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[Towertalk] Dipole setback on a cliff

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Subject: [Towertalk] Dipole setback on a cliff
From: (Bill Tippett)
Date: Wed, 17 Jul 2002 03:05:00 +0100
W4EF wrote:

>They have a 160 meter inverted-vee mounted on a 40' telephone pole
close to the edge of a ~500' mesa. Looking in the direction of the 500'
dropoff, what is the effective height of this antenna? Is it 40' or

        I'm not how YT would work for this but I think EZNEC can
handle it...even with one ground conductivity for the near earth and
another for the mesa valley (if that were a salt flat for example).
Intuitively, I would guess that the pattern would be a combination
of two patterns:

1.  A 40' dipole for TOA's above arctan (40/X), where X is the
distance from the tower to the dropoff boundary, and 

2.  A 540' dipole for TOA's below that same angle.  

In other words, it might look like a low dipole primarily, but 
could have some secondary low-angle components below the main lobe
(which radiates most of the energy straight up).

        Interesting question which EZNEC can easily handle since it
can handle two different ground heights and conductivities (but only
two).  The two grounds can be either a linear boundary (the cliff
problem you described) or a radial boundary (a circular boundary 
with the antenna at the center).  I'll let someone else do the model!

                                        73,  Bill  W4ZV

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