Oh, I didn't think of a trap. Simpler than a filter. But I have already
designed a 4-pole HP filter that will knock out all of the BC band,
which in this location is probably a good idea. There are some very
powerful AM BC stations in the Middle East. If my filter doesn't work
I'll fall back to a trap.
On 23 Oct 2014 15:10, Charlie T, K3ICH wrote:
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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