterization of both fabricated antennae using Network analyser

A characterization test is a means to describe (characterize) the actual behaviour of an

existing model. The goal of characterization tests is to help fabricators verify that the

modifications made to a reference model of a system did not modify its behaviour in

unwanted or undesirable ways. When creating a characterization test, one must observe what

outputs occur for a given set of inputs.

After we are done with fabrication of the antenna our next task is to characterize the

antenna. Since we are using 4:1 balun to match the antenna impedance, so for the

characterization purpose we have connected two n-type female connectors for each crossed

yagi.

To characterize the antennae we are using network analyser. Since we have fabricated

our antenna at centre frequencies 437Mhz and another for 145Mhz and so there are various

network analysers available with different specification in the market. There is a brief

summary about the network analyser.

electrical networks,

especially those properties associated with the reflection and transmission of electrical signals

known as scattering parameters (S-parameters).

Network analyzers are used mostly at high frequencies; operating frequencies can range from

9 kHz to 110 GHz. Special types of network analyzers can also cover lower frequency ranges

down to 1 Hz. These network analyzers can be used for example for the stability analysis of

open loops or for the measurement of audio and ultrasonic components.

The two main types of network analyzers are

• Scalar Network Analyzer (SNA) — measures amplitude properties only

• Vector Network Analyzer (VNA) — measures both amplitude and phase properties

A VNA may also be called a gain-phase meter or an Automatic Network Analyzer. An

SNA is functionally identical to a spectrum analyzer in combination with a tracking

58

generator. As of 2007, VNAs are the most common type of network analyzers, and so

references to an unqualified “network analyzer” most often mean a VNA. The three biggest

VNA manufacturers are Agilent, Anritsu, and Rohde & Schwarz.

A new category of network analyzer is the Microwave Transition Analyzer (MTA) or

Large Signal Network Analyzer (LSNA), which measure both amplitude and phase of the

fundamental and harmonics. The MTA was commercialized before the LSNA, but was

lacking some of the user-friendly calibration features now available with the LSNA.

Also, a category of network analyzers introduced by Agilent is a programmable network

analyzer (PNA).We have used HP 8720A network analyser for characterizing the antenna.

Fig: A microwave network analyzer (HP 8720A) showing a Smith chart

59

PORTS, CONNECTION, OPERATION

Front Panel of HP8720es Network Analyzer

The 8720ES is a powerful S parameter network analyzer. It can be used to evaluate the

forward and reverse gain and reflection coefficients of a system. The 8720 can be used to test

systems operating from 50 MHz to 20 GHz. Care must be taken when using this device due

to its extreme sensitivity.

Specifications

• Frequency Range: 50 MHz - 20 GHz

• Dynamic Range (sliding load):

• 50 MHz - 840 Mhz : 77 dB

• 840 MHz - 20 GHz : 100 dB

• Port Directivity (sliding load):

• 50 MHz - 2 GHz : 48 dB

60

• 2 GHz - 20 GHz : 44 dB

• Maximum Power : 5 dBm

• Port Impedance : 50 ohms

For Detailed Specifications please see the data sheet for the 8720ES. It includes detailed

information concerning different cable and calibration settings:

Ports/Connections

There are two primary ports on the 8720ES. These ports are found near the bottom of the

front panel. They are labelled "Port 1" and "Port 2." See the following picture for a view of

the ports. Port 1 is used for measuring S11 and Port 2 is used for measuring S22. If S12 and

S21 are to be measured, then both ports must be connected. In this case, Port 1 is the input

port and Port 2 is the output port.

HP8720ES Ports

The back panel of the 8720es has a number of connections. Not surprisingly, one finds the

power connector on the back. In addition, the GPIB port is found on the back of the device.

Other inputs, including an external trigger, are also located on the back panel

Operation

For basic S parameter measurements, connect the Port 1 and Port 2 to the DUT. If only S11

or S22 measurements are desired, it is possible to connect only one of the ports. Be very

careful and ensure that the cables are terminated with a non open circuited termination to

discharge any residual ESD! An adaptor kit is available to accommodate male or female

connectors.

Prior to running any tests, it is important to ensure that a good calibration state is loaded. This

can be done by pressing the save/recall button on the front panel of the device. To ensure the

accuracy of your test setup, it is a good idea to calibrate the system for the chosen

61

interconnects with the included calibration kit. This only takes a few minutes and is worth

doing to eliminate unexplainable results.

Calibration

The 8720ES is a very sensitive piece of equipment. As a result, proper calibration is vital to

obtaining good measured results. The calibration process is fairly simple, and a calibration kit

can be found in the lab. To initiate a calibration run, press the "cal" button on the front panel

and follow the on screen instructions.

There is a very high probability that the cables are broken at any given time. If the device

exhibits strange behaviour, check the cables.

• Protecting the input ports

To avoid damaging the input ports, make sure to short out the leads of any cables prior to

connecting them to the 8720ES. This can be done using the terminations in the calibration kit

(make sure you don't use the open circuit termination).

• Damaged Input Ports

If the machine is acting strange and you've verified that the cables are fine, then it is likely

that one of the input ports may be damaged

Characterization process for the crossed yagi .

Press the start button on the network analyser and leave it for at least half an hour to stabilize.

• Now connect the adapter to port number 1which ahs 50 ohm impedance.

• For setting up the frequency we press the start button and set the start frequency as

387Mhz and stop frequency as 487 MHz after that press the marker button on the

network analyser and enter the centre frequency at which we want to characterize in

our case it is 437 Mhz.

62

• Lets switch to format and press the format button to smith chart after pressing this

we get the smith chart with the marker at 437Mhz on the screen of network analyser.

• Now for the calibration of the network analyser we use three load one 50Ohm one

short load and one open.

• Then we press cal button which is the calibration button and go to calibrated value

then we need to press port 1 where we are calibrating and characterize the antenna.

• From the calibration kit we take open and short load and one 50ohm load.

• Now connect the open load with the adapter and check the all the fitting are tight.

• Press the open switch and done one fourth calibrations. Open this short one and

connect with the adapter and press the short button and when noise comes written it

means you can now open

• Connect the 50ohm impedance and you can see the marker button right pointing on

the middle of the smith chart. Press the load and one fourth calibration now if you

want you can press the format button and get back log match.

• We connect to the antenna and the plot is changing if we are changing the orientation

of antenna.

• One is advised to do this characterization in the open environment to avoid noise.

This is the basic characterization process of the Yagi antenna one should follow when

characterizing the antenna. Through the similar process with change in the start and stop

frequency and the centre frequency we characterized our 145 MHz antenna too. To find a

network analyser with required specification was not easy since the cost is too high. Although

we managed to characterized it in SAMIR at IIT BOMBAY

.

63

GROUNDSTATION

ANTENNAE

MOUNTING

INTERFACE

64

Antenna mounting interface

Our next task was to mount the fabricated and tested antenna on the roof of our college

building exactly above on the Advance Communication Lab. we are using rotor to rotate the

antenna. We have fixed our antenna on 30 degree elevation and have to rotate 180 degree

azimuth in order to track the Satellite. IITB students are using rotor to track the satellite and

using a rotor will easily solve the tracking issue Information about the rotor is given below:

Fig representing the rotor G-5400B

The Yaesu G-5400B provide 360 deg azimuth and 180 deg elevation control of medium and

large size unidirectional satellite antenna arrays under remote control from station operation

position. The two factory-lubricated rotator unit are housed in weatherproof melamine resin

coated die- cast aluminium. Rotor contains a thermal sensor to prevent damage from

overheating during period of high usage.

65

But the cost of rotor is around 30000 to 35000 rupees which is not in our budget and the fact

is we are making student ground station of total cost around 30000 including every

equipment and other expenditures. So we are using stepper motor instead of rotor controller

to rotate the antenna and track the satellite .Stepper motor will provide step wise rotation of

our antenna which would be fixed at 30 degree elevation and rotate 180 degree azimuthally.

We will adjust the step of stepper motor according to time period of satellite and Limits of

Visibility .When satellite will pass over our ground station under the Limit of Visibility

stepper motor will be synchronized with satellite using step wise rotation. Once a satellite

will get out of Limits of Visibility of our ground station antenna will go back to its original

position.

Stepper motor

A stepper motor (or step motor) is a brushless, synchronous electric motor that can divide a

full rotation into a large number of steps. The motor's position can be controlled precisely

without any feedback mechanism (see Open-loop controller), as long as the motor is carefully

sized to the application. Stepper motors are similar to switched reluctance motors (which are

very large stepping motors with a reduced pole count, and generally are closed-loop

commutated.)

Theory

A step motor can be viewed as a synchronous AC motor with the number of poles (on

both rotor and stator) increased, taking care that they have no common denominator.

Additionally, soft magnetic material with many teeth on the rotor and stator cheaply

multiplies the number of poles (reluctance motor). Modern steppers are of hybrid design,

having both permanent magnets and soft iron cores.

To achieve full rated torque, the coils in a stepper motor must reach their full rated

current during each step. Winding inductance and reverse EMF generated by a moving rotor

tend to resist changes in drive current, so that as the motor speeds up, less and less time is

spent at full current thus reducing motor torque. As speeds further increase, the current will

not reach the rated value, and eventually the motor will cease to produce torque.

66

Basic operation

Stepper motors operate differently from DC brush motors, which rotate when voltage is

applied to their terminals. Stepper motors, on the other hand, effectively have multiple

"toothed" electromagnets arranged around a central gear-shaped piece of iron. The

electromagnets are energized by an external control circuit, such as a microcontroller. To

make the motor shaft turn, first one electromagnet is given power, which makes the gear's

teeth magnetically attracted to the electromagnet's teeth. When the gear's teeth are thus

aligned to the first electromagnet, they are slightly offset from the next electromagnet. So

when the next electromagnet is turned on and the first is turned off, the gear rotates slightly to

align with the next one, and from there the process is repeated. Each of those slight rotations

is called a "step," with an integer number of steps making a full rotation. In that way, the

motor can be turned by a precise angle

Stepper motor characteristics

1. Stepper motors are constant power devices.

2. As motor speed increases, torque decreases. (most motors exhibit maximum torque

when stationary, however the torque of a motor when stationary is of little use, torque

is more important when the motor is actually spinning)

3. The torque curve may be extended by using current limiting drivers and increasing the

driving voltage (sometimes referred to as a 'chopper' circuit, there are several off the

shelf driver chips capable of doing this in a simple manner).

4. Steppers exhibit more vibration than other motor types, as the discrete step tends to

snap the rotor from one position to another, (this is important as at certain speeds the

motor can actually change direction).

5. This vibration can become very bad at some speeds and can cause the motor to lose

torque (or lose direction). You must pay attention to the torque of the motor)

6. The effect can be mitigated by accelerating quickly through the problem speeds range,

physically damping (frictional damping) the system, or using a micro-stepping driver.

7. Motors with a greater number of phases also exhibit smoother operation than those

with fewer phases (this can also be achieved through the use of a micro stepping drive

67

Stepper Motor Diagram:

Frame 1:

The top electromagnet (1) is turned on, attracting the nearest tooth of a gear-

shaped iron rotor. With the teeth aligned to electromagnet 1, they will be slightly offset

from electromagnet 2.

Frame 2:

The top electromagnet (1) is turned off, and the right electromagnet (2) is energized,

pulling the nearest teeth slightly to the right. This results in a rotation of 3.6° in this

example.

Frame 3:

The bottom electromagnet (3) is energized; another 3.6° rotation occurs.

68

Frame 4:

The left electromagnet (4) is enabled, rotating again by 3.6°. When the top

electromagnet (1) is again enabled, the teeth in the sprocket will have rotated by one

tooth position; since there are 25 teeth, it will take 100 steps to make a full rotation in

this example.

Mounting design

When we came to know that our antenna is working properly ,we must have to go for

antenna mounting .Since It is the main part of ground station.

Fig: mouting design interface diagram.

69

Operation of the design:

The above diagram shows the proposed antenna mounting interface which we will implement

in upcoming days. The two antennae(centre frequencies at 437Mhz and another at 145Mhz)

mounted on the top. The antenna will be mechanically fixed at 30 degree elevation and the

whole system will be rotated in 180 degree azimuthal with the help of stepper motor. The

main vertical rod which will be rotated by the stepper motor should be attached to a fix point

on the horizontal rod to provide stability to the system and the point can be calculated as

follows:

Assuming

The mass of 437Mhz antenna is m1

And the mass of 145Mhz antenna of m2

The distance form the437 Mhz antenna end is r1

The distance form the 145Mhz antenna end is r2

From the formula ,

m1*r1=m2*r2

And the another equation is,

r1+r2=r

where r=wavelength corresponds to 145MHz.

By knowing the value of masse of 437Mhz antenna and 145Mhz antenna and the horizontal

rod length we can calculate the point where we can joint our vertical rod to provide stability.

Bearing

Now to provide the stability of the system we are using two plain bearing to bear the system

so as the full load will be balanced by the bearing and we will keep the stepper motor in base

for the stepwise rotation.A bearing is a device to allow constrained relative motion between

two or more parts, typically rotation or linear movement. Bearings may be classified broadly

70

according to the motions they allow and according to their principle of operation as well as

by the directions of applied loads they can handle.

Fig: A cutaway example of a four-point contact ball bearing

Plain bearing

A plain bearing, also known as a plane bearing, is the simplest type of bearing comprising of

just a bearing surface and no rolling elements, therefore the journal (i.e., the part of the shaft

in contact with the bearing) slides over the bearing surface. The simplest example of a plain

bearing is a shaft rotating in a hole. A simple linear bearing can be a pair of flat surfaces

designed to allow motion; e.g., a drawer and the slides it rests on or the ways on the bed of a

lathe.Plain bearings, in general, are the least expensive type of bearing. They are also

compact, light weight, and have a high load-carrying capacity.

Description-Rubbing surfaces, usually with lubricant; some bearings use pumped lubrication

and behave similarly to fluid bearings.

Friction -Depends on materials and construction, PTFE has coefficient of friction ~0.05-0.35,

depending upon fillers added

Stiffness -Good, provided wear is low, but some slack is normally present

Speed -Low to very high

Life -Moderate (depends on lubrication)

Now next is the study of polarization measurement set up i.e. IC AD8302.

71

POLARIZATION

MEASUREMENT

SETUP

72

ANALOG IC AD 8302

This is the set up used for measurement of polarization angle difference and its intensity from the received signal from the two antennas.

GENERAL DESCRIPTION ABOUT AD8302

The AD8302 measures the magnitude ratio, defined here asgain, and phase difference

between two signals. A pair of matched logarithmic amplifiers provides the measurement,

and their hard-limited outputs drive the phase detector

Basic Theory

Logarithmic amplifiers (log amps) provide a logarithmic compression function that converts

a large range of input signal levels to a compact decibel-scaled output. The general

mathematical form is:

V OUT =VSLP log (VIN /VZ) (1)

73

where VIN is the input voltage, VZ is called the intercept (voltage), and VSLP is called the

slope (voltage). It is assumed throughout that log(x) represents the log10(x) function. VSLP is

thus the volts/decade, and since a decade of voltage corresponds to 20 dB, VSLP/20 is the

volts/dB. VZ is the value of input signal that results in an output of zero and need not

correspond to a physically realizable part of the log amp signal range. While the slope is

fundamentally characteristic of the log amp, the intercept is a function of the input waveform

as well. Furthermore, the intercept is typically more sensitive to temperature and frequency

than the slope. When single log amps are used for power measurement, this variability

introduces errors into the absolute accuracy of the measurement since the intercept represents

a reference level. The AD8302 takes the difference in the output of two identical log amps,

each driven by signals of similar waveforms but at different levels. Since subtraction in the

logarithmic domain corresponds to a ratio in the linear domain, the resulting output becomes:

V MAG =VSLP log (VINA /VINB) (2)

where VINA and VINB are the input voltages, VMAG is the output corresponding to the

magnitude of the signal level difference, and Unlike the measurement of power, when

measuring a dimensionless VSLP is the slope. Note that the intercept, VZ, has dropped out.

quantity such as relative signal level, no independent reference or intercept need be invoked.

In essence, one signal serves as the intercept for the other. Variations in intercept due to

frequency, process, temperature, and supply voltage affect both channels identically and

hence do not affect the difference. This technique depends on the two log amps being well

matched in slope and intercept to ensure cancellation. This is the case for an integrated pair of

log amps. Note that if the two signals have different waveform (e.g., different peak-to-

average ratios) or different frequencies, an intercept difference may appear, introducing a

systematic offset. The log amp structure consists of a cascade of linear/limiting gain stages

with demodulating detectors. Further details about the structure and function of log amps can

be found in data sheets for other log amps produced by Analog Devices.2 The output of the

final stage of a log amp is a fully limited signal over most of the input dynamic range. The

limited outputs from both log amps drive an exclusive-OR style digital phase detector.

Operating strictly on the relative zero-crossings of the limited signals, the extracted phase

difference is independent of the original input signal levels. The phase output has the general

form:

74

VPHS =V√[√(VINA) √(VINB)] (3)

where V√ is the phase slope in mV/degree and √ is each signal’s relative phase in degrees.

STRUCTURE:

Fig: General Structure

The general form of the AD8302 is shown in Figure. The major blocks consist of two

demodulating log amps, a phase detector, output amplifiers, a biasing cell, and an output

reference voltage buffer. The log amps and phase detector process the high frequency signals

and deliver the gain and phase information in current form to the output amplifiers. The

output amplifiers determine the final gain and phase scaling. External filter capacitors set the

averaging time constants for the respective outputs. The reference buffer provides a 1.80 V

reference voltage that tracks the internal scaling constants.

Each log amp consists of a cascade of six 10 dB gain stages with seven associated detectors.

The individual gain stages have 3 dB bandwidths in excess of 5 GHz. The signal path is fully

differential to minimize the effect of common-mode signals and noise. Since there is a total

of 60 dB of cascaded gain, slight dc offsets can cause limiting of the latter stages, which may

cause measurement errors for small signals. This is corrected by a feedback loop.

75

The nominal high-pass corner frequency, fHP, of this loop is set internally at 200 MHz but

can be lowered by adding external capacitance to the OFSA and OFSB pins. Signals at

frequencies well below the high-pass corner are indistinguishable from dc offsets and are also

nulled.

The difference in the log amp outputs is performed in the current domain, yielding by

analogy to Equation 2:

I LA = ISLP log(VINA /VINB)

Where ILA and ISLP are the output current difference and the characteristic slope (current) of

the log amps, respectively. The slope is derived from an accurate reference designed to be

insensitive to temperature and supply voltage. The phase detector uses a fully symmetric

structure with respect to its two inputs to maintain balanced delays along both signal paths.

Fully differential signalling again minimizes the sensitivity to common-mode perturbations.

The current-mode equivalent to Equation 3 is:

IPD= I√[√(VINA) √(VINB) 90°] (5)

Where IPD and I√ are the output current and characteristic slope associated with the phase

detector, respectively. The slope is derived from the same reference as the log amp slope.

PIN CONFIGURATION

76

PIN DISCRIPTION

Pin no Mnemonic Function 1,7 COMM Device Common. Connect to

low impedance ground

2 INPA High Input Impedance to

Channel A. Must be ac-

coupled

3 OFSA A capacitor to ground at this

pin sets the offset

compensation filter corner

and provides input

decoupling.

4 VPOS Voltage Supply (VS), 2.7 V

to 5.5 V

5 OFSB A capacitor to ground at this

pin sets the offset

compensation filter corner

and provides input

decoupling.

6 INPB Input to Channel B. Same

structure as INPA.

8 PFLT Low Pass Filter Terminal for

the Phase Output

9 VPHS Single-Ended Output

Proportional to the Phase

Difference between INPA

and INPB.

10 PSET Feedback Pin for Scaling of

VPHS Output Voltage in

Measurement Mode. Apply a

77

FEATURES

• Measures Gain/Loss and Phase up to 2.7 GHz

• Dual Demodulating Log Amps and Phase Detector Input –60 dBm to 0 dBm in a

50 _ System

• Accurate Gain Measurement Scaling (30 mV/dB)

• Typical Nonlinearity < 0.5 dB

• Accurate Phase Measurement Scaling (10 mV/Degree)

setpoint voltage for

controller mode

11 VREF Internally Generated

Reference Voltage (1.8 V

Nominal)

12 MSET Feedback Pin for Scaling of

VMAG Output Voltage

Measurement Mode.

Accepts a set point voltage in

controller mode.

13 VMAG Single-Ended Output. Output

voltage proportional to the

decibel ratio

of signals applied to INPA

and INPB

14 MFLT Low Pass Filter Terminal for the Magnitude Output

78

• Typical Nonlinearity < 1 Degree

• Measurement/Controller/Level Comparator Modes

• Operates from Supply Voltages of 2.7 V–5.5 V

• Stable 1.8 V Reference Voltage Output

• Small Signal Envelope Bandwidth from DC to 30 MHz

APPLICATIONS

• RF/IF PA Linearization

• Precise RF Power Control

• Remote System Monitoring and Diagnostics

• Return Loss/VSWR Measurements

Now we will go to the next task that is to study about ionosphere and total electron count of

ionosphere and its applications which is the main application of our project.

79

TEC

TOTAL ELECTRON COUNT

OF

IONOSPHERE

80

1. Introduction

The ionosphere is a complex part of the atmosphere, lying from about 60 km of altitude up to

several hundreds of kilometres. The ionising radiations of the sun and energetic particles

transported by the solar wind produce concentration of free electrons especially in the 250-

400 km high layer known as the F-region. Our aim is to calculate total electron count there.

TEC refers to the Total Electron Count of the Ionosphere. It refers to the total number of

electrons in a cylinder of unit area of cross section extending from the ground station up to

our satellite in space. Total electron content (or TEC) is an important descriptive quantity for

the ionosphere of the Earth. TEC is the total number of electrons present along a path

between two points, with units of electrons per square meter, where 1016 electrons/m² = 1

TEC unit (TECU). Total electron count (TEC) is a measure used to characterize the

conductivity of the ionosphere, which consists of ionized layers of the upper atmosphere.

Free electrons in the ionosphere affect the transmission of radio waves by absorbing and

reflecting, which slows the propagation of radio signals through the ionosphere.TEC values

are one of the most prominent sources of information for understanding the structure and

dynamic behaviour of the ionosphere. However, since there is a fraction of the ionosphere

(about 5%) above the altitude of our satellite, what we wish to measure is known as the

Ionosphere Electron Count. Estimates of the TEC can be used to correct for transmission

delays in GPS signals, which can be incorporated in GPS receivers for more accurate location

estimates.TEC is significant in determining the scintillation and group delay of a radio wave

through a medium.

Ionospheric TEC is characterized by observing carrier phase delays of received radio signals

transmitted from satellites located above the ionosphere, often using Global Positioning

System satellites. TEC is strongly affected by solar activity.

81

2. Importance of TEC

2.1 Error correction for GPS systems

These days GPS satellites are used on many fronts and their use for position determination

has proved to be important and useful for not only navigation but also several other research

applications. But in order to achieve a high level of accuracy from GPS (of the order of

millimetres), one must correct the carrier phase advance and pseudo range group delay that

are caused when GPS signals pass through the ionosphere. Free electrons in the ionosphere

affect the transmission of radio waves by absorbing and reflecting, which slows the

propagation of radio signals through the ionsphere.If we have a dual frequency GPS receiver,

we can then deal with this error by taking into account ionosphere's dispersive nature. But if

we are using a single frequency GPS receiver then we need some other method for getting rid

of this error. Most GPS companies provide an inbuilt GPS model that predicts TEC values on

the basis of previous history sheets. But the fact that we are situated near the equator makes

life a bit difficult for us because most of these models are based on data points obtained

predominantly from mid latitude regions and have very sparse data of equatorial region.

Further, the equatorial region is highly susceptible to fluctuations in TEC values due to

various reasons. So these models fail to deliver a satisfactory performance. An increase in

data points in this region can be used by GPS manufacturers for correcting this Ionospheric

error. The currently used global Ionospheric models can only model the monthly mean total

electron content to about ± 10 percent. These models cannot tell us about the day-to-day

variability of the TEC which can be 20 to 25 percent (1 sigma) of the monthly mean value.

So, even if the monthly mean TEC is modelled perfectly using one of the global Ionospheric

models, TEC predictions can be off by 20 to 25 percent. If the bias between the monthly

mean TEC and the predictions provided by these models is 10 percent of the monthly mean

values, then the performances of these models are considered to be excellent. The combined

effect of the error in the monthly mean predictions and the day-to-day variability of the TEC

will result in an overall performance of these models to be at about 22-27 percent (RMS) of

the Ionospheric delay. The performance can only be worse if we take into account the

potential effect of a solar or geomagnetic storm during medium or high solar activity times.

82

This is in addition to the fact that most of these single frequency GPS receivers use data

models that use values obtained from South American & African Stations. So anyways the

data for equatorial region is very sparse, so to have better accuracy of GPS in India we will

have to device methods that can give us additional information of day to day variability. We

wish to do it by supplementing the current database and the ongoing Indian projects for TEC

measurement with our TEC data.

After integrating the group and phase delay along the GPS signal path we will obtain a range

between satellite and receiver that is different from geometrical distance between them. This

difference is known as ionospheric error. Error is negative for carrier phase (phase is

advanced, measured range is shorter than the geometrical range) and positive for pseudo

ranges (i.e. phase is delayed, so measured range is greater than geometrical range).These two

are equal in magnitude but opposite in sign.

2.2 EIA (Appleton Anomaly) & ESF:

EIA, or equatorial ionization anomaly is characterized by a depression in ionization

densities (or trough) at the geomagnetic equator and two peaks (crests) on either side of the

equator at about 15° magnetic latitudes.

Possible reasons:

It is suggested that the trough exists because plasma produced by photo ionization at great

heights over the magnetic equator diffuses downwards and outwards to the north and south

leaving depletion at the equator.

Another explanation is that the mutually perpendicular east-west electric field and north-

south geomagnetic field give rise to an upward electrodynamics (E × B) drift of plasma

during the daytime. As the plasma is lifted to greater heights, it diffuses downward along

geomagnetic field lines towards higher latitudes under the influence of gravity and pressure

gradients and produces the anomaly. The EIA is also asymmetric about the geomagnetic

equator, caused by field aligned plasma flow due to factors like neutral winds.

An interesting feature in the geographic location of India is that the magnetic equator passes

through the bottom side tip of the country and the northern crest of the equatorial ionization

anomaly lies in the middle of the country, providing a unique opportunity for making studies

on the latitudinal variation equatorial ionization anomaly (EIA) which occurs at 5 – 25

83

geographic latitude. So as Mumbai falls in this region, we can easily get involved with this

experiment.

ESF,or equatorial spread-f

On certain nights, the spread in the width of the pulses reflected from the F-layer increases

abnormally, showing up as a diffuse trace, and this phenomenon is called Spread-F. ESF

actually refers to formation of areas in ionosphere that have abnormally low plasma density.

These bubbles are found to travel upwards in the ionosphere.

Link between EIA & ESF:

Studies have shown that ESF is closely linked with EIA. EIA as we know is intensification

of ionization crests in the late afternoon near +/- 20deg latitude. Formation of EIA is due to

flow of plasma along a fountain from the equator towards the crest, which is caused due to

Electric field generated in the atmosphere. This increase in ionization comes at the expense of

depletion in ionization of lower F layer (near the equator). So we get very steep plasma

gradients in the equatorial region, which are conducive to Rayleigh Taylor instability and

explains the relationship between EIA & ESF.

3. Variation of TEC values:

This randomness in the day-to-day variation in TEC may be attributed to the changes in the

activity of the sun itself and to the associated changes in the intensity of the incoming

radiations, and the zenith angle at which they fall on the Earth’s atmosphere, in addition to

the changes which take place in the Earth’s magnetic field and the equatorial electro jet (EEJ)

strength, added to the effects due to the dynamics of the neutral winds.

A logical explanation for day night variation in TEC can be given as:- During night time,

there is no solar radiation falling on the atmosphere, so there is nothing that is going to ionize

the atoms present in the ionosphere. But the opposite process of recombination of ions takes

place and so there is a decrease in ion density of atmosphere and hence reduced TEC is

observed. During afternoon when the solar radiation is maximum, ionization increases,

overtaking the process of recombination of ions and hence TEC value increases.

84

Variation in equatorial regions:

The daily variation in TEC at the EIA region shows its steep increase and reaches its

maximum value between 13:00 and 16:00 LT, while at the equator the peak is broad i.e. it

occurs for a longer duration of time and occurs around 16:00 LT. A short-lived day minimum

occurs between 05:00 to 06:00 LT at all the stations from the equator to the EIA crest region.

Beyond the crest region the day maximum values decrease with the increase in latitude, while

the day minimum in TEC is flat during most of the nighttime hours, i.e. from 22:00 to 06:00

LT, a feature similar to that observed in the mid-latitudes. Further, the diurnal variation in

TEC show a minimum to maximum variation of about 5 to 50 TEC units, respectively, at the

equator and about 5 to 90 TEC units at the EIA crest region.

Climatic variations in TEC:

The seasonal variation in TEC maximizes during the equinox months (equinox is that period

of time when sun passes over equator and lengths of day and night become equal , this occurs

on 21st March and 22nd September) followed by winter and is minimum during the summer

months.

Contour plots of the monthly average diurnal variation of TEC at four stations across India (taken from the results of GPS Aided Geo Augmented Navigation (GAGAN)

project of ISRO + AAI)

85

The lack of complete understanding of the above phenomena often leads to navigational

errors and communication losses. TEC measurements are necessary to improve our

understanding of the interaction between the solar wind and the earth’s ionosphere, which is

very crucial for the functioning of several space borne and ground based systems and also to

increase our capability for forecasting space weather events. Our venture will shed some light

on some of the complex phenomena occurring in space.

4. Orbit

The efficacy of our endeavour depends entirely on the orbit we get for the satellite. Most of

the Ionospheric phenomena occur at specific times of the day and we need the satellite to

have a pass over the ground stations at those times for us to obtain the relevant TEC values.

A circular orbit with an inclination of approximately 24 degrees would be optimum for the

mission since it will render several passes at different times of the day and these passes would

occur at different times of the day over the course of the lifetime of the satellite. The exact

value of the actual inclination would depend on the location of the ground stations, which

depends on the efficacy of the tomography algorithms. The satellite would be launched along

with a mainstream satellite of ISRO which would invariably put it in a polar sun synchronous

orbit. This would only allow us to measure the TEC values at the same local time. Given this

limitation, a 2:30 am-pm orbit would be optimum for the satellite as it would allow us to

measure the TEC values at a time when the EIA is at its peak.

5. Techniques for measurement of TEC from LEO satellite

The various techniques that can be used to measure TEC from LEO satellites are described

below:

1. Measurement of Faraday rotation (our method)

2. Measurement of group delay of received signal

86

3. Measurement of Doppler shift of received signal due to ionospheric fluctuations

4. Measurement of amplitude scintillations of received signal.

Technique of Measurement of Faraday rotation:

When a linearly polarized radio wave passes through an ionized medium with a magnetic

field in the direction of propagation, the plane of polarization rotates. This effect is called

Faraday rotation. The relation between the rotation angle and the TEC is given by

B = magnetic field of earth,

Θ = angle between the radio wave and line of sight,

Δφ = angle of rotation,

f = frequency of the wave.

Measurement process: In order to radiate linearly polarized waves from the satellite,a

monopole antenna is to be used. The radiation pattern of a monopole is such that the radiation is

always polarized in the direction assuming the monopole is oriented in the direction. Thus, we

know the initial polarization angle.

Measurement at the ground station:

We will be measuring the angle of polarization at the

ground station by using a crossed Yagi antenna and measuring the intensities of the signals at the

two feeds.

Resolution of the nπ ambiguity, use of two close frequencies: A major problem of the process

is the fact that there would always be an ambiguity of nπ in the angle measured at the ground

station. The formula for Faraday rotation gives an angle of about 110° for the rotation at a

frequency of 433 MHz even when the satellite is overhead. The maximum angle at this

frequency comes out to be about 600° which is pretty large. We propose to resolve this

ambiguity by use of two very nearby frequencies viz. 400 and 433 MHz and then measuring the

difference between the angles of polarization of the two waves. The difference between the

angles is given by:

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This is not very large and can be easily measured without any ambiguity as the maximum value

of this angle is only about 90°. We plan is to use the frequencies 400 and 433 MHz .

The following graphs illustrate the situation:

However, to measure the actual difference between the angles of rotation of the two

frequencies without any attitude data on ground will be impossible as the signals will have

some initial angle between them which will depend upon the yaw of the satellite and the

angle between the antennae. But if we keep the antennae perfectly parallel, then this initial

angle will be 0, irrespective of the yaw angle.

Advantages:

The advantages of this process are the following:

• Needs comparatively less hardware on board the satellite.

• Measurement is done only on carrier phase polarization angle which is an inherent

property of the signal unlike a modulated signal where errors might arise.

Disadvantages:

The disadvantages of this process are as follows:

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• The signal transmitted from the satellite must have a high degree of polarization

purity which is not so easy.

• >The final result will not be directly proportional to the TEC but will depend upon the

magnetic field along the path as well. Thus this dependence will have to be accounted

for by some other way.

• The angle between the antennae will have to be very small.

6. Ionospheric Tomography By Faraday rotation method, we can measure the TEC (Total Electron Content) at various elevation angles of the satellite. This gives us information about the integrals of electron density in various directions. From these integrals, we can derive the values of the electron densities at various positions of the ionosphere. This technique is called Ionospheric tomography.

Fig. 1: Geometry of the situation

Fig. 2: tomographic image

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FINAL

GROUNDSTATION

SETUP

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Ground station diagram.

Fig: final ground station set up

The total connection of the ground station is shown in the diagram.

• The signals from the antenna will be given to LNA’s(4) for amplification of signals

and bringing them to an appropriate level.

• Then the output will be given to polarization measurement setup which will give us

the phase difference between the signals and its intensity.

• The output will be stored in a PC at regular level and at periodic time intervals and the

data will be sent to IIT-B for further calculations and processing.

LNA AD 8302 SETUP

PC

ROTATING INTERFACE

USING

STEPPER MOTOR.

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COST ESTIMATION OF THE PROJECT:

COMPONENTS QUANTITY COST (Rs.)

CROSSED YAGI

FABRICATION

2 4000

LNA

4 800

AD-8302 POLARIZATION

MEASUREMENT SET-UP.

1 19000

CHARACTERIZATION - 1500

ANTENNA MOUNTING - 2000

STEPPER MOTOR 1 1200

WIRES , CONNECTORS. - 500

EXTRA - 1000

TOTAL - 30000

The final ground station will be seen working in oct-nov 2010 after the successful launch of

the IIT-B “Pratham” satellite

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References:

• aero.iitb.ac.in/Pratham/

• www.wikipedia.org

• Various project documents available on the Pratham website, like.

o Conceptual Design Report On Board Computing .

o CDR on Communication Subsystem.

o CDR on Report System Engineering.

o Overall Minutes Of CDR at ISAC.

o Project Design Report On Communication Subsystem.

• “Master Thesis” by Anura Wickramanayake.

• “Antenna Theory” by Balanis.

• Yagi Antenna Design" by P Viezbicke from the National Bureau of

Standards, 1968