ECLECTIC AETHER - Adventures with Amateur Radio

Broadband Loops

With a low Q broadband receive loop, we require an amplifier to boost the very low level signals that are presented at the loop feedpoint, up to a up to an acceptable level to feed a receiver. In addition the amplifier provides an impedance conversion, from that present at the loop feedpoint to something that is closer to a nominal value of 50 Ohms.

The Noise Figure (NF) of an amplifier and its linearity, often specified by its Intermodulation Distortion (IMD) performance, are important factors. But in addition to these considerations, the loop source impedance, amplifier input impedance, and any resultant mismatch loss, are key to achieving the best sensitivity and minimum amount of signal degradation.

Assuming a typical loop of 1m diameter with an inductance approximately 2 to 3uH, used over the frequency range of 20KHz to 30MHz.

At low frequencies<1MHz  the loop behaves as a current source, and the loop needs to have a low value of DC resistance matched by an amplifier with a low value of input impedance.

At mid frequencies, typically >1MHz and <10MHz the loop impedance is in the order of tens of ohms and nearly any amplifier with an input impedance in the range >10 Ohms & <200 Ohms will work to some degree. Many poor designs are judged on the basis of signals received in this frequency range, which can be misleading.

At higher frequencies when connected to the amplifier, a 1m diameter circular loop, tends to have a resonance at around 30MHz (or just above it), and so it effectively becomes a high impedance voltage source, which requires an amplifier with a much higher value of input impedance, typically peaking to values of around 500 to >1000 Ohms at 30MHz.

Note that if a larger loop, or one constructed from thin wire is used, it is likely that the self resonant frequency will be considerably lower than 30MHz, and this may have a resulting impact on the performance, especially above the resonant frequency.

In order to produce an amplifier which tracks these trends, most designers use a variety of tricks to vary the input impedance of the amplifier across the frequency range. In this respect, the input filter networks used in the amplifiers are key to implementing the change in input impedance with frequency.

Most input filters effectively 'tune' the input of the amplifier to provide a low Q resonant circuit with characteristics that track that of the loop in use. Very often the Q is controlled by means of resistors placed across the amplifier inputs in order to prevent too high a value of input Z at resonance. By careful selection of filter values in order to optimise the impedance tracking, it is possible to obtain 10dB improvement in sensitivity at 30MHz in comparison to an 'untuned' amplifier with no input filter network.

At first glance this sounds counter-intuitive, but it was only when I was testing various loop amplifiers with a real loop, energised by a small loop fed from a VNA, that I realised the significance of the input filter network, which I had previously ignored or omitted from the circuits I'd previously built. Although care was taken to minimise interaction with nearby objects during these tests, at the time I was concerned about their accuracy. However since then, I have also been able to perform extensive comparison A/B reception tests, which have confirmed my previous observations.

Minimising the loop inductance is still a key factor and I've found that on frequencies typically >1MHz, halving the loop inductance will provide up to 6dB increase in sensitivity. However using a fatter conductor may also allow you to use a larger loop for the same value of inductance, which is another way to increase the sensitivity, especially on the lower frequency bands.

For optimum performance on frequencies <1MHz a loop with a low value of resistance becomes increasingly important, and on HF frequencies the loop self resonance determines the  frequency response and sensitivity.

Ideally the amplifier input impedance would track the loop impedance, but this can be tricky 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, mainly due to the much higher natural noise floor on those frequencies, which helps offset the reduction in antenna gain.

The following set of graphs is derived from calculations to try and determine the sensitivity that can be achieved, in order to minimise signal degradation and maximise the received Signal to Noise ratio. The Antenna Factor of the loop is calculated and this is used to determine the signal level delivered into various load impedances, in comparison with the natural noise floor as defined by the standardised ITU noise curves.

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

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.

He defines Signal to Noise Degradation (SND) as being "the extent to which the external S/N is degraded by the receiver system" and a full description can be found in his blog.

The first graph shows the degradation in SND relative to the best case (lowest noise floor) 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 loop resistance and 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.

The first graph uses a logarithmic scale to better illustrate the low frequency range.

The next graph uses a linear frequency scale.

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.

Input Low Pass filter networks

The amplifier input impedance can be modified with frequency by the inclusion of an input filter, tuned to provide HF peak in impedance matching that of the loop in use. This can vastly improve the loop performance on the upper HF bands.

The next image shows the actual feedpoint impedance of a typical 1m diameter loop with an inductance value of approximately 3uH, where the self resonant frequency is conveniently at around 30MHz.

Note how the impedance rises from less than an Ohm at 1MHz, to >1K Ohm at 30MHz, and how this actual measurement differs from the many theoretical models that have been used to design loop amplifiers.

In an ideal world the input impedance of the amplifier would directly track that of the loop.

This is the network used in the LZ1AQ loop amplifier.

And here is a Smith chart showing a simulation of the LZ1AQ amplifier input impedance, kindly provided by Chin-leong.

It is shown both with and without the input Low Pass Filter. 

Without LPF, the input impedance is shown as a red dot (~3 ohm) over 1-30 MHz.

With LPF, the input impedance is the blue trace that stretches from 3 ohm at 1 MHz to 385 ohm at 30 MHz.

Note how similar it is to the previous Smith chart showing a typical loop feed point impedance.

The next plot shows the effect of including the input filter, as measured when connected to an actual loop. In this case there is nearly 10dB improvement in gain at 30MHz

This is the input filter used in the Wellbrook design.

And a comparison of a Wellbrook clone (shown in purple) with two LZ1AQ amplifiers (shown in red and orange) using different values of filter components.

Note that both of the filter networks shown are balanced and have a centre connection the the amplifier common ground. This helps maintain loop balance, and can improve the common mode rejection, especially at VHF, which helps to further reject FM broadcast band signals, that could otherwise overload the amplifier and produce unwanted IMD on the short wave bands.

Personally I don't like loop amplifiers that don't have some form of low pass filter on their input, as experience has shown that this often causes problems that may not be immediately apparent.

The amount of gain at the resonant frequency can be controlled by changing (or adding) resistors connected across the capacitors. I found it beneficial to have a bit more gain on the upper HF bands, where the natural noise floor tends to be lower, and signals are generally weaker.

In all cases it is important to match the amplifier input impedance curve with that of the loop in use, and it can be instructive to measure the input impedance of some of the more commonly used loop amplifiers in order to determine their suitability for the task.

Loop Amplifier input impedance

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.

The next two graphs show the input impedance curve of slightly different versions of LZ1AQ amplifiers, but note that these were measured without the input LPF in circuit.

Next the M0AYF differential transistor pair design. Note the increase in impedance at the LF end of the graph.
And finally the W6LVP loop amplifier, which varies considerably across the frequency range.

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.

See my other webpage which touches on this subject https://www.g8jnj.net/moebius-loop-antenna

Reducing the loop inductance can help move the self-resonance higher in frequency, which means that a larger diameter loop 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 https://www.g8jnj.net/hfloopantennas.htm