Tom:
The pattern is also a strong function of polarization and is a strong
function of the dielectric constant of the earth for vertically polarized
antennas.
John
----- Original Message -----
From: <n4kg@juno.com>
To: <TOWERTALK@contesting.com>
Sent: Saturday, February 02, 2002 9:12 AM
Subject: Re: [TowerTalk] High Antenna Modelling
>
> The pattern enhancements (peaks) and cancellations (nulls)
> are a result of combining the incident wave from the antenna
> with the ground reflection. It is strictly a function of geometry
> and wavelength (frequency).
>
> For angles where the reflections are IN PHASE, there is an
> enhancement. For angles where the reflections are OUT of PHASE,
> there is cancellation. Note that there is a 180 degree phase shift
> at the ground reflection.
>
> See the chapter on "The Effects of Ground" in the
> ARRL Antenna Book for a detailed explanation.
> Figure 2 depicts the phase difference between
> the incident and reflected waves using the concept
> of an 'Image Antenna' at an equivalent distance below
> ground.
>
> At 6 wavelengths above ground, the peaks and nulls alternate
> every 2.5 degrees over perfectly flat ground. In real life, small
> changes in slope can cause the nulls to 'fill in', if not completely,
> at least enough to become harder to detect when the peaks
> and nulls are close together, i.e., when the antennas are more
> than 4 wavelengths high (> 140 ft on 10M, 190 ft on 15M, etc).
>
> Tom N4KG
>
>
> On Sat, 02 Feb 2002 Don Havlicek <n8de@thepoint.net> writes:
> >
> > Comments inserted below:
> >
> > Pete Smith wrote:
> > >
> > > At 09:13 PM 2/1/02 -0500, Chuck wrote:
> > > >
> > > >In getting back to the basics, I submit that at heights over 4 to
> > 6
> > > >wavelengths above ground, the antenna is in "free space" and
> > > >that's why the "nulls"are filled in. Also, at these heights, in
> > free space
> > > >means you have no gain from ground reflection, so they are 5-6 db
> > > >down from lower antennas. How does one define free space? How
> > > >high above ground would you have to be to be considered in this
> > category?
> > >
> > > A lot higher than 4-6 wavelengths. I ran a 10m yagi in EZNEC at
> > 200 feet
> > > above ground, and both the deep nulls and reflection gain were
> > still fully
> > > evident.
> >
> > Obviously, the mathematical functions utilized within the modelling
> > program do NOT vary when the height above ground [or HAAT] is
> > 'excessive'.
> >
> > This is one of the reasons that we must NOT trust the software to
> > give
> > EXACT answers, but rather to give 'idealized' situational results of
> > the
> > MATHEMATICS involved [not shouting ... just emphasis].
> >
> > Don-N8DE [BSc, MAt, ABD - 'all but dissertation' - Mathematics]
> >
> > >
> > > 73, Pete N4ZR
> > >
> >
> >
> ________________________________________________________________________
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> > The
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> > Order online at http://store.eham.net.
> > -----------------------------------------------------------------------
> >
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________________________________________________________________________
Where do you get ICE bandpass filters & beverage matching boxes? The
same place that pays for the hosting of this list: The eHam Store.
Order online at http://store.eham.net.
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