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[TowerTalk] High Antenna Modelling

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Subject: [TowerTalk] High Antenna Modelling
From: (Pete Smith)
Date: Sat, 02 Feb 2002 10:45:54 -0500
At 09:12 AM 2/2/02 -0600, wrote:
>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).

As Tom's explanation of null formation suggests, some foreground slope
angles and distances from the antenna may actually deepen a given null or

I think there is probably another phenomenon at work with very high
antennas, and that is a "smearing" of the arrival angle of signals due to
ionospheric irregularities.  We talk about a signal arriving at 8 degrees,
for example, but the physical reality may be that its energy is spread over
7-9 degrees instead.  With a very high antenna with closely spaced lobes
and nulls, all of the energy of an arriving signal is unlikely to fall only
in as null.  This could explain the perception that the nulls have

73, Pete N4ZR

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