*/However I do not have any powerful transmitters in the neighbourhood ./*
I do, and that simple circuit generates many 2nd-order intermod products
from the local MW AM stations. The loudest signal at my QTH is on 580
kHz, at about 600 mV/m. The Burhans designs (such as the AMRAD circuit,
but with a J-310) are definitely cleaner. The best results so far have
been from a Burhans variation published in The Lowdown about a year ago.
I believe John Reed was the author, and the front end uses a 3:1
transformer in place of the usual 1:1 version. This yields about 6 dB
less gain, but the extra feedback does help. He follows the FET with a
push-pull bipolar amp that has a very high 2nd-order IMD intercept.
In my testing of these circuits last winter, I always used a variable
attenuator between the preamp and the receiver to make sure that the
receiver was not the culprit. I finally gave up when I realized that the
AC noise pickup was always better on the loops, and that I could never
eliminate the 2nd-order IMD products, only reduce them. Many grounding
and choking variations were tried.
I do plan to go back to this project as time permits this winter, and
will report any successes from my noisy and RF-infested back yard.
John Andrews, W1TAG
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