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

Jacob,

The antenna is fed by 50 ft. of RG-8 coax, all but about 15
ft of which is coiled and lying on the cement floor of my
steel building shop. I have made measurements through the
coax with the Autek RF1, MFJ259B, and Palomar RX100 Noise
Bridge. The most recent measurements were with the MFJ259B
Impedance Analyzer with its ground terminal grounded. Also,
the SWR variation across the band is duplicated on the
Kenwood TS570's meter, with and without the stake
connection. When in use, the transceiver is grounded by the
same 4 ft long #14 wire as when I was doing the
measurements.

While I know that shield currents from a mismatch can be an
issue, I believe the 15 fit is too short for this effect to
be a factor at 75 m. Also, the fact that the ground stake
improves the SWR all across the band suggests that the
effect is at the antenna.

Doug
----- Original Message -----
From: "Jacob Schanker" <schanker_at_frontiernet.net>
To: "D. B. Miron" <dbmiron_at_paulbunyan.net>;
<nec-list_at_gweep.ca>
Sent: Wednesday, November 05, 2003 7:34 PM
Subject: 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
>
> --
> The NEC-List mailing list <nec-list_at_gweep.ca>
> http://www.gweep.ca/mailman/listinfo.cgi/nec-list
>
>

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