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Re: [TowerTalk] 4 square for 80

To: towertalk@contesting.com
Subject: Re: [TowerTalk] 4 square for 80
From: Jim Brown <jim@audiosystemsgroup.com>
Reply-to: jim@audiosystemsgroup.com
Date: Mon, 26 Sep 2016 17:47:41 -0700
List-post: <towertalk@contesting.com">mailto:towertalk@contesting.com>
On Mon,9/26/2016 7:41 AM, Rik van Riel wrote:
The thing about resistive common mode chokes is, the higher
their impedance, the lower the amount of power lost.

Right.

Put a choke with over 1000 ohms of impedance into an antenna
system with 35 ohm impedance, and the losses will be quite
small.

Wrong. Power dissipated in the choke will depend on its resistive impedance and how it relates to the common mode circuit. That can be a very complex calculation. Also, the 35 ohm number you're citing has little to do with the calculation -- it's in the DIFFERENTIAL circuit. Several years ago, N2IC posted a question (maybe here) about power handling and dissipation for a choke. I didn't pay enough attention to his application and gave him a bad answer, which W8JI caught.

AND how much common mode power a given choke can dissipate depends a lot on its thermal mass and its ability to shed that heat to the environment. A choke wound with RG8/213 size coax on five - six 2.4-in diameter #31 cores has a lot more thermal mass that a choke bifilar wound with #12 on a single #31 core, so the physically larger choke will handle more power than the smaller one, even if both have the same common mode impedance.

73, Jim K9YC

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