Hi Raphael,
Signal to noise degradation (SND) is a concept that was suggested by Owen Duffy.
It is a mechanism that can be used in order to determine if a receive system (including the antenna) is good enough to detect the weakest of signals relative to the system noise floor.
In turn this can be used to compare the receive performance relative to the ITU defined noise curves for particular locations.
This is useful, because it provides a means of assessing if a receive system is likely to be adequate when being used in a particular environment.
The idea is that the system should have sufficient sensitivity that the weakest received signals are only degraded by 1dB by the receive system when used under conditions defined by the ITU noise curves. It provides a value to aim for when designing a system, and makes allowance for the operating conditions and frequencies.
Using these figures and the calculators provided by Owen it is possible to determine if an antenna such as a small loop has sufficient sensitivity to only be constrained by the local noise floor. Obviously the noise floor in real life will vary from location to location, but by using the ITU noise curves as a reference point, you can at least estimate what sort of performance is likely to be obtained. The larger the value of SND the greater the received signals are degraded.
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I do not have figures for shielded loop antennas. I have experimented with many, but I have not been able to find any that seem to offer any advantages over a single unshielded loop. The problem being the additional capacitance that the shielding adds, which in turn lowers the loops self resonant frequency. If the loop amplifier is well balanced, and has a low enough value of input impedance, it will have a sufficiently high value of common mode rejection, and a screen is not required.
Fat low inductance loop(s) are the best option for small broadband loop antennas.
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The self resonant frequency is more of an issue in terms of managing the input impedance of the amplifier than the actual performance of the loop. The loop will still perform in some form on frequencies above the self resonant point, but it's difficult to? manage the impedance excursions if they are in the middle of the frequency range of interest, rather than just at one end. It's convenient to have the self resonant frequency just above 30MHz, as the loop remains inductive on lower frequencies and it can be easily matched to the amplifier across the required frequency range just by using a simple L/C/R network.
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I'm not claiming to know everything about this, my maths is pretty poor and most of my comments are based on observations and experiments that I have designed and made in order to test various theories, and try to better understand how things work.
Owen Duffy's notes and email exchanges have helped me enormously in this respect, and I appreciate the effort he has gone to in order to try and 'debunk' many of the myths and incorrect information that abounds (on the web in particular). It's particuarly interesting when he tries to reconcile actual measurements with modelled figures, which very few other people bother to do (or have the skill to be able to do it).
Regards,
Martin
