|To:||"Jim Lux" <email@example.com>, "W3YY" <firstname.lastname@example.org>,"TowerTalk List" <email@example.com>|
|Subject:||Re: [TowerTalk] Tower height increments|
|From:||"Guy Olinger, K2AV" <firstname.lastname@example.org>|
|Date:||Sat, 16 Oct 2004 12:04:05 -0400|
----- Original Message ----- From: "Jim Lux" <email@example.com>
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.
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.
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