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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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