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Vlf Antennas VLF ANTENNAS by Evren EKMEKÇİ 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 long range communication and navigation systems caused 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. Skin Effect High background noise levels. Communication needs large amount of power at the output of the transmitter. 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 rms tangentical electric field 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- where and 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-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. 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 Pictures Triatic Type Antenna Pictures Cutler, Maine Antenna Installation Pictures Goliath Antenna (1) Pictures Goliath Antenna (2) Pictures Goliath 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 MAGNETIC ANTENNAE FOR ULF Detection and recording of Schumann resonances and other electromagnetic phenomena at frequencies below 50 Hz. By Hans Michlmayr The emphasis here is on the detection of natural electromagnetic radiation , with my particular interest being the Schumann Resonances from 8 ....... 45 Hz. Schumann resonances are generated by the numerous lightning discharges around the world, injecting shock energy into the spherical space ("cavity") enclosed between the earth's surface and the ionosphere. This mechanism is much the same as generating microwaves within a metal cavity,like a waveguide, by means of small electric spark discharges (spark transmitter).The earth-ionosphere cavity is physically extremely large, therefore its resonant frequency is not in the microwave region, but virtually below the audio frequencies.The fundamental frequency is ~ 7.8Hz , with several harmonics and other "wave-guide modes" making up the range of 7.8, 14, 20, 26, 33, 39 and 45 Hz.The height of the ionosphere varies according to the (local) time of day, etc., and this alters the exact frequencies. The Schumann resonances are fairly broad, unlike man-made signals, which are normally nice and sharp. The reception of Schumann resonances proved to be surprisingly difficult, and in the course of the efforts to build equipment capable of reliable performance a number of systems were constructed and operated . These are out lined as follows, together with technical details and observational results. THE ANTENNAS One is simply a loop in the ground. A 40-core telephone cable was buried in a 400mm deep trench, shaped as a 25 x 53 metre rectangle. All the cores are connected in series (with a centre connection after 20 turns), resulting in a rectangular coil of 20 + 20 turns, with an effective window area of 53,000 squ.metres. The overall loop inductance is 427mH, total resistance 546 Ohms. Although it is capable of picking up signals up to 25kHz into the VLF region, its principal use is for frequencies below 50Hz. Next I constructed a 200 turn, 4m dia. "Octoloop" (left image). Its effective window area is 2650 squ. metres . The winding was made with a length of 100 pair telephone cable, several groups of cores can be selected to function as a 30 turn up to a 200 turn coil. The Octoloop was operated at various times as high as VLF, and as low as 2 Hz. Its principal use was the range 1 - 8 kHz, but more recently it was actually my first antenna with which I was able to detect and record the Schumann resonances. The major drawback of the Octoloop is its sensitivity to slight movement by wind., causing "microphonics" . Finally I had to construct the type of device with which the "professionals" use to study the Schumann resonances, namely a large induction coil. I wound my coil of 69,300 turns on a 800mm length of 50mm dia. PVC pipe, using along threaded rod through the coil former (pipe). This was at one end clamped into the chuck of an electric drill, which in turn was held in place in a bench vise. The opposite end of the threaded rod was located through a hole in an improvised bearing bracket bolted to the workbench. Finally the rod's end connected to a mech. turns counter. The electric drill was powered by a VARIAC transformer, the best voltage for my particular drill turned out to be around 60...70 Vac. I started and stopped this set-up with a foot switch. It took several hours of continuous high speed winding to get about 8Kg of 0.3 mm dia. enamelled copper onto the former. The finished coil's resistance is 3.64 kOhms , its inductance 10.52 H . 10 lengths of 3mm thick flat steel bar of various widths ( 16...40mm) , each 2 metres long, were then put through the pipe centre to increase the coil's magnetic permeability. Standard low noise 0-50 Hz field preamplifier. This steel mass just about fills the available clear inside coil aperture completely. I could not measure its final inductance, but I guess it must be several kH (!!).
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