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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