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

Doug:

It sounds to me like you may be having measurement problems. Can you tell us
how you are doing the feedpoint impedance measurement? What kind of Z meter
or bridge? Are you using coax, how long, is the feedpoint between the
antenna and a separate ground stake, or between a longer and shorter part of
the antenna? I am thinking unbalanced RF currents giving false indications.

Regards,

Jacob Z. Schanker, P.E.
65 Crandon Way
Rochester, NY 14618

Tel: 585 442 3909
Fax: 585 442 2182
j.schanker_at_ieee.org

"D. B. Miron" <dbmiron_at_paulbunyan.net> writes:
> 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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