Hi Tom & all,
Thanks for the input, thought provoking as always. I neglected to mention
that the model I'm using is a short 46' vertical with a 120' T wire for a
top hat running along the x axis. I moved the model antenna up so the
radials were 2' above high accuracy ground model and then up 150' with
similar results. Both showed skewing of the pattern. I haven't modeled the
top hat with 4 symmetrical wires yet, as I don't have a way to support 4 in
real life. This maybe what's causing the effect I'm seeing in the models.
The antenna I currently have up is a inverted L made of #14 copper. The
vertical portion is 46' tall with the top horizontal section being 90' and
the end dropping down to 30'. I'm building the radial system under this and
plan on switching the L out to the above T configuration. It's interesting
watching the resistance at the feedpoint as radials are added. I measured
the following on the L with a MFJ-269:
# 126' radials resistance resonant Freq
0 56 ohms 1.89
4 42 1.86
8 30 1.84
12 23 1.83
Looks like it's going to take lots more radials to get close to the models
feed resistance prediction of approx 12 ohms. BTW I'm just laying the
radials on the surface and holding them in place with wire staples made out
of stiff wire. Sure beats the thought of buried wires.
73,
Tony
KB8JVH
----- Original Message -----
From: "Tom Rauch" <w8ji@contesting.com>
To: <eric@k3na.org>; "Tony Reynolds" <kb8jvh@reydata.com>;
<topband@contesting.com>
Sent: Sunday, September 05, 2004 8:36 PM
Subject: Re: Topband: radial layout and pattern skew
> > 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
>
>
>
>
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