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Re: [RFI] Common mode choke on 4 Square feedlines without skewing phasin

To: rfi@contesting.com
Subject: Re: [RFI] Common mode choke on 4 Square feedlines without skewing phasing?
From: Jim Brown <jim@audiosystemsgroup.com>
Reply-to: jim@audiosystemsgroup.com
Date: Tue, 19 Feb 2019 16:25:29 -0800
List-post: <mailto:rfi@contesting.com>
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@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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