[Elecraft] New P3 TX Monitor....unusual behavior with SWR (at times)

[Randy Farmer]

I find this thread quite interesting. I too have been dealing with VSWR 
"spurts". My TX lineup is similar to Phil's, but I don't have the TX 
Monitor. After a recent station rewiring job I found I was having a 
great deal of trouble with indicated VSWR jumping to high levels with 
the amplifier on. The problem was especially bad on 80 meters -- 
anything above about 300W would randomly give a high VSWR fault on the 
KAT500. Since the KPA500 was feeding the suddenly changed load it too 
would usually fault out. The sensor for my LP-100A is on the antenna 
side of the KAT500, and its VSWR indication was rock solid during these 
events. My station is in an extremely difficult RFI environment, with 
seven antennas crammed onto a 0.2 acre city lot. I immediately suspected 
that common mode RF was getting into the reverse power sensor circuit in 
the KAT500.

I installed a Balun Designs choke on the antenna side of the LP-100A 
sensor and the KAT500 stopped giving the high VSWR indications right 
away. That's the good news. The bad news is that now the KPA500 is doing 
it, but at considerably higher power levels. I can run around 400W out 
before the KPA500 begins to randomly show VSWR faults. The KAT500 is 
completely stable at full power. For now, I'm living with this situation.

So I would suggest the troubles you're seeing may be the result of 
common mode RF problems on the transmitter feedline. It could be that 
the addition of the extra electrical length of the TX Monitor sensor and 
its associated cabling changed things just enough to put a common mode 
voltage node near the KAT500 output port. You might try adding an 
additional piece of coax somewhere in the lineup and see if that changes 
the situation. RF is funny stuff.

73...
Randy, W8FN

On 8/12/2015 10:10 AM, Phil Hystad wrote:
> Mike,
>
> Thanks for the comment.  I only have one K3 with one P3.  My configuration is:
>
>       K3/P3 —>  KPA500 —> TX Mon Sensor —> LP100A Sensor —> KAT500 —> 80-meter Dipole
>
> I first noticed the spike with the RED LEDs on both the KAT500 and the KPA500.  I was not looking
> at the SWR display on the P3 although the second time I did notice that it too spiked (max reading).
>
> Like I said, I was operating CW and when I sent just the first dit of a QNI into a CW net the spike occurred.
> I stopped, and then keyed again and everything was normal.  The second time that I did this, I thought that
> maybe the spike was momentary and continued keying but the second or third key down action (actually, a paddle)
> resulted in fault with the KPA500.
>
> Last night, I did not have any problems at all because I disconnected the TX Mon sensor.  I will continue
> experimenting though as I don’t have too many data points yet to draw conclusions.
>
> 73, phil, K7PEH
>
>
>
>
>> On Aug 12, 2015, at 7:28 AM, Mike VE3YF <Mike at ve3yf.com> wrote:
>>
>> Phil:
>>
>> I have the same setup but only 1 x P3. A K3 and P3 on my left setup and a K3 on the right setup using a MK2R+ and I noticed what you mention only once. I was on 80m with
>> the Right K3 and I saw a brief Power and SWR spike on the Left P3 and K3. I might have had a bit of SWR on the standalone Right K3. I have not noticed it happen since, it might
>> be a band/swr specific issue ie, as I was operating around 3800 and the antenna is designed for the CW and RTTY portion of the band. The Left side radio was on 160m Sloper
>>
>>
>>
>>
>> 73 De Mike
>> VE3YF
>>
>> http://www.ve3yf.com