[Elecraft] Inter-K3 interference

[Greg Troxel]

Howard Hoyt via Elecraft <elecraft at mailman.qth.net> writes:

> 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.

At my club's 2019 FD, I made some quick measurements of the transfer
function between antennas.  These stations were most of 100m apart (plus
another 300m away).
While it varied, the rough situation was

  100W TX, 50 dBm
  received signals S9+50 dBm, -23 dBm

  more or less 73 dB loss from transmitter to receiver

and with a K3 (not S/upgraded that year, I think) things were ok.  I
looked at data for transmit phase noise and RMDR, ARRL review data and
Jim's TXNoise.pdf, and concluded we had very little margin.  However I
was thinking about operating in the same band at the same time, not
40->20.

>From one particular pair of antennas, received power was 0 dBm, S9+73,
for only 50 dB of loss.  I remember that combination of stations/antennas being
a little trouble, but it was with a multiband antenna at the CW
station, closer to a multiband antenna at the digital station, and the
CW station also had beams that were a) farther away and b) pointed away,
so we ~never experienced that -50 dB isolation, usually being more like
-70 dB ish.

S9+20 for the 2nd harmonic seems high; that's only 10 dB less than what
I was seeing for the fundamental most of the time.  I would suggest
trying to measure the loss for the fundamental, perhaps with a KX3 at 1W
transmitting to avoid damage.   If you come out at -40 or -30 dB loss, I
don't really expect things to work out.

But maybe your antennas are only 20m apart and parallel....

> 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.

What happens if you drop transmit power from 100W to 50W?   Does your
noise go down 3 dB, or more?

73 de n1dam