Small untuned loop for receiving – is an amplifier necessary?

A correspondent reading recent articles on active loops for receiving asks:

I have a 30″ square loop of #12 wire that I use for receiving, and when I attach it to the receiver on 40m, the audio output voltage goes up three times or more. Do I need an amplifiers, or will it worsen things?

It is possible to determine the ambient noise temperature from the true noise power change over that of a matched termination.

The equivalent noise temperature of the receiver is implied by its Noise Figure when it is terminated with a matched termination. Noise due to an open circuit or short circuit input is not defined.

The correspondent re-measured with a termination, and as it turned out, the results were much the same, so lets work the case of voltage increasing by a factor of three.

Without going any further, we can calculate the degradation in External S/N by the receiver, total noise power is proportional to (3^2) times internal noise, so S/N degradation is 10*log(9/(9-1))=0.51dB… very little.

It is true that an amplifier is unlikely to improve things and will be likely to degrade things because of intermodulation distortion that is inherent in them, more so if it overloads on broadband signal input.

But let’s go on to estimate the ambient noise figure Fa.

It is really important for this process that the AGC does not change the receiver gain, and there is no overload or clipping. The latter means DO NOT SWITCH THE AGC OFF, the S meter deflects, you need extra input attenuation to keep things linear.

Now lets assume the receiver has a Noise Figure of 6dB (most modern HF transceivers are in that ball park).

We need to estimate the gain of the antenna, we will use Calculate small loop Antenna Factor.

Ok, terminated in 50Ω, the untuned small loop has a gain of -43.4dBi. So, it captures only a very small portion of the external noise, but even so it delivers sufficient to the receiver to increase the output voltage by a factor of 3.

The noise floor of a 2kHz effective noise bandwidth receiver with noise figure of 6dB is -135dBm. The total noise equivalent input power with output voltage raised by a factor of 3 is -135+20log(3)=-125.5dBm. If we allow for the antenna gain of -43.4dBi, the receiver input power with a lossless isotropic antenna would be around -125.5–43.4=-82.1dBm which is about S7.5 on the common ham scale… so a quite high noise level.

Lets use Ambient noise calculator to find the ambient noise.

Above is the input form for the scenario.

Above is the result, Fa is 58.4dB which turns out to be a pretty high noise level.

From the above chart (ITU-R P.372-12 (7/2015)), we can see that the predicted ambient noise figure Fa in business precincts is around 55dB, and at the lower limit, Galactic noise is about 35dB.

So, the correspondent’s ambient noise is at the high end of expectations, indeed higher that you would expect in most residential areas so it begs the question whether there is some strong local noise source that can be reduced.

Conclusions

A small passive loop may be sufficient to achieve small S/N degradation on low HF bands in scenarios where the ambient noise level is high to extreme.

If the passive loop is sufficient to obtain small S/N degradation, then an amplifier may well worsen things.

The real problem may be the quite high ambient noise level, and that may be resolveable.

References / links