I'd go for a series tuned trap to ground. That would seem to the least
disruptive, if that's the right word, to your system.
I remember working at Vectrol in Rockville MD fairly close to an AM transmit
antenna site. We designed & built big three phase SCR controllers and of
necessity, had to isolate any oscilloscope's chassis ground. You could
always see the low level AM signal on the scope. A similar trap worked, but
it would have required dozens of them to cover all the scopes etc..
Luckily, when you're looking a 60Hz waveforms, we just ignored the BCI.
73, Charlie k3ICH
----- Original Message -----
From: "Vic Rosenthal 4X6GP/K2VCO" <[email protected]>
To: "Elecraft Reflector" <[email protected]>
Sent: Thursday, October 23, 2014 6:58 AM
Subject: [Elecraft] Interesting K3 QRM problem
I live about 3.3 miles from a 50 kW BC station on 1080 kHz. It is on 24
hours a day, transmitting in Arabic, not that that matters.
I have two antennas: an R8 vertical and a Pixel Loop receiving loop.
With either antenna, I hear a wideband noise on my K3 that is synchronized
with the program material of the BC station on the 14, 10 and 7 MHz bands.
I first determined that it was not affected at all by the attenuator in
the K3. So I assume that it is not caused by front-end overload in the K3.
I noticed that if I listen on the Pixel Loop via the K3's RX ANT input the
noise completely goes away when I unplug the R8 from the K3's ANT 1
connector!
I also noticed that the noise is reduced when I bypass my amplifier or my
SWR/wattmeter, which are connected between the ANT 1 connector and the R8.
Reduced, but not eliminated.
I conclude that what is happening is that the BC RF is being picked up by
the R8, and the diodes in the wattmeter and the amplifier and the K3
finals are creating IMD products which I am hearing. The K3 seems to be
the major contributor.
I am thinking that maybe the IMD products are being re-radiated by the R8
and picked up by the Pixel Loop.
I tried one more experiment: I have a BC band filter which is used with
the MFJ antenna analyzer when there are nearby BC signals. Sure enough,
putting this in series with the K3 ANT 1 connector kills the noise. Of
course I can't transmit through it.
Has anyone had a similar problem? I am thinking that the solution is a
highpass filter or trap at this spot. I don't operate on 160, so it
shouldn't be too hard to make.
--
73,
Vic, 4X6GP/K2VCO
Rehovot, Israel
http://www.qsl.net/k2vco/
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