[Elecraft] NOT the feedline

[Jim Brown]

On 6/1/2020 6:45 AM, 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.

Alan,

What you describe basically a lousy common mode choke, applied to an 
antenna that, because it is so badly unbalanced, has a LOT common mode 
current. The primary function of such a choke is to prevent common mode 
noise picked up on that feedline from coupling to the antenna, and from 
there to the receiver. I don't know of a way to EFFECTIVELY choke such 
an antenna. The application of ANY choke to such an antenna is an 
unnatural act -- it does nothing useful.

I strongly suggest that you study the material on my website about how 
common mode chokes work. These concepts have been part of the ARRL 
Handbook and/or Antenna Book for nearly 10 years.

k9yc.com/publish.htm

The word "balun" is used to describe nearly a dozen very different 
things. What COULD work is a two winding transformer wound on a low loss 
ferrite toroid, like Fair-Rite #61 or #67 material. #61 will likely 
handle 100W from 160M to 10M without overheating. #67, which has much 
lower loss above about 17M, may be required at the 400W level, and would 
certainly be used at 1 kW and above.

Making the windings bifilar provides a capacitive path for common mode 
current, degrading its effectiveness. This is minimized by placing the 
windings on opposite sides of the toroidal core.

73, Jim K9YC