[TowerTalk] Feedline (choke) question

[Jim Lux]

On Tue, 30 Sep 2025 00:17:11 -0700, Jim Brown <jim at 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.