[Elecraft] Fw: Interesting K3 QRM problem

[Russ Tobolic]

In this recent discussion here on K3 QRM from strong local broadcast stations,  W4TV brought up the noise generated in the T/R switching diodes.   In doing some experimentation with a bandpass flter in the loop between the RX antenna "out" and "in" on the KXV3A I have discovered something which has baffled me.   I have a 2KW station on 1480 kc 1.5 miles north of me and a 20KW station on 1300 kc about 5 miles south of me.  The 1300 station is directional north and I am right in the boresight.  I am experiencing +30 to +40 dB over S9 intermod from these two stations on 1840 kc.  An intermod calculator shows a 5th order intermod at 1840 from the two stations.   I normally run QRP at 5W cw and what I have discovered is that when I crank up the power above 15W (activating the KPA3), the intermod drops to about +5 to +10 dB over S9.  This behavior is very consistent when switching between low and high power.  

Can anyone explain this or is something broke in my K3?

Thanks,
Russ, N3CO





On Thursday, October 23, 2014 4:52 PM, "Joe Subich, W4TV" <lists at subich.com> wrote:
 



> Maybe, and if you're right, my suggestion may not help. But you
> misunderstood my suggestion. There is an insert point between the
> T/R switch and the RX input. That's the RX loop I was talking about
> -- you insert the filter there.

The insert point is *after* the PIN diode T/R switches for both the HPA 
(KPA3) and 10W LPA.  Given Vic's description, it is likely the noise is
being generated in one or more of the PIN diodes in the T/R switching
and placing the highpass filter in the insert point will not resolve
the noise.  Vic, you can bypass (disable) the KPA3 to see if the noise
generation is in the KPA3 T/R switch.

73,

    ... Joe, W4TV