> With the mostperfect resistive termination, I agree that it seems to
> make no difference whether the resistor is on top or down at the
> ground rods.
>
> However, when I model Beverages with EZNEC and use a complex
> termination (to produce a perfect null off the backside), I see a
> difference in F/B ratio of around 9 dB with the better case being when
> the resistor is ontop.
We have to be careful looking at very deep nulls with models. We
often forget they are intentional shortcuts that use approximations
designed to represent a system far too complex to calculate exactly.
Because of that, we can "design things" in models that would never
occur or be noticeable in the real world, just by satisfying the
model's approximations to achieve a mathematical result. Looking at
null depth in a large antenna near earth is probably one of the best
examples, as is earth loss with an antenna close to nonhomogeneous
soil.
If moving a resistor and capacitor (that have exactly equal current
at each terminal) just a few feet in a 600foot long antenna resistor
does that much, imagine the large effect of standing next to the
antenna where things really would change!
73, Tom W8JI
W8JI@contesting.com
