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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