[TowerTalk] Feedline (choke) question

Tue Sep 30 02:11:15 EDT 2025

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 :-).

73, Mike W4EF..............

On 9/29/2025 3:40 PM, David Gilbert wrote:
>
>> The original postulation from Brian, K6STI, was that connecting the 
>> shield of the coax to ground at a point where any common mode current 
>> might exist would shunt it to ground.  I've claimed that's a fallacy, 
>> because ground is NOT an RF sink and RF is not a static charge that 
>> can be just bled off somewhere.  RF is AC, and any connection to 
>> ground is bidirectional.  At most, connecting the shield to ground 
>> just adds another element (most likely capacitive) to the overall 
>> network.  The current doesn't just disappear into the earth.
> Dave   AB7E
>
>>
>> On Sun, 28 Sep 2025 08:33:30 -0500, Kelly Taylor via 
>> TowerTalk<towertalk at contesting.com> wrote:
>>
>> Question: If the Earth WAS an RF sink, why would you want it to be?
>>
>> Any RF “sunk” into the ground is RF that’s not available to radiate. 
>> Better to design a system to put as much RF as possible into the air, 
>> no?
>>
>> Silly me… ;-)
>>
>> 73, kelly, ve4xt
>>
>>
>> Sent from my iPhone
>>
>>> On Sep 28, 2025, at 06:30, Jim Lux wrote:
>>>
>>> 
>>>
>>>
>>>
>>>
>>> The field from the antenna (and from the feedline, if there's any 
>>> current in the shield, or it's unbalanced) certainly does interact 
>>> with the soil under the antenna (and houses, trees, etc.).
>>> The question is "how much" (which NEC can answer, as long as you're 
>>> willing to accept the "uniform soil property" model).
>>> And that depends mostly on "how close is it" - after all, 
>>> instruments called sounders fly on spacecraft and measure the EM 
>>> properties of the soil below at a distance of hundreds or thousands 
>>> of km: whether on Earth, Mars (MARSIS, SHARAD), or Europa (REASON). 
>>> They work at 9 MHz, and REASON also has a VHF mode.
>>>
>>> We can even get a sort of worst case - There's a paper by Dave 
>>> Rutledge and Michael Muha that that has some simple equations for a 
>>> dipole laying on the ground. For very dry soil with epsilon 3 + 
>>> 0.005j, the signal propagating into the soil is about 5-6 dB greater 
>>> than the signal propagating into space. It roughly goes as n^3 
>>> (where n is the index of refraction - sqrt(epsilon), so 
>>> epsilon^1.5). George Hagn at SRI spent quite a while trying to 
>>> measure soil properties with dipoles at various heights above the 
>>> ground.
>>>
>>> It's behind the IEEE Paywall, but it might be available elsewhere:
>>> D. Rutledge and M. Muha, "Imaging antenna arrays," in IEEE 
>>> Transactions on Antennas and Propagation, vol. 30, no. 4, pp. 
>>> 535-540, July 1982, doi: 10.1109/TAP.1982.1142856.
>>>
>>> When it comes to "drive a rod" vs "radials" (or some form of 
>>> counterpoise), one way to look at it is that the radials make a 
>>> "higher conductivity" soil, and there's all kinds of interesting 
>>> trades about wires in the ground vs wires on ground vs wires above 
>>> ground, which lots of people have looked at: Rudy N6LF has done a 
>>> lot of experiment at frequencies of interest to hams; J.R. Wait has 
>>> published dozens of papers on the electromagnetics of wires close 
>>> to, or immersed in, a dielectric. As the phrase has it, this is a 
>>> "well studied problem".
>>>
>>> Of some considerable interest is that the soil is not homogeneous 
>>> and the RF propagates quite a ways below the surface (hence the 
>>> effectiveness of sounders at doing subsurface imaging). So it's very 
>>> much a "build it and try it" (which is where the 120 radial thing 
>>> comes from: that's enough that empirically, it doesn't matter what 
>>> kind of soil is under that dense radial field)
>>>
>>>
>>>
>>> On Sat, 27 Sep 2025 21:24:42 -0700, David Gilbert  wrote:
>>>
>>> I already did with the example of a floating portable setup. Current
>>> requires an E-field to push it. You could connect a grounded wire to a
>>> point on the coax shield and it wouldn't shunt any common mode current
>>> to ground because there is no E-field (voltage) for it.
>>>
>>> An earth ground is a grounding point for lightning strikes because the
>>> current that flows in lightning is the result of charge buildup (an
>>> E-field) between clouds and ground.
>>>
>>> Earth ground affects transmitted RF because the radiated RF impinges on
>>> the earth and is absorbed and reflected, the ratio between the two 
>>> being
>>> affected by the parameters of the earth (conductivity and 
>>> permittivity).
>>>
>>> There is no such E-field between the coax and the earth due to the
>>> common mode current on the coax shield. Even if you view the earth as
>>> some sort of super large capacitor, it would require an E-field to push
>>> current into it.
>>>
>>> The earth is NOT an RF sink.
>>>
>>> Dave AB7E
>>>
>>>
>>>
>>>> On 9/27/2025 12:55 PM, Brian Beezley wrote:
>>>> "That's a fallacy. It simply isn't."
>>>>
>>>> Dave, it would be helpful if you'd supply your reasoning.
>>>>
>>>> In many ways I regard ground as just another conductor. However,
>>>> unlike a wire, it is normally without resonance effects. That's the
>>>> "current sink" aspect. Current will flow from a wire into ground if
>>>> you make a connection. If you're using a ground rod, the impedance at
>>>> the connection depends on the rod length, rod diameter, and the
>>>> characteristics of the soil. If the soil is uniform, reflections don't
>>>> occur, unlike for a wire of finite length. The current dissipates as
>>>> it spreads within the ground, which acts like an infinitely long wire
>>>> with a traveling wave. However, when ground strata are distinct and
>>>> well defined, resonance can occur. An example shown for the stratified
>>>> ground calculator described in the writeup below exhibits strong
>>>> resonance. A water table 200 feet below a desert surface magnifies
>>>> surface ground conductivity by a factor of 10, which is pretty
>>>> amazing. I think such situations are rare because I suspect most
>>>> variation in ground characteristics occurs gradually rather than as
>>>> distinct strata, which is necessary for resonance.
>>>>
>>>> https://k6sti.neocities.org/sg
>>>>
>>>> Brian
>>>>
>>>> _______________________________________________
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