VLF LF MF and HF ANTENNAS

CLASSIFICATION ACCORDING TO

FREQUENCY BANDS

VLF/LF Antennas and Antenna Systems

VLF BandEM waves penetrate well into the sea water. (Communications with submerged submarines)

Low atmospheric attenuation.

Appropriate for long range communication.

VLF AntennasGround and Sky waves

Frequeny range: 3-30 KHz

Antennas : very large

Power: kW levels and even more

Some Problems Associated with VLF Antenna SystemsSmall Bandwidth (usually less than 200 Hz)

Small radiation resistance.

High cost.

Antenna system covers a large area.

Need for very high power levels for transmission.

LF Antennas

Ground and Sky waves

Frequeny range: 30-300 KHz

Antennas: large

Power: kW levels and even more

Some DisadvantagesHigh cost

Large Dimensions

Trouble with efficiency, power capacity, bandwidth

VLF and LF antennas are electrically small antennas :

problem: high capacitive reactance and small antenna radiation resistance

remedy: top loading

Top-loadingTop-loading increases gain bandwidth (by decreasing reactance)

In VLF large top-loading supported by towers

A simple VLF/LF Transmitting Antenna

VLF / LF Ground SystemsRadial-wire: radial wires buried in the ground

Multiple-star: small radial-wire systems forming a star topology

Basic Theory: The Vertical Electric Monopole Antenna

Vertical Electric Monopole Antenna

Assume uniform electric current I along a vertical monopole of effective height he :

electric fieldmagnetic field

Vertical Electric Monopole Antenna-Radiated Power-The vertical electric field in terms of radiated power is:

Vertical Electric Monopole Antenna-Equivalent Antenna Circuit-

Vertical Electric Monopole Antenna(Radiation Efficiency)whereandEffective power = (power capacity of the transmitter) x (antenna system efficiency) antenna total loss resistance

Vertical Electric Monopole Antenna-Antenna Bandwidth-The 3 dB bandwidth b in (c/s) for a single resonant circuit is:

f : resonant frequencyQ: the circuit reactance resistance ratio X/R0 R0: Total series resistance

Multiple Tuned VLF AntennasTo have sufficiently large bandwidths:

Huge antenna systems must be built.orSeveral small multiple-tuned elements must be be used.

Multiple Tuned VLF Antennas

Multiple Tuned VLF AntennasGround losses are reduced.

Radiation resistance and efficiency are increased.

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

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

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

Tuning and retuning the system is difficult.

each antenna has to be matched to the transmitter.

Triatic Type Antenna

Cutler, Maine Antenna Installation

Goliath Antenna

Goliath Antenna

References

(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 & figures:

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

**MF ANTENNAS & ANTENNA SYSTEMS

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

The towers may be guyed or self-supporting.

APPLICATION AREASAM BroadcastingMaritime RadioCoast Guard CommunicationDirection Finding

CHARACTERISTICS OF RADIATORSMaximum radiation in the horizontal planeAntennas taller than one-half wavelength have a minor lobe

Characteristics of the RadiatorsRequirement for metallic ground plane to minimize lossesVertical polarization is preferred due to superior propagation characteristics

Other features of the radiatorsShunt fed radiatorsTop loaded radiatorsSectionalized radiators

Circuits for MF antenna systemsAntenna tuning units for matching purposesPhase shifter networks for directional antenna systemsPower dividing networks

PHASE CONTROL NETWORK

ANTENNA TUNING UNIT

T-LINE

T-LINE

T-LINE

PHASE CONTROL NETWORK

ANTENNA TUNING UNIT

POWER DIVIDER NETWORK

FROM

TRASMITTER

PHASE CONTROL NETWORK

Ground Systems120 buried (/4 length) copper wiresExtending radially outward120-180 cm depth is sufficientIndividual ground systems are required for each tower of the array. Copper-mesh ground system may also be used.

Ground SystemsA typical ground system for a two-element directional antenna

HF Antennas & Antenna Systems

HF Antennas and Antenna Systems Frequency Range: 3 to 30 MHz ( 10 to 100 meters; in wavelength)

For medium- and long- distance communications and broadcoasting

Characteristics of HF Antennas:Signals are distorted as the ionosphere is neither regular nor smooth.

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

Types of HF Antennas:

Non-Resonant HF AntennasLong-wire AntennaVee AntennaRhombic Antenna

Resonant HF AntennasMonopole AntennaDipoles and Slot AntennasLoop Antennas Log Periodic HF AntennasEarly Log-Periodic AntennaLogarithmic Dipole Antenna

Directional HF AntennasEnd-fire ArraysBroadside ArraysCircular Arrays

Non-Resonant HF Antennas:wave propagates along the radiator in one direction onlyremaining power is absorbed in a matched load

TYPES

Long-wire AntennaVee AntennaRhombic Antenna

**Long-wire AntennaA long terminated wire radiator

**Vee AntennaSingle mast (one wire radiator terminated in a resistive loadat the far end).Radiation pattern exhibits large side lobes near the main beam.The efficiency is low (almost half of the total input power may be exhausted in the matched load.

**Rhombic Antenna4 radiating wires of equal length mounted on four mastsone of the wires are load-matched.high directivity the large rhombics are used for long-range communications.

**Resonant HF Antennas:Monopole AntennaElevated-feed MonopoleDouble-cone MonopoleInverted-L and T Antenna

Dipoles and Slot Antennas

Loop Antennas

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

Dipole Antennas

Loop Antennas

Usully used for reception and direction finding.

The Log-Periodic AntennaFed from the vertex.Signal travells along the structure until reaches its resonant region.The signal radiates from the resonant region

Directional HF Antennas:End-fire ArraysHorizontal Array of DipolesRCA Fishborne AntennaSeries Phase Array

Broadside ArraysBroadside Dipole ArrayWide-Band Curtain Array

Circular Arrays

End-fire ArraysHigher directivity.Provide increased directivity in elevation and azimuth planes.Generally used for reception.Impedance match difficulty in high power transmissions.Variants are:Horizontal Array of DipolesRCA Fishborne AntennaSeries Phase Array

Broadside ArraysBeam steering by phase variation is possible.

Circular ArraysUsed for direction finding.Consists of 30 100 elements, with equi-spaced and fed from a central source goniometer.Band-width seperation is possible:

ReferencesJASIK, H.: Antenna Engineering Handbook; Mc Graw Hill, 1961

Y.T., LEE S.W.: Antenna Handbook; Van Nostrand Reinhold, 1988.

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

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