A simple explanation might be that the takeoff angle at which you
empirically measured was not close to 90 degrees.
The difference in gain between perfect ground and medium ground, predicted
by Eznec for 40m dipole 18 feet up, monotonically increases as with takeoff
angle. The difference has a maxima of 3.08 db at 90 degrees and minima of 0
db at 0 degrees. Intermediate values: 2.75db at 60deg, 1.97db at 30 deg and
1.3db at 10 degree takeoff.
Add to this, the potential prediction error, and the results may well be
Rajiv Dewan, N2RD
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because of excessive spam mail that originates from
----- Original Message -----
From: "Richard Karlquist" <firstname.lastname@example.org>
To: <W8JI@contesting.com>; <email@example.com>
Sent: Thursday, July 11, 2002 2:20 AM
Subject: RE: [Towertalk] Models and real world
> Here is some more food for thought
> w.r.t. modeling of ground by NEC.
> I got interested in NVIS by the
> recent article in QST. I put up
> five 40 meter antennas to check
> this out: a ground mounted vertical,
> a dipole at 18 feet, an inverted vee
> at 30 feet, an inverted vee at 60 feet,
> and a dipole at 18 feet over a 230 foot
> diameter ground screen. Beyond 300 miles,
> these antennas were all too close to call.
> Closer than 300 miles, the vertical was
> terrible (no surprise). The 60 foot vee
> was generally 3 dB worse than the lower
> antennas. The 18 foot antennas were
> the same as the 30 foot antennas except
> within 100 miles, when the 18 foot
> antennas were 3 dB better than the 30
> footer. What was interesting was that
> the two 18 foot high dipoles were too
> close to call, even though one was over
> bare dirt, and the other was over a
> ground screen with much higher conductivity
> than even salt water.
> According to NEC, the bare dirt 18 footer
> should have lost 3 dB due to ground losses.
> I just didn't see this 3 dB, even though
> I could see it clearly in the other cases
> mentioned above. Go figure.
> Rick Karlquist N6RK
> > > W8JI wrote:
> > > >In my experience models tend to fall in line more on 40
> > meters and up
> > > >with real-world results, while on 160 results vary the most.
> > >
> > For example, our antenna modelling programs assume the earth is a
> > perfectly homogeneous media and represent the earth as a single
> > boundary layer with certain characteristics. The programs we often
> > use do not even consider groundwave, and so show zero radiation at
> > zero angles and blend the pattern to that point.
> > This would especially be true with things heavily dependent on the
> > ionosphere, real earth, or any other "soup" of varying
> > characteristics, where we might be lucky to get within ten
> > dB. Models
> > give you direction, not absolute answers.73, Tom W8JI
> > W8JI@contesting.com
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