[Elecraft] Fw: Interesting K3 QRM problem

[Vic Rosenthal 4X6GP/K2VCO]

The intermod is generated in the T/R switch, which is before the RX 
IN/OUT circuitry, so putting a filter in the loop won't help. It must be 
between the T/R switch and the antenna, which means it has to be able to 
handle transmitter power.

There is a T/R switch in the KPA100 and one in the driver stage (which 
is the final in the QRP K3). Possibly the two T/R switches behave 
differently.

On 10 Nov 2014 18:17, Russ Tobolic wrote:
> 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

-- 
73,
Vic, 4X6GP/K2VCO
Rehovot, Israel
http://www.qsl.net/k2vco/