[Top] [All Lists]

Re: Topband: radial layout and pattern skew

To: <>, "Tony Reynolds" <>,<>
Subject: Re: Topband: radial layout and pattern skew
From: "Tom Rauch" <>
Reply-to: Tom Rauch <>
Date: Sun, 5 Sep 2004 20:36:44 -0400
List-post: <>
>    EZNEC 3 is not capable of modeling radials at 3-inches
above ground.
>    A 3-inch separation between ground and radial is below
0.001 wavelength at 160m.  Even with NEC4 one should avoid
modeling wires
> within 0.001 wavelength of the ground (unless these wires
are vertical connections through the ground interface).
>    NEC4 models indicate you will not see a significant
pattern variation with azimuth in your situation.
>    More importantly, since radials can't go into the dense
woods, just build what you can and get on the air.
> -- Eric K3NA

I agree with Eric.

The common Eznec programs use NEC-2 engines, and there is
considerable question about accuracy of models with
conductors near earth. There was several dB difference
between a low dipole model and measured FS in a paper Jack
Belrose presented.

NEC-2 also treats earth as a homogeneous media, and programs
commonly available to Hams do not include ground wave (and
very low angle) response.

Since the ground behavior greatly influences pattern and
since the pattern does not accurately reflect the real world
(even take off angle isn't accurate for verticals near
earth), I'd be really careful accepting any model as
"accurate". Quite frankly, it can't be accurate.

Even with that warning in mind,  I just modeled a 1/4 wl
vertical (using Eznec +V4.0) with half the radials removed.
I changed the model from having 360 degree radial coverage
to only 180 degrees, which is surely a worse case.

Even in this worse case, with 10mS/m soil skew was only
about .7dB
With 30mS/m earth, skew was only .4dB

>From practical experience, I had the pleasure of working at
a ground system at WAAM in Ann Arbor, Michigan. Someone had
actually cut and plowed up all radials in a direction where
FS was too high.

When I added the radials back in and readjusted phase to
licensed values, a proof showed the pattern  got slightly
stronger with no discernable pattern shift. The end result
of removing the radials was only that FS dropped a small
amount in all directions, and the array became unstable.

When I corrected a phase metering error, the pattern went
back in spec.

We had a similar problem at a station near Perrysburg Ohio
that was licensed to cover Bowling Green Ohio. Once again,
the only real thing we found was the whole pattern dropped
very slightly.

My conclusion was if it makes any difference at all the
difference is so small that you can ignore it.

73 Tom

Topband mailing list

<Prev in Thread] Current Thread [Next in Thread>