Antennas for the 73 kHz band

At such low frequencies, antenna design is a real challenge. The wavelength of a 73 kHz signal in free space is 4.1 kilometres.

Also, a good transmitting antenna may not be ideal for receiving.

Transmitting antennas

Any practical antenna will have dimensions that are only a tiny fraction of the wavelength. Its radiation resistance may be in the order of a few tens of milliohms. This means that any practical antenna is likely to be very inefficient, with a gain of below -20 dB. Yes, that's minus twenty dog biscuits.

Fortunately, the RA seems to have realised this when deciding the maximum allowable power level. Because the limit is on Effective Radiated Power (ERP) rather than transmitter output power, it will be possible to compensate for an inefficient antenna by using higher transmitter powers. With an antenna efficiency of say, 1 per cent, it would be legal to use a 100 Watt transmitter.

Many huge professional VLF antennas, using arrays of masts each many hundreds of feet tall, with earth mats covering many acres, achieve efficiencies in the order of only 1 per cent. Even though the 73 kHz band is in the LF (30 - 300 kHz) part of the spectrum rather than the VLF (3 - 30 kHz), 1 per cent seems a tall order for most amateurs. So in practice it may be be necessary to sink many hundreds of watts into the antenna before the legal limit is reached.

Measuring the efficiency of this kind of antenna is almost impossible. It is possible to estimate the efficiency at the design stage, but this is neither easy nor precise, and is not a ``measurement'' in the true sense anyway.

There is only one way to measure the efficiency properly. That is to measure the field strength at a known distance from the antenna. At these low frequencies, the relationship between transmitter power, antenna efficiency and field strength is a simple one. The predominant mode of propagation is the ground wave. The field strength of the ground wave is inversely proportional to distance. At distances of up to a few tens of kilometres, the effect of soil conductivity and permittivity on the received strength will be negligible - a fraction of a dB in fact.

The relationship between ERP and field strength at a given distance will be constant. Anyone with a stronger signal than the theory indicates can safely be assumed to be using an illegal amount of power!

Receiving antennas

With reception, the main problem at such low frequencies is likely to be interference. The Cave Radio and Electronics Group has found that even in the subterranean quiet of a cave, a loop aerial performs significantly better than a whip in this respect. In a typical amateur's home, the interference levels are likely to be much higher.

A ferrite rod aerial will work, but a loop of copper wire wound as a traditional frame aerial may work better.

Last updated 30th April 1996.