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Re: [TowerTalk] Balun for Force 12 6 Element 20 Meter Monobander

To: towertalk@contesting.com
Subject: Re: [TowerTalk] Balun for Force 12 6 Element 20 Meter Monobander
From: jimlux <jimlux@earthlink.net>
Date: Tue, 21 Jul 2020 17:10:08 -0700
List-post: <mailto:towertalk@contesting.com>
On 7/21/20 4:40 PM, Jim Brown wrote:
Hi Jim,

Choking impedance is driven by both noise and power handling. The fundamental principle on which my designs are based is that the choke should have a high value of RESISTIVE impedance at the frequency(ies) of interest. Reactive impedance can be cancelled by the common mode impedance of the feedline.

Because the choke is resistive, it dissipates power due to the common mode current. And because power is proportional to the resistance but to the square of voltage or current, dissipation falls twice as fast as choking Z. So Z must be high enough to minimize the dissipation. That's why some of my designs for higher power consist of two chokes in series.

Aha.. that's the other design consideration, then - not just "no current on outside of coax" but also "don't melt the choke" And as you say big R helps.



I have learned, through extensive work in the lab, that Fair-Rite #31 is the only ferrite material suitable for use at HF. The reason is, that because it is a very special MnZn mix, the dimensional resonance component of its impedance combines with the circuit resonance to make the resonance much broader than with NiZn materials like #43, #52, and #61. It is the higher Q resonance of the NiZn, combined with the wide tolerance of ferrite parts, that every single choke must be measured to find it's resonance, AND that the resonance is too narrow to cover adjacent bands. In other words, the wide tolerance of the ferrite material moves the resonance too much for a cookbook like mine to be published.

So that allows broadband chokes - as I was winding two at lunch time today to cover 10-80, following your cookbook designs.





73, Jim K9YC

On 7/21/2020 3:30 PM, jimlux wrote:
And that leads to a "choking impedance should be around 5k" which is 100x.

But if it's 4k, it's still good. If it's 3k, it's still good.

I think the value of Jim's cookbook is not so much the choking impedance, per-se, but that he's

a) bounded the core variability (designed for worst observed case performance of the core) b) also taken into account side effects like resonance and leakage capacitance

The latter is MUCH less affected by core properties (I think, Jim can correct me if I'm wrong).

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