[TowerTalk] Feedline (choke) question

[Jim Brown]

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