Well, some of it does reradiate from subsurface (hence sounders and other
"radars").
And in things like a Beverage, the currents in the dielectric under the wire
form an essential part of the antenna.
What's interesting is that the loss in the soil isn't all that high. The big
effect for a propagating wave/field is the impedance discontinuity between
"air" and "soil".
On Sun, 28 Sep 2025 08:33:30 -0500, Kelly Taylor via TowerTalk
<towertalk@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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>>
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