[Towertalk] Dipole setback on a cliff

[Richard Karlquist]

FWIW, a couple of weeks ago I took
the mobile up to the top of a mesa.
It was a really great location for
VHF:  I could receive FM broadcast
signals from 200 miles away with
armchair copy on the car radio.
These disappeared off the mesa.
On the ham rig on 17 meters,
however, it was nothing special.
Even for signals in the same
direction as the FM signals I
was receiving.  The ham antenna
was a 13 foot whip (full 1/4
wave on 17 meters.)  I was
located 25 feet from a vertical
drop of 100's of feet.

Rick Karlquist    N6RK
richard@karlquist.com
www.n6rk.com
www.karlquist.com

> -----Original Message-----
> From: towertalk-admin@contesting.com
> [mailto:towertalk-admin@contesting.com]On Behalf Of Bill Tippett
> Sent: Tuesday, July 16, 2002 7:05 PM
> To: W4EF@dellroy.com; towertalk@contesting.com
> Subject: [Towertalk] Dipole setback on a cliff
>
>
> 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
> 500'?
>
> 	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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