[Elecraft] NOT the feedline

[Bob McGraw K4TAX]

I agree with Jim, K9YC  on these points.

** OCFD antennas are noted to have lots of common mode current issues.   
These are a chore to tame.

** Most commercial baluns or common mode chokes are poor designs, of 
inadequate material, designs copied from other poor designs but packed 
in a pretty box of different color, size, and shape with a high price.

One of the best applications for a 4:1 balun is with a single band 
folded dipole made of equal wire diameter or size. Depending on height 
above ground the feed point Z is between 200 and 300 ohms.  The use of a 
4:1 give an impedance of 50 to 75 ohms.    Any other usage of a 4:1 
balun is more of a compromise to a disaster.

73

Bob, K4TAX


On 6/1/2020 9:41 PM, Jim Brown wrote:
> 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
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