----- Original Message -----
From: "Michael Tope" <W4EF@dellroy.com>
To: "Jim Lux" <email@example.com>; "RICHARD BOYD" <firstname.lastname@example.org>;
"Towertalk" <email@example.com>; "Michael Harris"
Sent: Friday, December 03, 2004 9:46 PM
Subject: Re: [TowerTalk] Stacking
> When you do that calculation for a 1/4 ground mounted
> vertical, what do you use for the effective height when
> doing the ray trace for the distance to the reflection zone?
> I posed that question on the topband reflector (in the context
> of siting a 160 meter antenna near salt water) and nobody
> seemed to have a good answer. Using the ray tracing
> simplification the calculation is rather simple, but the answer
> you get depends strongly on where you define the phase
> center of the antenna. On the site I had in mind, it could
> make a big difference because of the limited extent of the
> salt water in certain directions.
> 73 de Mike, W4EF............................................
A very good question, and one to which I don't have a real good answer. In
the classic monopole case, there are no reflections out in the Fresnel zone,
because the phase center height is zero. All of the pattern is determined
very close in to the antenna (the image of the monopole, and then any
reactive near field absorption effects).
On an elevated vertical dipole, or a "ground plane" vertical, the phase
center can probably be taken at the feedpoint. I'm in the process of
looking at modeled phase patterns of various practical antennas to see if
that's a reasonable assumption. For low angles, it's probably reasonable.
The whole "reflections of vertically polarized signals from the earth" is
sort of a peculiar thing, made more so by the fact that I'll bet most
notionally vertically polarized HF antennas have a significant horizontal
polarized component, at least in some directions. As many have pointed out
recently, it's really hard to decouple the feedline (and even if decoupled,
it's still there in the reactive field), so it might radiate. And, if you
have a tophat that's not idealized, perhaps it radiates some horizontally
polarized component. Given the vagaries of propagation, it's plausible that
someone using the vertical would work someone actually relying on a spurious
H pol component that's 10-15 dB down.
See: http://www.mscomputer.com for "Self Supporting Towers", "Wireless Weather
Stations", and lot's more. Call Toll Free, 1-800-333-9041 with any questions
and ask for Sherman, W2FLA.
TowerTalk mailing list