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"
<li...@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/
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