VLF LF MF and HF ANTENNAS

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VLF LF MF and HF ANTENNAS

• CLASSIFICATION ACCORDING TO
• FREQUENCY BANDS

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• VLF/LF Antennas and Antenna Systems

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VLF Band

• EM waves penetrate well into the sea water.
• (Communications with submerged submarines)
• Low atmospheric attenuation.
• Appropriate for long range communication.

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VLF Antennas

• Ground and Sky waves
• Frequeny range 3-30 KHz
• Antennas very large
• Power kW levels and even more

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Some Problems Associated with VLF Antenna Systems

• Small 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.

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LF Antennas

• Ground and Sky waves
• Frequeny range 30-300 KHz
• Antennas large
• Power kW levels and even more

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Some Disadvantages

• High cost
• Large Dimensions
• Trouble with efficiency, power capacity, bandwidth

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• VLF and LF antennas are electrically small
  antennas
• problem high capacitive reactance and small
  antenna radiation resistance
• remedy top loading

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Top-loading

• Top-loading
• increases gain bandwidth (by decreasing
  reactance)
• In VLF large top-loading
• supported by towers

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A simple VLF/LF Transmitting Antenna
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VLF / LF Ground Systems

• Radial-wire
• radial wires buried in the ground
• Multiple-star
• small radial-wire systems forming a star
  topology

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Basic Theory The Vertical Electric Monopole
Antenna
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Vertical Electric Monopole Antenna

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

electric field
magnetic field
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Vertical Electric Monopole Antenna-Radiated
Power-
The vertical electric field in terms of radiated
power is
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Vertical Electric Monopole Antenna-Equivalent
Antenna Circuit-
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Vertical Electric Monopole Antenna(Radiation
Efficiency)
and
where
antenna total loss resistance
Effective power (power capacity of the
transmitter) x (antenna system efficiency)
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Vertical Electric Monopole Antenna-Antenna
Bandwidth-

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

f resonant frequency Q the circuit reactance
resistance ratio X/R0 R0 Total series resistance
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Multiple Tuned VLF Antennas

• To have sufficiently large bandwidths
• Huge antenna systems must be built.
• or
• Several small multiple-tuned elements must be be
  used.

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Multiple Tuned VLF Antennas
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Multiple Tuned VLF Antennas

• Ground 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.

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Triatic Type Antenna
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Cutler, Maine Antenna Installation
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Goliath Antenna
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Goliath Antenna
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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 Pages1091 - 1096
vol.3 (3) Technology Conference,
1991.IMTC-91.Conference Record. ,8th IEEE , 14-16
May 1991 Pages330 - 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
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MF ANTENNAS ANTENNA SYSTEMS
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INTRODUCTION

• Usually Vertical radiators operating in the MF
  band (300-3000 kHz).
• The towers may be guyed or self-supporting.

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APPLICATION AREAS

• AM Broadcasting
• Maritime Radio
• Coast Guard Communication
• Direction Finding

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CHARACTERISTICS OF RADIATORS

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

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Characteristics of the Radiators

• Requirement for metallic ground plane to minimize
  losses
• Vertical polarization is preferred due to
  superior propagation characteristics

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Other features of the radiators

• Shunt fed radiators
• Top loaded radiators
• Sectionalized radiators

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Circuits for MF antenna systems

• Antenna tuning units for matching purposes
• Phase shifter networks for directional antenna
  systems
• Power dividing networks

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Ground Systems

• 120 buried (?/4 length) copper wires
• Extending radially outward
• 120-180 cm depth is sufficient
• Individual ground systems are required for each
  tower of the array.
• Copper-mesh ground system may also be used.

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Ground Systems

• A typical ground system for a two-element
  directional antenna

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HF Antennas Antenna Systems
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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

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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.

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Types of HF Antennas
Non-Resonant HF Antennas Long-wire Antenna Vee Antenna Rhombic Antenna Resonant HF Antennas Monopole Antenna Dipoles and Slot Antennas Loop Antennas Log Periodic HF Antennas Early Log-Periodic Antenna Logarithmic Dipole Antenna Directional HF Antennas End-fire Arrays Broadside Arrays Circular Arrays
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Non-Resonant HF Antennas

• wave propagates along the radiator in one
  direction only
• remaining power is absorbed in a matched load
• TYPES
• Long-wire Antenna
• Vee Antenna
• Rhombic Antenna

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Long-wire Antenna

• A long terminated wire radiator

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Vee Antenna

• Single mast (one wire radiator terminated in a
  resistive load
• at 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.

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Rhombic Antenna

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

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Resonant HF Antennas

• Monopole Antenna
• Elevated-feed Monopole
• Double-cone Monopole
• Inverted-L and T Antenna
• Dipoles and Slot Antennas
• Loop Antennas

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Monopole Antennas

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

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Dipole Antennas

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Loop Antennas

• Usully used for reception and direction finding.

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The Log-Periodic Antenna

• Fed from the vertex.
• Signal travells along the structure until reaches
  its resonant region.
• The signal radiates from the resonant region

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Directional HF Antennas

• End-fire Arrays
• Horizontal Array of Dipoles
• RCA Fishborne Antenna
• Series Phase Array
• Broadside Arrays
• Broadside Dipole Array
• Wide-Band Curtain Array
• Circular Arrays

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End-fire Arrays

• Higher 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 Dipoles
• RCA Fishborne Antenna
• Series Phase Array

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Broadside Arrays

• Beam steering by phase variation is possible.

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Circular Arrays

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

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References

• JASIK, 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.