[Elecraft] Interesting K3 QRM problem

[Vic Rosenthal 4X6GP/K2VCO]

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"
> <k2vco.vic at gmail.com>
> To: "Elecraft Reflector" <elecraft at mailman.qth.net>
> 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/