Dear Daniele,
The loop antenna I have been using is similar in size and inductance to the
one in your link, but the preamp design is quite different. The "Easyloop"
appears to be designed as a "natural radio" receiver antenna. For this
application, one wants good sensitivity extending to the low audio frequency
range. Since the loop output basically decreases as the frequency is
reduced, very high gain is required at low frequencies to maintain
sensitivity. This is obviously not good where a high level of 50Hz H-field
is present, and for the "VLF communications receiver" application, high
sensitivity at low audio frequency is not needed anyway.
So in my preamp design (see attachment) , I rolled off the low-frequency
gain below about 3kHz. This results from the loop antenna response when
loaded with the preamp input resistance, and the effect of C5/R7 in the
second stage using IC1a. Also, high frequencies above about 22kHz are
attenuated due to the low-pass filter formed by the loop inductance, C8 and
the preamp input resistance. For use with software clipper/noise blanker, I
have added a 9kHz bandpass filter, which further attenuates lower frequency
noise, and also higher frequency utility stations. I have not had any
problems with overloading due to mains noise or utilities with this design.
(BTW, the ZTX690B can be replaced by a BC337 with negligible reduction in
performance).
Of course, filtering cannot remove mains harmonics, etc. that are within the
signal frequency range. These may be reduced by:
Changing the antenna (a E-field whip will be better than a loop if the local
QRM is predominantly H-field, the loop will be better if it is mainly
E-field)
Moving the antenna (often, there are large variations in QRM level over
distances of only a few metres)
Using the directional nulls of a loop - it may be benificial to experiment
with moving the antenna when trying to null QRM, because the apparent
direction of the local QRM field also often varies over short distances.
Cheers, Jim Moritz
73 de M0BMU
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