N7RT wrote:
> Hi All,
> I have been following this thread for a while and have a question
> about how accurate the results are for a ground plane antenna are
> using EZNEC 5. Modeling a 160 mtr ground plane with 4 radials over a
> high accuracy ground (height 10 feet), ground conductivity 0.005
> mS/mtr, dielectric constant 13 only yields ground losses ~2 ohms as
> compared to free space. This does not fit well with my experience.
> It's more like 10 to 15 ohms of ground loss. I have never solved this
> problem.
If you just modeled a vertical with low elevated radials and did not add
any additional loss to the model, the answer is likely to be
significantly in error. This is another case where NEC significantly
underestimates ground loss.
Here is another example. Read this message from out of the archives:
http://lists.contesting.com/archives//html/Topband/1998-03/msg00118.html
W8JI claims to have measured a difference of 4 dB between 4 and 64
radials, 6 ft above ground, on an 80 meter vertical. Try to explain
that with NEC. EZNEC says (without fudging the model) the difference
should be 0.09 dB over average ground. When you see an answer like
that, that should raise an eyebrow even if you didn't have W8JI's data.
The EZNEC help file claims that the data for a horizontal wire over a
real ground should be good down to about 0.005 wavelengths (1.35 ft in
this case). I don't think so.
This is the reason that I always include extra loss resistors in my
models of verticals. The value of those I determine from measured data
(not mine). That way I get answers that agree more closely with
measured data. As a matter of fact, for the verticals and Ls I have
built and measured, the feedpoint impedance agrees almost exactly with
the measured data when modeled like this. I'm not sure if the gain is
correct or not, but it is closer to some of the measured data I have
seen (but it is still not exact). The same problem happens with a very
low dipole, but since most people don't care about a very low dipole,
that isn't much of a limitation.
In the case of the measurements W8JI made, there is something else going
on that I can't explain. To get 4 dB difference, you have to add 65
ohms of loss to the antenna. That is not a reasonable number. The
feedpoint impedance would not be close to the measured number. Near
field calculations also show only about 0.1 dB difference, not much
difference from the far field number, so that isn't the answer. I still
don't have any explanation for this data. Maybe you really can't model
ground loss by inserting a resistor at the base of the antenna(?).
Suggestions appreciated.
Jerry, K4SAV
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