If you use a decent combination balun such as ;
https://www.balundesigns.com/model-4116-4-1-hybrid-balun-1-5-54mhz-3kw/
if you have any metal structure in range of the field, or
https://www.balundesigns.com/model-4115ocf-4-1-for-ocf-dipoles-1-5-54-mhz-5kw/
for a total current 4:1 balun. These are very robust at high power, and
no other feedpoint RF common mode choke is required.
I ran a K3 , amp and ocf dipole like this, with above balun earlier this
decade with great success, had top 4a4a dxpedition total band vk points
with this one antenna. Also the windom and ocf are different animals.
The true windom uses the vertical section of its feedline as part of the
antenna, such that any ocf dipole with a feedpoint common mode choke
cannot be referred to as a 'windom'
Adrian Fewster
On 1/6/20 11:45 pm, Alan - G4GNX wrote:
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
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