VLF ANTENNAS

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VLF ANTENNAS

• by Evren EKMEKÇI

Middle East Technical University May 2004
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Content

• VLF Band
• VLF Antennas
• About VLF Antennas
• Advantages and Disadvantages of VLF Antennas
• Example Vertical Electric Monopole Antenna
• Multiple Tuned VLF Antennas
• Some Applications on VLF Antennas

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VLF Band (I)

• VLF (Very Low Frequency) Band take place from 3
  kHz to 30 kHz in the frequency spectrum.

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VLF Band (II)

• Question
• Why do we use VLF Band?
• Answer
• Increased demand for very reliable long range
  communication
• and navigation systems caused to use of VLF band.

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VLF Band (III)

• EM waves penetrate well in the sea water.
• Low atmospheric attenuation.
• Appropriate for long range communication.
• High background noise levels.
• Communication needs large amount of power at the
  output of the transmitter.

Skin Effect
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VLF Antennas

• They operates on VLF Band.
• They are electrically small. This simplifies
  analysis.
• They are physically large structures.
• Generally have a number of towers 200-300 m high.
• Generally cover areas of up to a square kilometer
  or more.
• Support worldwide communicatipn.
• The principal objective is to radiate specified
  amount of power over a
• sufficient bandwidth of frequency.

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Problems with VLF Antennas

• Bandwidth is less than 200 Hz.
• Small radiation resistance.
• They are expensive structures.
• Antenna system covers a large area.
• Designing an efficient transmitting antenna is
  difficult.
• High power levels are needed for transmission.

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Vertical Electric Monopole Antenna-Antenna Model-
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Vertical Electric Monopole Antenna-E and H
Fields-

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

rms vertical electric field
rms tangentical 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 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 (I)

• To have sufficient bandwidths
• Huge antenna systems can be built.
• or
• Several small multiple-tuned elements can be
  used.

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Multiple Tuned VLF Antennas (II)

• 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 shunt off servicing, the others
  still can be operated.
• The effective ground loss with multiple-tuning
  will be less than for a single element.
• Tuning and retuning after the system is disturbed
  is difficult.
• Each antenna has to be matched to a transmitter.

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Multiple Tuned VLF Antennas (III)
Goliath Antenna
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Some Applications of VLF Antenna (I)

• Submarine
• Requires EM Wave at VLF because of skin effect.
• Propagation in sea water is almost vertical so
  only electric and magnetic type of dipoles can be
  used.
• Transmitted wave will be attenuated in the
  sea-water so output power must be high enough to
  reach receiver.

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Some Applications of VLF Antenna (II)

• Underground Mine Communication
• Especially it is designed for the event of mine
  disaster.
• Provide wireless communication between earths
  surface and miner.
• Normal radio frequency get attenuated rapidly so
  VLF Band is used.
• VLF Loop antenna can be used for this purpose.

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Some Other Applications

• Water resource exploration.
• Geological mapping.
• Human body.

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Pictures Simplified VLF Transmitting Antenna
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PicturesTriatic Type Antenna
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Pictures Cutler, Maine Antenna Installation
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PicturesGoliath Antenna (1)
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PicturesGoliath Antenna (2)
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PicturesGoliath Antenna (3)
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References
1 VLF Radio Engineering, Arthur D. Watt,
Pergamon Press, 1967 2 High Power Very Low
Frequency/Low Frequency Transmitting Antennas,
Peder Hansen, Military Communications Conference,
1990. MILCOM '90, Conference Record, 'A New Era'.
1990 IEEE, 30Sept.-3Oct.1990 Pages1091 - 1096
vol.3 3 A New System For Measurement of Low
Frequency Radio Transmitting Antenna Parameters
in Near Real Time, Steven C. Tietsworth,
Instrumentation and Measurement 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,
December 1993. 5 A Multiple Tuned Multiple FED
Broadband MF Antenna, John S. Belrose, Antennas
and Propagation Society International Symposium,
2002. IEEE  ,Volume 2 , 16-21 June 2002
Pages264 - 267 vol.2 6 Fundamental Relations
in the Design of a VLF Transmitting Antenna,
Harold A. Wheeler, Antennas and Propagation, IEEE
Transactions on legacy, pre - 1988 , Volume
6, Issue1, Jan1958 Pages120 - 122 7 Analysis
of VLF Loop Antennas on the Earth Surface for
Underground Mine Communication, A.K. Gogoi,
R.Raghuram, Antennas and Propagation Society
International Symposium,1996.AP-S.Digest ,Volume2
, 21-26July1996 Pages962 - 965 vol.2 8
Electromagnetic Fields in Human Body Due To VLF
Transmitter, Ronold W. P. King, Antennas and
Propagation Society International Symposium,
1996.AP-S. Digest,Volume3 21-26July1996
Pages1802 - 1805 vol.3 9 Fundamental
Limitations of a Small VLF Antenna For
Submarines, Harold A. Wheeler, Antennas and
Propagation, IEEE Transactions on legacy, pre -
1988 , Volume 6 , Issue1, Jan1958 Pages123 -
125
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References for Pictures
1 http//hawkins.pair.com/nss.shtml 2
http//www.tpub.com/neets/book17/77.htm
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Thank You For Your Attention!
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