[RFI] Common mode choke on 4 Square feedlines without skewing phasing?

[Jim Brown]

Tony,

Here are my recommendations for 75 ohm transmitting chokes wound with 
FEP or PTFE mini-coax RG179 on 2.4-in o.d. #31 toroids.  Turns are wound 
close-spaced (touching on the inside diameter).  As previously noted, 
this coax has fairly high loss, so the choke should not see more than 
the approximately 360W it would see when divided equally between four 
verticals, and even at that power level, it MUST be exposed to air.  
Note that I am NOT saying that they CAN handle this much power -- that's 
up to others to verify.

160M:  27 turns   12K Ohms Rs
  80M:  24 turns  13.5K Ohms Rs
40M:  22 turns  10K Ohms Rs

To develop these recommendations, I wound Grant's coax on the same four 
cores at the limits of recently purchased cores used for my recent 
RG400, THHN, and Teflon pair 2018 Cookbook, tabulated the data, and 
chose worst case (lowest) values for Rs on each band.

73, Jim K9YC

On 2/18/2019 1:15 PM, Jim Brown wrote:
> On 2/18/2019 11:40 AM, tony.kaz at verizon.net wrote:
>> Thanks for the input. I use dumped power to check the bandwidth. 
>> Okay, will
>> use RG400 for the chokes.
>
> That's 50 ohm coax, so not ideal for your application.  KZ1W sent me 
> some miniature TFE coax to measure, the size of RG179, which is 75 
> ohms with FEP jacket. That would be a better choice. Because it's so 
> small (0.1-in o.d.), dissipation is fairly high, so it will be 
> important that it be exposed to air.  I'm currently developing winding 
> guidelines for it. Because it's so much smaller than RG400, they are 
> very different.