[Elecraft] Inter-K3 interference

[Jack Brindle]

How much power? You will probably need to reduce power for 40 meters.

At K6EI we ran a similar scenario, all K3s, with antennas in close proximity, and triplexes on 20/15/10 for two separate C3 tribanders, one for CW, one for SSB.
On 80 we had in-line 80m dipoles. That is the only band where we had problems, with SSB able to hear the CW stations at times.
It really helped that we were running only 5 watts.

Sounds like you need BPFs on the transmit side of the rigs, one for 40, one for 80. Having them for all bands would probably be a good idea if you are running 100 watts.
This is the same problem those of us who have contest SO2R stations face. You can run a power/db analysis to find out exactly how much you need to knock off the TX to make the RX work the way you wish. It is quite doable, but you don’t have much time. If you have a contester among your group, you might see if they would be willing to bring in some of their filters (and switching). A pair of 5B4AGN BPFs (which include switching) would be ideal.

73,
Jack, W6FB


> On Jun 27, 2025, at 7:57 PM, 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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