[TowerTalk] Tower height increments
Guy Olinger, K2AV
olinger at bellsouth.net
Sat Oct 16 12:04:05 EDT 2004
----- Original Message -----
From: "Jim Lux" <jimlux at earthlink.net>
> and the peak elevation angle for propagation on 40 is only 3
> degrees above the horizon
While I agree with the admonition to consider the effects of tower
height on the lower takeoff angles...
It is neither proven nor universally accepted that on 40m the "best"
or "peak" takeoff angle is three degrees for all amateur radio needs.
That three degree figure probably comes from VOACAP or VOACAP derived
sources. Those programs have an entirely different agenda that is only
a portion of typical ham radio uses and needs. VOACAP has broadcast
derived blind spots that have to be worked around carefully.
Particularly, VOACAP heavily weights takeoff angles that are
continuously successful over long times and calendar intervals. A one
hour strong opening somewhere, that would be a ham's delight, is
largely dunned by VOACAP.
VOACAP assumes that a broadcaster can actually construct high, large
and efficient enough antennas, and run high enough power levels to
make use of such openings.
The experience at large contest stations with switchable antennas
confirms that higher angles, even in the 20's, are significant on 40m
over the course of an evening, depending on conditions, path to DX and
distance.
It was discovered, only last year, that VOACAP systematically
discarded any data regarding a fourth incoming angle, that combined
with a low starting angle (such as one or three degrees), made it
incapable of reporting the existence of significant high incoming
angles. It is a restriction that can be removed by completely recoding
the application and its spin-offs in something other than its
sixties-based Fortran.
A two element 40m beam at 70' will prove to be an excellent antenna if
it's not lossy and the f/b is decent.
Particularly if you mainly want to work DX, the 70' height provides a
rejection of high angles (30 and up) and rejection of close in (NVIS)
signals and QRN which can be extremely helpful.
You may be beaten out by a 5 element wide spaced quad, but hey...
73, and good luck.
Guy.
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