[Elecraft] NOT the feedline

[Adrian]

My experience is that the Balun-Designs 4:1 current balun works 
brilliantly on a well designed ocf antenna with decent height and 
proportions to

provide good band sync at the same impedance point, where the different 
band sine waves intersect, at the approx 1/3 total length feedpoint. 
shown in below from ;

https://archive.org/details/UnderstandingAndBuildingTheOCFDipole



  However these days I use delta loops, beams and verticals.

Adrian Fewster


On 2/6/20 1:01 pm, Bob McGraw K4TAX wrote:
> 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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