VLF LF MF and HF VLF LF MF and HF ANTENNASANTENNAS

CLASSIFICATION ACCORDING TO CLASSIFICATION ACCORDING TO

FREQUENCY BANDSFREQUENCY BANDS

VLF/LF Antennas and VLF/LF Antennas and Antenna SystemsAntenna Systems

VLF BandVLF Band

EMEM waves penetrate well in waves penetrate well intoto the sea water the sea water.. (Communications with submerged submarines)(Communications with submerged submarines)

Low atmospheric attenuationLow atmospheric attenuation..

Appropriate for long range communication.Appropriate for long range communication.

VLF AntennasVLF Antennas

• Ground and Sky wavesGround and Sky waves

• Frequeny range: 3-30 KHzFrequeny range: 3-30 KHz

• Antennas : very largeAntennas : very large

• Power: kW levels and even morePower: kW levels and even more

Some Problems Associated with VLF Some Problems Associated with VLF Antenna SystemsAntenna Systems

• Small Small Bandwidth Bandwidth (usually (usually less than 200 Hzless than 200 Hz))

• Small radiation resistanceSmall radiation resistance..

• High cost.High cost.

• Antenna system covers a large area.Antenna system covers a large area.

• Need for very high Need for very high power levels for transmission.power levels for transmission.

LF AntennasLF Antennas

Ground and Sky wavesGround and Sky waves

Frequeny range: 30-300 KHzFrequeny range: 30-300 KHz

Antennas: largeAntennas: large

Power: kW levels and even morePower: kW levels and even more

Some DisadvantagesSome Disadvantages

High cosHigh costt

Large Large DimensionsDimensions

TTroublroublee with efficiency with efficiency, power capacity, , power capacity, bandwidthbandwidth

VLF and LF antennas are VLF and LF antennas are “electrically small” antennas“electrically small” antennas : :

problem: high capacitive reactance and problem: high capacitive reactance and small antenna radiation resistancesmall antenna radiation resistance

remedy: top loadingremedy: top loading

Top-loadingTop-loading

Top-loadingTop-loading

increases gain bandwidth (by decreasing reactance)increases gain bandwidth (by decreasing reactance)

In VLF large top-loadingIn VLF large top-loading

supported by towerssupported by towers

A simple VLF/LF Transmitting A simple VLF/LF Transmitting AntennaAntenna

VLF / LF Ground SystemsVLF / LF Ground Systems

Radial-wire:Radial-wire:

radial wires buried in the groundradial wires buried in the ground

Multiple-star:Multiple-star:

small radial-wire systems forming a star topologysmall radial-wire systems forming a star topology

Basic Theory: The Basic Theory: The Vertical Electric Vertical Electric Monopole AntennaMonopole Antenna

Vertical Electric Monopole AntennaVertical Electric Monopole AntennaAssume uniform electric current I along a vertical monopole of Assume uniform electric current I along a vertical monopole of

effective height heffective height he :e :

electric field

magnetic field

Vertical Electric Monopole AntennaVertical Electric Monopole Antenna-Radiated Power--Radiated Power-

The vertical electric field in terms of radiated power is:

Vertical Electric Monopole AntennaVertical Electric Monopole Antenna-Equivalent Antenna Circuit--Equivalent Antenna Circuit-

Vertical Electric Monopole AntennaVertical Electric Monopole Antenna(Radiation Efficiency)(Radiation Efficiency)

where and

Effective power = (power capacity of the transmitter) x (antenna system efficiency)

antenna total loss resistance

Vertical Electric Monopole AntennaVertical Electric Monopole Antenna-Antenna Bandwidth--Antenna Bandwidth-

The 3 dB bandwidth The 3 dB bandwidth bb in (c/s) for a single resonant circuit is in (c/s) for a single resonant circuit is::

f : resonant frequency

Q: the circuit reactance resistance ratio X/R0

R0: Total series resistance

Multiple Tuned VLF AntennasMultiple Tuned VLF Antennas

To have To have sufficientsufficiently largely large bandwidths bandwidths::

Huge antenna systems must be built.Huge antenna systems must be built.

oror SeveralSeveral small multiple-tuned elements small multiple-tuned elements must must

bebe be used be used. .

Multiple Tuned VLF AntennasMultiple Tuned VLF Antennas

Multiple Tuned VLF AntennasMultiple Tuned VLF AntennasGround losses are reduced.Ground losses are reduced.

Radiation resistance and efficiency are increased.Radiation resistance and efficiency are increased.

Instead of one and vulnerable antenna, several and smaller Instead of one and vulnerable antenna, several and smaller elements can achieve the same bandwidth-efficiencyelements can achieve the same bandwidth-efficiency product.product.

If one element is If one element is out of serviceout of service, the others , the others can can still operate.still operate.

The effective ground loss with multiple-tuning will be less than The effective ground loss with multiple-tuning will be less than for a single elementfor a single element..

Tuning and retuning the system is difficult.Tuning and retuning the system is difficult.

eeach antenna has to be matched to ach antenna has to be matched to thethe transmitter. transmitter.

Triatic Type AntennaTriatic Type Antenna

Cutler, Maine Cutler, Maine Antenna InstallationAntenna Installation

Goliath AntennaGoliath Antenna

Goliath Antenna Goliath Antenna

ReferencesReferences

(1) “VLF Radio Engineering”, A. D. Watt, Perg. Press, 1967(2) “High Power Very Low Frequency/Low Frequency Transmitting Antennas”, P Hansen, Military Communications Conf., 1990. MILCOM '90, Conference Record, 'A New Era'. 1990 IEEE, 30Sept.-3Oct.1990 Pages:1091 - 1096 vol.3(3) Technology Conference, 1991.IMTC-91.Conference Record. ,8th IEEE , 14-16 May 1991 Pages:330 - 334(4) “Multiple Tuned VLF Antennas”, Manfred Schopp, IEEE Transactions

on Broadcasting, Vol. 39, No.4, Dec. 1993. References for the photos & figuresReferences for the photos & figures::

[1] http://hawkins.pair.com/nss.shtml[2] http://www.tpub.com/neets/book17/77.htm

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MFMF ANTENNAS & ANTENNAS & ANTENNA SYSTEMS ANTENNA SYSTEMS

INTRODUCTIONINTRODUCTION

• Usually: Usually: Vertical radiators operating in Vertical radiators operating in the the MF band (300-3000 kHz)MF band (300-3000 kHz)..

• The towers may be guyed or self-The towers may be guyed or self-supportingsupporting..

APPLICATION AREASAPPLICATION AREAS

• AM BroadcastingAM Broadcasting

• Maritime RadioMaritime Radio

• Coast Guard CommunicationCoast Guard Communication

• Direction FindingDirection Finding

CHARACTERISTICS OF CHARACTERISTICS OF RADIATORSRADIATORS

• Maximum radiation in the horizontal planeMaximum radiation in the horizontal plane• Antennas Antennas taller than one-half wavelength have a taller than one-half wavelength have a

minor lobeminor lobe

Characteristics of the Characteristics of the RadiatorsRadiators

Requirement for metallic ground plane to Requirement for metallic ground plane to

minimize lossesminimize losses

Vertical polarization is preferred due to Vertical polarization is preferred due to

superior propagation characteristicssuperior propagation characteristics

Other features of the Other features of the radiatorsradiators• Shunt fed radiatorsShunt fed radiators

• Top loaded radiatorsTop loaded radiators• Sectionalized Sectionalized radiatorsradiators

Circuits for MF antenna systemsCircuits for MF antenna systems• Antenna tuning units for matching purposesAntenna tuning units for matching purposes• Phase shifter networks for directional antenna systemsPhase shifter networks for directional antenna systems• Power dividing networksPower dividing networks

ANTENNA TUNING

UNIT T-LINE

ANTENNA TUNING

UNIT PHASE CONTROL

NETWORK T-LINE

POWER DIVIDER

NETWORK

FROM TRASMITTER

PHASE CONTROL NETWORK

Ground SystemsGround Systems

• 120 buried (/4 length) copper wires120 buried (/4 length) copper wires• Extending radially outwardExtending radially outward• 120-180 cm depth is sufficient120-180 cm depth is sufficient• Individual ground systems are required Individual ground systems are required

for each tower of the for each tower of the array. array. • Copper-mesh ground system may also Copper-mesh ground system may also

be used.be used.

Ground SystemsGround SystemsA typical groundA typical ground system for a two-element directional system for a two-element directional

antenna antenna

HF Antennas & HF Antennas & Antenna SystemsAntenna Systems

HF Antennas and Antenna HF Antennas and Antenna SystemsSystems

Frequency Range: 3 to 30 MHz Frequency Range: 3 to 30 MHz

( ( 10 to 100 meters; in wavelength10 to 100 meters; in wavelength) )

For medium- and long- distance For medium- and long- distance communications and broadcoastingcommunications and broadcoasting

Characteristics of HF Antennas:Characteristics of HF Antennas:

Signals are distorted as the ionosphere is neither Signals are distorted as the ionosphere is neither regular nor smooth.regular nor smooth.

High powers and high antenna gains may be High powers and high antenna gains may be needed for communication.needed for communication.

Types of HF Antennas:Types of HF Antennas:

Non-Resonant HF AntennasNon-Resonant HF AntennasLong-wire AntennaLong-wire AntennaVee AntennaVee AntennaRhombic AntennaRhombic Antenna

Resonant HF AntennasResonant HF AntennasMonopole AntennaMonopole AntennaDipoles and Slot Dipoles and Slot AntennasAntennasLoop AntennasLoop Antennas

Log Periodic HF AntennasLog Periodic HF AntennasEarly Log-Periodic AntennaEarly Log-Periodic AntennaLogarithmic Dipole AntennaLogarithmic Dipole Antenna

Directional HF AntennasDirectional HF AntennasEnd-fire ArraysEnd-fire ArraysBroadside ArraysBroadside ArraysCircular ArraysCircular Arrays

Non-Resonant HF Antennas:Non-Resonant HF Antennas:

• wave propagates along the radiator in one wave propagates along the radiator in one direction onlydirection only

• remaining power is absorbed in a matched load remaining power is absorbed in a matched load

TYPESTYPES

Long-wire AntennaLong-wire Antenna Vee AntennaVee Antenna Rhombic AntennaRhombic Antenna

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Long-wire AntennaLong-wire Antenna

A long terminated wire radiatorA long terminated wire radiator

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Vee AntennaVee Antenna Single mast (one wire radiator terminated in a resistive loadSingle mast (one wire radiator terminated in a resistive load

at the far end).at the far end). Radiation pattern exhibits large side lobes near the main beam.Radiation pattern exhibits large side lobes near the main beam. The efficiency is low (almost half of the total input power may be The efficiency is low (almost half of the total input power may be

exhausted in the matched load.exhausted in the matched load.

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Rhombic AntennaRhombic Antenna• 4 radiating wires of equal length mounted on four masts4 radiating wires of equal length mounted on four masts• one of the wires are load-matched.one of the wires are load-matched.• high directivity high directivity • the large rhombics are used for long-range communications.the large rhombics are used for long-range communications.

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Resonant HF Antennas:Resonant HF Antennas:

Monopole AntennaMonopole Antenna• Elevated-feed MonopoleElevated-feed Monopole• Double-cone MonopoleDouble-cone Monopole• Inverted-L and –T AntennaInverted-L and –T Antenna

Dipoles and Slot AntennasDipoles and Slot Antennas

Loop AntennasLoop Antennas

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Monopole AntennasMonopole AntennasOutside half-wave resonance, elevation pattern breaks up into main Outside half-wave resonance, elevation pattern breaks up into main lobes as input impedance becomes very high. Efficiency decreaseslobes as input impedance becomes very high. Efficiency decreases

Dipole AntennasDipole Antennas

Loop AntennasLoop Antennas

Usully used for reception and direction findingUsully used for reception and direction finding..

The Log-Periodic AntennaThe Log-Periodic Antenna Fed from the vertex.Fed from the vertex. Signal travells along the structure until reaches its resonant region.Signal travells along the structure until reaches its resonant region. The signal radiates from the resonant regionThe signal radiates from the resonant region

Directional HF Antennas:Directional HF Antennas:

End-fire ArraysEnd-fire Arrays• Horizontal Array of DipolesHorizontal Array of Dipoles• RCA Fishborne AntennaRCA Fishborne Antenna• Series Phase ArraySeries Phase Array

Broadside ArraysBroadside Arrays• Broadside Dipole ArrayBroadside Dipole Array• Wide-Band Curtain ArrayWide-Band Curtain Array

Circular ArraysCircular Arrays

End-fire ArraysEnd-fire Arrays

Higher directivity.Higher directivity. Provide increased directivity in Provide increased directivity in

elevation and azimuth planes.elevation and azimuth planes. Generally used for reception.Generally used for reception. Impedance match difficulty in Impedance match difficulty in

high power transmissions.high power transmissions. Variants are:Variants are:

Horizontal Array of DipolesHorizontal Array of Dipoles RCA Fishborne AntennaRCA Fishborne Antenna Series Phase ArraySeries Phase Array

Broadside ArraysBroadside Arrays

Beam steering by phase variation is possible.Beam steering by phase variation is possible.

Circular ArraysCircular Arrays Used for direction finding.Used for direction finding. Consists of 30 – 100 elements, with equi-spaced and fed Consists of 30 – 100 elements, with equi-spaced and fed

from a central source – goniometer.from a central source – goniometer. Band-width seperation is possible:Band-width seperation is possible:

ReferencesReferences

JASIK, H.: Antenna Engineering Handbook; Mc Graw Hill, JASIK, H.: Antenna Engineering Handbook; Mc Graw Hill, 19611961

Y.T., LEE S.W.: Antenna Handbook; Van Nostrand Reinhold, Y.T., LEE S.W.: Antenna Handbook; Van Nostrand Reinhold, 19881988..

RUDGE, A.W., MILNE K., OLVER A.D., KNIGHT P.: Handbook RUDGE, A.W., MILNE K., OLVER A.D., KNIGHT P.: Handbook of Antenna Design (Volume 2); Peter Peregrinus, 1983.of Antenna Design (Volume 2); Peter Peregrinus, 1983.