ECLECTIC AETHER - Adventures with Amateur Radio

Broadband Loops

With a broadband loop we are trying to convert the minute loop current into a signal that is suitable to feed our receiver.

On frequencies where the loop presents an extremely low feedpoint impedance (typically the LF bands for a small say 1m diameter loop) it is better to use an amplifier with a low input impedance.

On the higher frequencies where the loop resistance is in the region of tens of ohms and the inductive reactance could be much higher, you can obtain better results by using an amplifier with a higher value of input impedance. 

The problem with broadband loops is trying to find a terminating impedance that provides the best performance over the required frequency range. Ideally the amplifier input impedance wold track the loop impedance, but this is difficult to achieve, so we have to decide upon the frequencies where we need the best performance. 

My personal preference is to concentrate on the HF bands, as LF band performance tends to be just about adequate anyway.

Owen Duffy produced a whole set of notes regarding loop sensitivity and I have followed his calculations to plot my own graphs for a typical small loop.

The basis of the calculations is to determine the Antenna Factor of the loop and then calculate the signal level into various load impedances and compare the sensitivity against the level of the ITU noise curves.

In my case I chose to use a 1m diameter loop and an amplifier with a 1dB noise figure.

The first graph shows the degradation in S/N relative to the best case ITU noise curves.

For example at 1MHz with an amplifier having a 5 Ohm input impedance, the best we can hope for is to be able to receive signals 15dB above the noise floor. If we increase the amplifier input impedance to 50 Ohms this drops to 10dB above the noise floor.

On the lower frequencies below 1MHz the results with a low value of amplifier input impedance are better, in fact if we were to extend the graph below 100KHz it would be clear that the amplifier with the 1 Ohm input impedance would be the best performer. Designs like the LZ1AQ have low values of input impedance.

At 10MHz and above the amplifiers with the higher values of input impedance are the better performers. Designs like the M0AYF have moderate values of input impedance.

Loop inductance is the main issue that degrades the overall performance of broadband loops.

The next series of graphs show the improvement in S/N performance when using different values of amplifier input impedance with different diameter conductors at different frequencies. I have used extreme values of wire diameter in order to show the effect of using 'Fat' loops made from flat plate conductors or multiple loops connected in parallel.

In all cases, regardless of amplifier input impedance, decreasing the loop inductance improves the S/N ratio.

The two graphs shown below show similar results with different sizes of loop conductor and 3 Ohm and 50 Ohm amplifier impedances.

Note that I have used the ITU best case Quiet Rural noise curves as the basis for comparison. In this case it is noticeable when the loop sensitivity is worse than the natural noise floor over most of the frequency range, even when using a very 'Fat' loop with a low value of inductance.

However in an urban environment, where a loop is being used to try and mitigate the effects of local noise sources, which could easily be 20 or 30dB higher than the rural noise floor, then the loop sensitivity may be adequate.

I believe these calculations and graphs to be accurate, and my measurements and tests on many different types of broadband loops seem to confirm the theoretical values I have obtained. However my maths have never been top grade, and although I have a reasonable level of confidence in the results,  I would be more than happy for someone else to do the same work and prove, or disprove, my findings.

Loop Amplifiers

It can be instructive to measure the input impedance of some of the more commonly used loop amplifiers.

Here is a selection of graphs produced by Steve Ratzlaff, AA7U, which he has kindly allowed me to include on this page.

Ideally we would like to see a low value of input impedance at the low frequency end of the spectrum, increasing to a much greater value at the high frequency end of the spectrum in order to compensate for the significant change that occurs in the value of loop reactance. Ideally the input impedance would track that of the loop in order to provide the best Signal to Noise ratio across the required frequency range.

The Welbrook ALA1530 is probably the best example of this among the featured charts, the others perhaps less so.

Loop Inductance

Loop inductance can be reduced by connecting multiple loops in parallel, or by making the conductor larger.

The following graph shows the reduction is loop inductance relative to a single loop of 0.95m dimeter made from LDF4-50 coax.

A single loop had an inductance of 2.5uH.

As a rule of thumb, the optimum spacing for two loops seems to be around 1 radius spacing, which is similar to that used for Helmholtz coils

In this case using 0.95m diameter loops, the best compromise seems to be at approximately 400mm spacing.

As more loops are added they approximate to a conductor consisting of one continuous flat strip

Tuned Loops

WRT tuned loops, even passive tuned loops can produce better results than broadband amplified loops, and for a similar size are capable of sufficient sensitivity to be able to hear the natural noise floor. However they are not quite so convenient to use if you are interested in observing large swathes of spectrum.

Reducing the loop inductance can help move the self-resonance higher in frequency, which means that a larger diameter loop a can be used on the higher frequency bands, which in turn will provide additional improvement in gain on the lower frequency bands.

I have a small tuned loop that I use to locate interference sources and it is capable of very good sensitivity on receive.

The construction details are about 2/3 down this webpage.