Low Frequency Aerials

What are The Low Frequencies?
Somewhat confusingly, this term is sometimes applied to low audio frequencies such as the base notes on a musical keyboard. This article is not concerned with these, but only with Low Radio Frequencies. Low (radio)

Frequencies are sub-divided into:

a) Low Frequencies, those lying between 300kHz and 30kHz,
b) Very Low Frequencies, lying between 30kHz and 3kHz,
c) c) Ultra Low Frequencies lying between 3kHz and 300Hz,
d) d) Super Low frequencies lying between 300Hz.and 30Hz, and
e) Extremely Low Frequencies lying between 30Hz and 3Hz.

There are two main distinctions between these Low frequencies and the High Frequencies. Firstly, the propagation is usually confined to the 70km high zone between the earth and the ionospheric ‘D’ layer, (the lowest ionised layer in the atmosphere), and secondly the wavelength is usually much greater than any practical antenna.

What are the Low Frequencies used for?
Because of their good and reliable propagation over long distances Low Frequencies are used for air navigation beacons, public broadcasting, (e.g.Radio-4), World-Wide hyperbolic navigation stations, and World Wide shipping and general weather forecasts. At the VLF, to ELF end of the spectrum, they were used historically for communicating with submerged submarines. (E.g. GBR on 16kHz). During the Cold War the Americans built antenna arrays 10’s of km long and transmitted at a few 10’s of Hz (avoiding harmonics of European and American mains frequencies) to deeply submerged submarines. The data rate must have been rather low, and “receive only” by the submarines. These days, it is more common for a submarine to raise a special buoy up to the surface from its submerged position and send and receive data via a high speed microwave link to and from a passing earth satellite. Except for amateurs, and the occasional broadcast by the Swedish station SAQ on 17.2kHz, Morse Code is seldom used on the LF bands. Various forms of “minimum frequency shift” or phase shift RTTY seem to be most common. (The only transmissions I have been able to read satisfactorily are SAQ and the German station DDH47 on 147.3kHz with 80Hz shift, giving the North Sea shipping forecast in German and English).

Aerials at LF are of two main types: unterminated wires, and loops. Because the aerial size, (of whatever type), is so much smaller than the wavelength, it has virtually no directivity. In fact the Aerial Gain compared with a dipole is usually many negative dBs. This doesn’t matter much on reception because the ambient natural noise level is high and the lack of sensitivity can be made up for by electronic amplification in the receiver. (Most of the natural ambient noise at the lower frequencies comes from thunderstorms with their attendant lightning flashes. These cover a wide spectrum peaking around 10kHz and, because of the excellent propagation over long distances at LF, all the lightning flashes within a radius of many thousands of miles can be received).

On transmission the inefficiency of the aerial system is of supreme importance, as in most cases only a tiny fraction of 1% of the RF power fed into the aerial system is radiated. All Aerial systems effectively generate an antiphase image in the ground. As the aerial size, (and therefore its height) is small compared to the wavelength, it is very near its antiphase image. The pair therefore cancel out any horizontal component of radiation. Also, as any high angle radiation cannot get through the “D” layer, the only effective radiation is vertically polarised. This may be regarded as fortuitous as vertical polarisation is optimum for the low altitude ground wave.

Because the requirements of transmitting and receiving aerials are so different, they are frequently quite separate. The transmitting aerial should be located as high as possible over good conducting ground against which it is tuned, and the receiving aerial is often located well away from likely sources of interference such as mains wiring.

Receiving aerials seldom present any real problems, and small tuned or untuned loops of a few turns a metre or so in diameter, or so called “Active Antennas” or E-Field probes placed in a quiet location at a distance are common. Ferrite rod aerials can also be used. The small loop aerial or ferrite rod has the advantage that it can be turned either to maximise the received signal or to minimise QRM and/or QRN. It is worth noting that frequencies of less than about 20kHz are directly receivable on a computer with suitable software. A long piece of wire connected to the computer’s microphone socket is all that is needed. (This needs care however to avoid damaging the computer if the wire is subject to static).

Wire aerials
Transmitting Aerials at LF present a real problem on account of their low efficiency and their consequent narrow bandwidth and high ‘Q’. On unterminated wire aerials, the high ‘Q’ results in a very high voltage on all parts of the aerial. (Because the aerial is so short compared to the wavelength, the voltage hardly varies along its length. The current however can be large at the input end and of course zero at the free end). With 100W input to a 50m length of wire on 136kHz well over 10kV has been measured on the aerial wire. Very thin wire will produce a corona along its length, and this is particularly intense at the far end. Because the height of any aerial at LF is necessarily small compared to the wavelength, even aerials with a long horizontal section and a short vertical section radiate vertical polarisation. It’s only the vertical bit that radiates, the horizontal part acting only as a “capacitive hat” thereby increasing the current in the vertical bit. It might be said that the (rather limited ) objective of the transmitting aerial at LF is to get at least some of the RF input radiated in some direction.
Because of the small height of an aerial in terms of the wavelength and the need to radiate vertical polarisation, you can forget dipoles and balanced systems. All LF transmitting aerials are unbalanced and tuned against ground. In fact it is often said that the figure of merit of a LF aerial is “total length of copper wire times its height”. Because of the low frequency and the high voltages present on the aerial wire and tuning system, variable capacitors tend to give way to various sorts of variable inductance systems known as “Variometers”.

Loop Aerials
All loop aerials at LF are effectively “Magnetic Loops” because of their small size and thus the current is virtually constant at all points round the loop. In contrast to the free wire, the current is large, (in the order of 10s of amps for 100W input), and the voltage small round the loop. It is therefore paramount to keep the resistive losses low: Sizable loops have been made of co-ax braid or copper water pipe. Because there is virtually no phase difference between one end of the loop and the other, the only directivity is due to the magnetic field passing through the centre of the loop. Forget “Quads” and “Delta Loops” where the different phases of current in different parts of the loop can be arranged to contribute in some preferred direction as opposed to radiating all round. At LF the phase is effectively the same at all points and all loops are effectively small magnetic loops. There is little point in mounting an LF loop horizontally as its magnetic field would then be vertical, and the electric field horizontal that, as we have seen, is ineffective when the loop is near the ground. Loops should therefore be mounted vertically. Their chief disadvantage is that they then have a null in the horizontal radiation pattern. Their radiation pattern is the familiar “figure of 8 on its side”, just like a dipole. If the loop is large enough to be effective it is usually too large to be rotated. Loops do have one advantage over a straight wire; because the near field is mainly magnetic, less power is absorbed by adjacent structures like buildings and trees that mainly respond to electric field.

Although the lowest frequency Amateur Radio band is 135.7 to 137.8kHz, there is plenty of commercial and government activity around and below it.



Secured By miniOrange