Lecture Set Three-Microwave Antennas

8/11/2019 Lecture Set Three-Microwave Antennas

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kyu microwave antennas ET422/2014

Microwave antennas

An antenna is a component that radiates and receives the RF or microwave power. It is a

reciprocal device, and the same antenna can serve as a receiving or transmitting device. Antennas

are structures that provide transitions between guided and free-space waves. Guided waves are

confined to the boundaries of a transmission line to transport signals from one point to another ,

while free-space waves radiate unbounded. A transmission line is designed to have very little

radiation loss, while the antenna is designed to have maximum radiation.

The antenna is a key component in any wireless system, as shown in below

The RF/microwave signal is transmitted to free space through the antenna. The signal propagates

in space, and a small portion is picked up by a receiving antenna. The signal will then be

amplified, down converted, and processed to recover the information.

Different categories of antennas

Wire antennas: These include dipoles, monopoles, loops, yagi yuda arrays etc. wire antennas

are characterized by low gains and used at lower frequencies .They have advantage of light

weight, low cost and simple design

Aperture antennas:

Include open ended waveguides, rectangular or circular horns, reflectors etc. Aperture antennas

are most commonly used at microwave frequencies and have moderate to high gains

Antenna arrays:

They consist of a regular arrangement of antenna elements with feed network. They aremotivated by two reason: Beam steering and beam nulling. Pattern characteristics such as beam

pointing angle and sidelobe levels can be controlled by adjusting the amplitude and phase

distribution of the array elements.


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Antenna characteristics and parameters

These parameters provide information about the properties and characteristics of an antenna

1. Radiation pattern

An antenna radiation pattern or antenna pattern is defined as a mathematical function or agraphical representation of the radiation properties of the antenna as a function of space

coordinates. In most cases, the radiation pattern is determined in the far field region and is

represented as a function of the directional coordinates. Radiation properties include power flux

density, radiation intensity, field strength, directivity etc.

For an antenna, the

a. Fieldpattern ( in linear scale) typically represents a plot of the magnitude of the electric or

magnetic field as a function of the angular space.

b. Powerpattern ( in linear scale) typically represents a plot of the square of the magnitude of

the electric or magnetic field as a function of the angular space.

c.

Power pattern ( in dB) represents the magnitude of the electric or magnetic field, in

decibels, as a function of the angular space.


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A major lobe (also called main beam) is defined as the radiation lobe containingthe direction of

maximum radiation

A minor lobe is any lobe except a major lobe

Aside lobe is a radiationlobe in any direction other than the intended lobe. (Usually

a side lobe is adjacent to the main lobe and occupies the hemisphere in the direction

of the main beam.)


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A back lobe is a radiationlobe whose axis makes an angle of approximately 180with respect

to the beam of an antenna. Usually it refers to a minor lobe that occupies the hemisphere in a

direction opposite to that of the major (main) lobe.

Minor lobes usually represent radiation in undesired directions, and they should be minimized.

Side lobes are normally the largest of the minor lobes.

The level of minor lobes is usually expressed as a ratio of the power density in the lobe in

question to that of the major lobe. This ratio is often termed the side lobe ratio or side lobe level

Isotropic,Directional And Omnidirectional Patterns

An isotropic radiator is defined as a hypothetical lossless antenna having equal radiation in all

directions. Although it is ideal and not physically realizable, it is often taken as a reference for

expressing the directive properties of actual antennas. A directional antenna is one having the

property of radiating or receiving electromagnetic waves more effectively in some directions

than in others

This type of a pattern is designated as omnidirectional, and it is defined as one having an

essentially nondirectional pattern in a given plane (in this case in azimuth) and a directional

pattern in any orthogonal plane (in this case in

elevation). An omnidirectionalpattern is then a special type of a directionalpattern

Field Regions

The space surrounding an antenna is usually subdivided into three regions: (a) reactive near-

field, (b) radiating near-field (Fresnel) and (c) far-field (Fraunhofer) regions as shown in Figure

below. Although no abrupt changes in the field configurations are noted as the boundaries are

crossed, there are distinct differences among them. The boundaries separating these regions are

not unique, although various criteria have been established and are commonly used to identifythe regions.

Reactive near-field region is defined as that portion of the near-field region immediately

surrounding the antenna wherein the reactive field predominates. For most antennas, the outer

boundary of this region is commonly taken to exist at a distance R < 3

0.62 /D from the

antenna surface, where is the wavelength andD is the largest dimension of the antenna. For a

very short dipole, or equivalent radiator, the outer boundary is commonly taken to exist at a

distance/2 from the antenna surface.

Radiating near-field (Fresnel) region is defined as that region of the field of an antenna

between the reactive near-field region and the far-field region wherein radiation fields

predominate and wherein the angular field distribution is dependent upon the distance from the

antenna. If the antenna has a maximum dimension that is not large compared to the wavelength,

this region may not exist.

Far-field (Fraunhofer) region is defined as that region of the field of an antenna where the

angular field distribution is essentially independent of the distance from the antenna. If the


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antenna has a maximum overall dimensionD, the far-field region is commonly taken to exist at

distances greater than 2D2/ from the antenna,being

the wavelength.

Far f ield propert ies

The EM field in far field satisfies the following properties:

1. The Electric and magnetic fields are orthogonal

2. The ratio of the E and H fields is a constant and equal to the intrinsic impedance of the

medium

Thus EH

3. The fields in far field region are plannar.

Radian and Steradian

The measure of a plane angle is a radian. One radian is defined as the plane angle with its vertex

at the center of a circle of radius r that is subtended by an arc whose length is r. Since the

circumference of a circle of radius r is C = 2r, there are 2 rad (2r/r) in a full circle.

The measure of a solid angle is asteradian. Onesteradian is defined as the solid angle with its

vertex at the center of a sphere of radius r that is subtended by a spherical surface area equal tothat of a square with each side of length r. Since the area of a sphere of radius r isA = 4r2,

there are 4 sr (4r2/r2) in a closed sphere.

Radiation power density

Electromagnetic waves are used to transport information through a wireless medium or a guiding

structure, from one point to the other. It is then natural to assume that power and energy are


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associated with electromagnetic fields. The quantity used to describe the power associated with

an electromagnetic wave is the instantaneous Poynting vector defined as

W=EXH

Where

W = instantaneous Poynting vector (W/m2)E= instantaneous electric-field intensity (V/m)

H= instantaneous magnetic-field intensity (A/m)

Since the Poynting vector is a power density, the total power crossing a closed surface can be

obtained by integrating the normal component of the Poynting vector over the entire surface. In

equation form,

The time average Poynting vector (average power density) can be written as

Based upon the definition of (2-8), the average power radiated by an antenna (radiated

power) can be written as


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Radiation Intensity

Radiation intensity in a given direction is defined as the power radiated from anantenna per

unit solid angle. The radiation intensity is a far-field parameter, and it can be obtained by

simply multiplying the radiation density by the square of thedistance.

In mathematical form it is expressed as

The total power is obtained by integrating the radiation intensity over the entire solid angle of 4.

Thus

Beam Width

The beam width of a pattern is defined as the angular separation between two identical points on

opposite side of the pattern maximum.

One of the most widely used beamwidths is theHalf-Power Beamwidth (HPBW )HPBW definition: In a plane containing the direction of the maximum of a beam, the angle

between the two directions in which the radiationIntensity is one-half value of the beam..

Another important beamwidth is the angular separation between the first nulls of the pattern, and

it is referred to as theFirst-Null Beamwidth (FNBW).


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Antenna Efficiency

Resistive losses due to non perfect and dielectric materials exist in all antennas. Such losses

result into a difference between the power delivered to the input of the antenna and the power

radiated by that antenna

We define radiation efficiency of an antenna as the ratio of the desired output power to thesupplied input power

1rad in loss loss

rad

in in in

p p p p

p p P

The total antenna efficiency e0 is used to take into account losses at the input terminals and

within the structure of the transmission line antenna system. Such losses may be due,

1. Reflections because of the mismatch between the transmission line and the antenna

2.I2R losses (conduction and dielectric losses)


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Gain

Another useful measure describing the performance of an antenna is thegain. Although

the gain of the antenna is closely related to the directivity, it is a measure that takes into the

antenna efficiency as well as its directional capabilities

Gain of an antenna (in a given direction) is defined as the ratio of the intensity, in a given

direction, to the radiation intensity that would be obtained if the power accepted by the antenna

were radiated isotropically. The radiation intensity corresponding to the isotropically radiated

power is equal to the power accepted (input) by the antenna divided by 4. In equation form

this can be expressed as

Note that the total radiated power (Prad) is related to the total input power (Pin)by Prad=ecd Pin

where ecdis the antenna radiation efficiency.

Effective area/Aperture:

Effective area (aperture), in a given direction is defined as the ratio of the available power at

the terminals of a receiving antenna to the power flux density of a plane wave incident on theantenna from that direction, the wave being polarization matched to the antenna. If the direction

is not specified, the direction of maximum radiation intensity is implied.

In general, the maximum effective aperture (Ae) of receiving antenna is related to the gain of the

antenna as,


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2

4e rA G

Input Impedance

Input impedance is defined as the impedance presented by an antenna at its terminals or the

ratio of the voltage to current at a pair of terminals or the ratio of the appropriate components of

the electric to magnetic fields at a point.

The ratio of the voltage to current at these terminals, with no load attached, defines the

impedance of the antenna as A A AZ R jX

whereA

Z =Antenna impedance at input terminals

AR =Antenna resistance at input terminals

AX =Antenna reactance at input terminals

Microwave AntennasHorn Antenna

The horn antenna is a transition between a waveguide and free space. A rectangular waveguide

feed is used to connect to a rectangular waveguide horn, and a circular waveguide feed is for the

circular waveguide horn. The horn antenna is commonly used as a feed to a parabolic dish

antenna, a gain standard for antenna gain measurements, and as compact medium-gain antennas

for various systems. Its gain can be calculated to within 0.1 dB accuracy from its known

dimensions and is therefore used as a gain standard in antenna measurements.

For a rectangular pyramidal horn, shown in Fig. below, the dimensions of the horn for optimum

gain can be designed by setting

where A and B are dimensions of the horn and leand lhare the slant lengths of the horn


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Parabolic Dish Antenna

A parabolic dish is a high-gain antenna. It is the most commonly used reflector antenna for

point-to-point satellites and wireless links. A parabolic dish is basically a metal dish illuminated

by a source at its focal point. The spherical wave front illuminated by the source is converted

into a planar wavefront by the dishFor an illumination efficiency of 100%, the effective area equals the physical area

where D is the diameter of the dish.

Radiation from parabolic dish antenna

Microwave propagation

In free space, electromagnetic waves propagate in straight lines without attenuation or other

adverse effects. Free space however is just an idealization that is only approximated when


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microwave energy propagates through the atmosphere or in the presence of the earth. In practice,

the performance of the communication system may be adversely be affected by effects such as

reflection, refraction, attenuation or diffraction and scattering

Attenuation: caused by absorption of microwave energy by water vapor and molecular oxygen

Ground effects: the most obvious effect of the presence of the ground on microwave

propagation is reflection from the earths surface. A receiver may be illuminated by both a direct

wave from the transmitter and a wave reflected from the ground. The reflected wave is smaller in

amplitude than the direct wave because of the larger distance it travels, the fact that it usually

originates from the side lobe region of the transmit antenna.

Plasma Effects: Plasma is a gas consisting of ionized particles. The ionosphere consists of

ionized particles due to solar radiation. Depending on the density of ions and frequency, wave

may be reflected, aborbed or transmitted by the plasma medium.

Microwave Biological effects and safety

The proven dangers of exposure to microwave radiation are due to thermal effects .The body

absorbs RF and microwave energy and converts it to heat; as in the case of the microwave oven,

this heating occurs within the body and may not be felt at low levels. such heating is more

dangerous in the brain, the eye and stomach organs.

Excessive radiation can lead to cataracts, sterility and cancer.

This makes it important to determine the safe radiation levels so that users of microwave

equipment will not be exposed to harmful power levels.

The most recent standards for human exposure as given by IEEE:In the RF microwave frequency range of 100 MHz to 300 GHz, exposure limits are set on the

power density (w/cm2) as a function of frequency. The recommended safe power density limit is

as low as 0.2 mw/cm2

at the lower end of the frequency range since the fields penetrate the body

more deeply at lower frequencies. At frequencies above 15 GHz the power density limit rises to

10 mW/cm2, since most of the power absorption at such frequencies occurs near the skin surface.

Other countries have different exposure limits some of which are a function of exposure time.

A separate standard applies to microwave ovens in the United States; Law requires that all

microwave ovens be tested to ensure that the power level at 5cm from any point of the oven does

not exceed 1 mW/cm2.

Exercise

1. A parabolic reflector antenna used for reception with DBS system is 0.45m in diameter

and operates at 12.4 GHz. Find the operating wavelength and the far field distance for

this antenna.

2. Distinguish between reflection, refraction, scattering and diffraction


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3. A 6 GHz common carrier microwave communication link uses a tower mounted antenna

with a gain of 40dB and a transmitter of power 5W.Evaluate the radiation hazard of this

system at a distance of 20m from the antenna.

4. A microwave antenna is characterised by Electric field intensity

sinoE A

r

field.Calculate power radiated at apoint located in far field

Lecture Set Three-Microwave Antennas

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