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
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.
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 is the network used in the LZ1AQ loop amplifier.
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.
Loop inductance can be reduced by connecting multiple loops in parallel, or by making the conductor larger.
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 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