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

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


VLF Band (I)

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


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.


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

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.


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.


Vertical Electric Monopole Antenna-Antenna Model-


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

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-


where and

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

antenna loss resistance

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

Multiple Tuned VLF Antennas (I)

To have sufficient bandwidths:

Huge antenna systems can be built.

or

Several small multiple-tuned elements can be used.


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.


Multiple Tuned VLF Antennas (III)


Goliath Antenna

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.


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.


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)

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

References for Pictures


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


Thank You For Your Attention!

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

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