I don't know why the server always does that my posts. Sorry. Last try. Then I
give up.
If you have an antenna modeling program like EZNEC you can perform a simple
experiment to illustrate a strange behavior of ground loss.
The EZNEC example "Elevrad1" is a 1/4-wave vertical with 4 radials about 2 cm
above ground. I re-scaled the antenna to 40 meters while keeping the radial
height the same.
.
The "common" or "average" ground that everybody talks about is conductivity = 5
milliSiemens per meter (mS/m) and relative permittivity = 13. With this ground
EZNEC gives an antenna gain of -2.46 dB at a takeoff angle of 26 degrees.
If conductivity = 30 mS ("very good ground") the gain improves to -0.47 dB
while the elevation angle lowers to 22 degrees. This is what we expect.
Now, we want to see what happens as the conductivity gets *worse*. Here are the
EZNEC results, where the 1st column is conductivity in mS and 2nd column
antenna gain in dB.
30 -0.47
15 -1.68
10 -2.25
8 -2.44
5 -2.46
4 -2.30
2 -1.52
1 -0.78
0.1 +0.12
0.01 +0.20
See what's happening? There's a point where the loss hits maximum, but then the
system gain actually improves as the ground worsens more.
In my experience poorer ground also has lower permittivity. Setting
permittivity = 4, the losses do increase (so that there are no positive values)
*but the trend remains*.
It's also true that the elevation angle increases as the ground gets worse, but
it stays in the 20 - 30 degree range and doesn't get crazy or anything.
This suggests that ground loss is sort of an impedance-matching problem.
Clearly, as your ground gets really bad your antenna can still work just fine,
even better in some cases!
Totally unintuitive. But upon further reflection it makes sense that an
"insulator" as ground is okay, otherwise an antenna in free space wouldn't
work. I never meant to suggest something crazy like that one not use radials; I
only wanted to point out a peculiar characteristic of verticals above real
ground.
I will re-state my original thesis this way: If you have perfectly conducting
ground, there is no ground loss. If you have perfectly insulating ground, there
is no ground loss. There's always some ground conductivity in between those
extremes at which the loss is maximum. This value depends on the frequency.
Al W6LX
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