NEC-LIST: NEC-LIST:Ground stake effect.

Good day all,

I have designed and constructed an experimental
volume-loaded vertical dipole for the HF amateur bands from
75 m on up. It has a coil element (1.75 turns at 1.2 m
radius) for 75 m and a straight tube for 40 m on up. The
radiator to be used is relay-selected. There are both top
and bottom volumes and the feedpoint is near the bottom
volume. The antenna sits on wood blocks about 0.1 m above
ground.

Modeling this thing in NEC (wire list available on request),
using Arie's latest NEC2DSXxx.exe, shows little difference
whether the antenna is grounded by a wire down from the
bottom center to ground or not. When I had an iron stake
about a foot into the ground, this was what I observed, no
difference. When I replaced the stake with a copper one
with a little over 7 ft. in the ground, it raised the input
resistance at resonance a factor of 3 on 75 m and a factor
of 2 on 40 m. The resonant frequency did not shift
significantly. If I model the antenna as being further off
ground, I start to get this kind of spread, but the
ungrounded resistance goes down as the distance goes up,
which is to be expected, and the resistance with the ground
wire on goes up with distance, which is also to be expected
as the electrical height of the antenna is then greater.

Can anyone tell me their experience with stake grounding of
verticals? Any ideas on the modeling problem?

Thanks,
Doug Miron

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