[Elecraft] NOT the feedline

[Alan - G4GNX]

I have a similar issue with ferrite 'balun' heating, although mostly 
tested on 40 metres. I'm using an OSCFD (Windom) which has a main 4:1 
balun at the feed point, but also has a sleeve 'balun' consisting of 8 
ferrites wrapped in a plastic sleeve, about 3 feet from the feed point. 
If I run the K3S at 100W, via the KPA500 in standby, then through the 
KAT500 tuner, I see no problems.

On increasing the power to 200W by using the KPA500 in Operate and about 
10W drive from the K3S, during a SSB 'over' the temperature of the 
KPA500 rises and after a few minutes, the SWR readings on the KPA500 and 
KAT500 start to rise. One of the Elecraft guys actually very kindly went 
through my fault log and came to the conclusion that the fault is with 
the antenna/feeder and I agree with him. What is strange is that if I 
leave a solid carrier running at 200W, I can't easily reproduce the 
issue and I wonder if its something being affected by peak excursions 
with SSB?

When things do start to go wrong, I can usually perform a manual tune 
with the KAT500 and the fault will then not (mostly) be apparent.

Anyway, I'm in the process of replacing the entire feeder with Westflex 
103 and much bigger ferrites. I will also replace the main balun which 
is a 400W device for a 1KW device, as I think that running the UK max of 
400W may be pushing the spec of a 400W balun. Also, the balun itself or 
connections to it may be faulty.

The point now is that I have discovered some damage to the outer 
covering of the sleeve 'balun' which may be the result of ferrites 
overheating or may be the cause of movement of the ferrites thus causing 
instability. Either way, heftier devices seem to be the order of the 
day.

Although the KPA500 temperature rise is somewhat expected, I feel it's a 
bit higher over a short period than I would like and may be due to 
reflected power.

73,

Alan. G4GNX


------ Original Message ------
From: "David Olean" <k1whs at metrocast.net>
To: "Peter Dougherty" <lists at w2irt.net>
Cc: elecraft at mailman.qth.net
Sent: 01/06/2020 13:25:18
Subject: Re: [Elecraft] NOT the feedline (was KPA-1500 faulting on 6m; 
SWR issues)

>Hello Peter
>
>I suspect RFI as well. I do not have a KPA1500, so my advice is not worth much, but the fact that the fault only occurs when a signal is being radiated sends me a red flag.  I had a similar problem here with my SPE amplifier power output. I was measuring it on an LP-100A wattmeter.  It turns out that 1.8 MHz energy from the SPE amp radiated from my vertical antenna and  was getting into a new wire doublet antenna and coming back down the ladder line into the shack. The ladder line was about 7 inches from the power coupler for the LP-100A. RF caused the readings to go wacky and it looked like my amplifier was going postal!  The problem was high rf fields next to the power meter coupler.
>
>Ferrite balun heating can be seen with a VSWR meter if you set the power at some intermediate level, key the amp up with a steady carrier, and watch for VSWR creeping up.  Loose pins in coax can also be a problem. They can heat up and disconnect or start arcing. Those problems do not typically show VSWR creep. They happen all at once usually.
>
>The KPA1500 works fine into a dummy load. (No RFI)  That is a big clue.   Good luck and I hope you find the problem. Maybe an RF sniffer would detect the ingress point?
>
>73
>
>Dave K1WHS
>