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

[Jim Brown]

When I posted this work on another reflector, KK9A responded that he 
thought RG302 was a better choice, and I responded that I wouldn't 
question that judgement call. The issue, of course, is that the center 
is silver-coated copper coated solid steel, and the bend radius noted. 
If wound tight to the core and with turns closely spaced, guidelines for 
RG400 are probably "good enough," and at 375 W keydown, I wouldn't sweat 
the bend radius issue.

As to the hot summer day -- according to Fair-Rite's data sheet, the 
impedance of #31 material has a positive temperature coefficient below 
10 MHz up to at least 80C, and RG179 is good for 200C. I'll leave that 
engineering problem to others.

73, Jim K9YC

On 2/19/2019 3:43 PM, Grant Saviers wrote:
> I think your bend radius number should be 30mm (1.18") one time per 
> Hubner & Suhner not 0.175".  About the same as RG142 50 ohm.
>
> Grant KZ1W
>
> On 2/19/2019 15:21 PM, John K9UWA wrote:
>> What if the outside temperature is already 95 to 100 degrees and then 
>> we start
>> heating up this RG-179 with the loss it has?    So my question is is 
>> there another
>> 75 ohm coax that could be wrapped properly around some type of Mix 31 
>> core as
>> well as having less loss and therefore less heat?
>> Maybe it isn't quite 10K ohms Rs on 160m or 13.5K Ohms Rs on 80m?
>> Maybe RG-302/U ?? Bending radius is 0.175" even though it has a solid 
>> center
>> conductor. Power handling is better than RG-179 and loss is a bit 
>> less. Total
>> diameter is 0.202" power handling listed at 1700 watts at 400 mhz.
>> On an FT-240 #31 core ID is 1.4" perhaps you could get to 17-18 turns 
>> on that
>> core. Yes that isn't 27 turns for 160m or 24 for 80m Perhaps two 
>> stacked FT-240
>> #31 cores would make the 10K Ohms numbers?
>>
>> John k9uwa
>>
>>
>>>
>>> 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
>> John Goller, K9UWA & Jean Goller, N9PXF
>> Antique Radio Restorations
>> k9uwa at arrl.net
>> Visit our Web Site at:
>> http://www.JohnJeanAntiqueRadio.com
>> 4836 Ranch Road
>> Leo, IN 46765
>> USA
>> 1-260-637-6426
>>
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