[Elecraft] Inter-K3 interference

[Victor Rosenthal]

I live about 1 km from a site that used to have a 50 kW AM transmitter
operating on about 1 MHz. I was using a vertical antenna at the time.
I had problems with it breaking through on various bands. The attenuator in
my K3 did not help. I discussed this with someone at Elecraft and it was
suggested that the signal was strong enough to override the bias on the t/r
switch diodes, which of course are before the attenuator.
I made a 2.5 MHz high pass filter and it solved the problem. Later I
switched to a horizontal dipole, and I could operate without the filter.

Victor 4X6GP

On Sat, Jun 28, 2025, 04:01 Howard Hoyt via Elecraft <
elecraft at mailman.qth.net> wrote:

> Hi folks,
>
> For Field Day we have a K3 with the upgraded synth and a K3S, each has a
> 3rd order Cauer elliptical bandpass filter but they are operating with
> antennas in close proximity.  The only interference we can see is when
> either one is transmitting on 40m and the other receiving on 20m.
> Obviously we are listening far away from 2nd harmonic, which comes in at
> S9+20 dB, but we get S7 interference everywhere else in the 20m band
> with what sounds like composite or phase noise.
>
> It would seem that, if the transmit composite noise was being bandpass
> filtered along with the desired transmit signal, then 20 m should be ~60
> dB down off the passband.  We have tried adding a second bandpass to
> each with zero change to received noise, and if it was indeed received
> noise you would expect it to be reduced even further.
>
> It is not common-mode currents, several large Mix31 cores with multiple
> turns on feedlines did not attenuate it a bit.  The rigs are on separate
> power supplies and have no galvanic connection to each other, even with
> the ground lifted there is no change.
>
> Any ideas?
>
> Thanks in advance,
> Howie / WA4PSC
>
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