>but how far from a vertical antenna does soil
>conductivity continue to affect field strength?
Several tens of wavelengths as ground reflections interfere with the
direct radiation from the antenna to form the pattern...
But good moist conductive earth at the base of the antenna could
alleviate the need for a big radial field for effective ground current
There are only two ways to determine whether or not you need more
radials, though, no matter how good your ground is.
Method 1, large rural locations: Put some in. Measure field strength.
Add more radials? Did it help? Put more in, measure field
Method 2, small urban/suburban locations: Put some in. Did you run
out of space yet? No? Add more radials. Did you run out of space
If you test for diminishing returns in field strength, you know for
sure you are done putting in radials and can just decide how many more
dB you can get out for your work and money.
Otherwise, locally good ground *helps* but you'll never know for sure
how much. Everything besides measuring, including modeling with
NEC-4, seems to be a guess.
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