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VLF ANTENNAS. by Evren EKMEKÇİ. Middle East Technical University – May 2004. 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 ANTENNAS

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VLF ANTENNAS

by Evren EKMEKÇİ

Middle East Technical University – May 2004


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

Middle East Technical University – May 2004


VLF Band (I)

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

Middle East Technical University – May 2004


VLF Band (II)

Question:

Why do we use VLF Band?

Answer:

Increased demand for very reliable longrangecommunication

and navigationsystemscaused to use of VLF band.

Middle East Technical University – May 2004


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

Middle East Technical University – May 2004


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.

Middle East Technical University – May 2004


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.

Middle East Technical University – May 2004


Vertical Electric Monopole Antenna-Antenna Model-

Middle East Technical University – May 2004


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

Middle East Technical University – May 2004


Vertical Electric Monopole Antenna-Radiated Power-

The vertical electric field in terms of radiated power is:

Middle East Technical University – May 2004


Vertical Electric Monopole Antenna-Equivalent Antenna Circuit-

Middle East Technical University – May 2004


Vertical Electric Monopole Antenna-Radiation Efficiency-

and

where

antenna loss resistance

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

Middle East Technical University – May 2004


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

Middle East Technical University – May 2004


Multiple Tuned VLF Antennas (I)

To have sufficient bandwidths:

  • Huge antenna systems can be built.

    or

  • Several small multiple-tuned elements can be used.

Middle East Technical University – May 2004


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-efficiencyproduct.

  • 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.

Middle East Technical University – May 2004


Multiple Tuned VLF Antennas (III)

Goliath Antenna

Middle East Technical University – May 2004


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.

Middle East Technical University – May 2004


Some Applications of VLF Antenna (II)

Underground Mine Communication:

  • Especially it is designed for the event of mine disaster.

  • Provide wireless communication between earth’s surface and miner.

  • Normal radio frequency get attenuated rapidly so VLF Band is used.

  • VLF Loop antenna can be used for this purpose.

Middle East Technical University – May 2004


Some Other Applications

  • Water resource exploration.

  • Geological mapping.

  • Human body.


Pictures Simplified VLF Transmitting Antenna


PicturesTriatic Type Antenna


Pictures Cutler, Maine Antenna Installation


PicturesGoliath Antenna (1)


PicturesGoliath Antenna (2)


PicturesGoliath Antenna (3)


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 Pages:1091 - 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 Pages:330 - 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 Pages:264 - 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, Issue:1, Jan1958 Pages:120 - 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 ,Volume:2, 21-26July1996 Pages:962 - 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,Volume:3 21-26July1996 Pages:1802 - 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 , Issue:1, Jan1958 Pages:123 - 125

Middle East Technical University – May 2004


References for Pictures

[1] http://hawkins.pair.com/nss.shtml

[2] http://www.tpub.com/neets/book17/77.htm

Middle East Technical University – May 2004


Thank You For Your Attention!

Middle East Technical University – May 2004


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