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Re: [TowerTalk] Feedline (choke) question

To: towertalk@contesting.com
Subject: Re: [TowerTalk] Feedline (choke) question
From: Jim Brown <jim@audiosystemsgroup.com>
Reply-to: jim@audiosystemsgroup.com
Date: Tue, 30 Sep 2025 00:17:11 -0700
List-post: <mailto:towertalk@contesting.com>
On 9/29/2025 11:11 PM, Michael Tope wrote:
Dave,

Say you have a dipole up 50ft and you have some common-mode current on the coax shield (perhaps the choke at the feed point is not very good or worse yet, there isn't one). If you bond the coax shield to an 8ft ground rod at the point where the coax reaches the ground, you will create a circuit path for RF current to flow that is in parallel with the shield of the coax leading back toward the transmitter. Now an 8 ft ground rod (depending on soil properties) isn't a very good RF ground, so the amount of RF current that is shunted there might not be much, but it won't be zero. Moreover, if I start attaching radials to that ground rod the RF impedance of that grounding point will drop and more of that shield current will be diverted away from the shield leading back to the transmitter.> > Lightning protection is, at least in part, about shunting fast
rise-time
currents either from direct hits or induced currents from nearby strokes. The other part is about having everything in the electrical system bonded to a common point so the voltage on everything rises together during the stroke (potential difference is the enemy). Bleeding of static charge is related, but not the same thing (that may save your receiver front-end). If you look at the spectrum of a lightning stroke there is a lot of energy  in the MW and HF bands. A robust RF ground should make a good lightning ground and vice versa). Seems like there is also (at least with a direct hit) a DC component since there is a net movement of charge. I've always wondered about the insulated THHN in the radial system for my HF vertical and the DC component. Perhaps that's where the ground rods come in :-).
Some thoughts on this. First, not only is that rod not a very good RF ground, it often isn't even a good conductor at DC and power frequencies. Here in the Santa Cruz mountains, I've measured in the range of 30K ohms DC for ~ 550 ft distance between the two ends of one of my Beverages, realizing that it includes termination resistance.

I've measured more than 1K ohms at DC between two driving rods roughly 15 feet apart by my tower (before connecting them to the tower). Another observation. When Ward and I were working on his Power and Grounding book, we agreed that the common mode path between a tower and a shack is an inductor, and that there is no good reason for lightning safety to bond the tower to the premises ground system. There IS, however, a requirement to bond anything at that tower connected to mains power.

All of this long ago caused me to see the purpose of the best practical ground at the tower, combined with that inductive path to the premises as shunting some of that that HF energy in lightning to ground.

I agree that radials will add some capacity coupling to the earth, but it's distributed capacitance connected by wire that is behaving as part of an antenna, and current distribution is like on any other part of an antenna.

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.

I recall a wonderful demo at a Chicago IEEE EMC workshop that connected an HP generator and voltmeter by 10 ft or so of RG58, and bonded both generator and voltmeter to a wide metal plate. There ammeters in series with the shield and the plate. At DC, all the current was in the plate, but by mid-frequency audio, an increasing fraction of the current was in the coax shield, and by 10 kHz or so, there was no current in the plate.

73, Jim K9YC

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