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Re: [TowerTalk] Feedline (choke) question

To: Jim Lux <jim@luxfamily.com>
Subject: Re: [TowerTalk] Feedline (choke) question
From: Kelly Taylor via TowerTalk <towertalk@contesting.com>
Reply-to: Kelly Taylor <ve4xt@me.com>
Date: Sun, 28 Sep 2025 08:33:30 -0500
List-post: <mailto:towertalk@contesting.com>
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 <jim@luxfamily.com> 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 <ab7echo@gmail.com> 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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