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Re: [TowerTalk] Feedline (choke) question

To: towertalk@contesting.com
Subject: Re: [TowerTalk] Feedline (choke) question
From: "Jim Lux" <jim@luxfamily.com>
Date: Tue, 30 Sep 2025 10:12:08 -0400
List-post: <mailto:towertalk@contesting.com>
        


 


On Tue, 30 Sep 2025 00:17:11 -0700, Jim Brown <jim@audiosystemsgroup.com> wrote:

<big snip>

The late Henry Ott taught in his EMC workshops that above a few kHz, the
impedance of a conductor with even a very large cross sectional area is
dominate by inductance.

… yes - the inductance is only weakly dependent on the shape and strongly 
dependent on 
a) the length (~1 uH/meter), and,
b) loops (where the magnetic field of one part of the conductor interacts with 
the magnetic field of another).

The whole thing about “strap” vs “wire” is not about inductance, but resistance 
-> The strap has more surface area, so at higher frequencies, where the current 
travels in the surface, skin effect has less effect.  Oh, and mechanical 
convenience: It is much easier to make a bolted connection to a rectangular 
cross section than a round one. 

Now an interesting aspect strikes me as I write this.  To a certain extent, you 
want to “dissipate” the RF current (whether from your Tx, or from a lightning 
strike).  There’s probably a nice balance between inductance and resistance. 
you want to dissipate the energy over a large enough area that nothing gets too 
hot (lightning)  (the “smoking rod” is a thing in high current faults)

So you can’t get rid of the inductance of the feedline shield (or an equivalent 
current path in the soil). So there is ALWAYS going to be a potential 
difference between the ends at anything above DC. But do you care? What you 
care about is the potential difference between, say, the chassis of your rig 
and you, or between the two conductors going into your receiver (one of which 
is often at chassis potential).   But if the antenna system is at 10,000 volts 
relative to the shack, do you care, as long as the voltage *at the shack* isn’t 
10kV. 



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