Re: [TowerTalk] Tower height increments

["Guy Olinger, K2AV" <olinger@bellsouth.net>]

----- Original Message ----- 
From: "Jim Lux" <jimlux@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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