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Date: Tue, 03 Oct 2006 09:48:32 -0400
From: John Andrews <w1tag@w1tag.com>
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Subject: Re: LF: Re: miniwhip miniresults...?
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> */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