Re: [amsat-bb] Antenna Horizon and Fresnel Zone

[Anthony Monteiro <aa2tx@xxxxxxxxxxxxxxxxxxxxxxxxxxx>]

At 04:24 PM 7/11/00 -0400, you wrote:
>Just thought you might be interested in a test we did today concenring how
>much higher you have to be above your roof edge to see the "radio" horizon
>versus the "visual horizon".
...
>So The difference of raising the antenna just 7 feet above the visual
>line-of-sight gained us about 6 to 8 dB!  We have not calculated the
>fresenel zone for this situation, but it is CLEARLY there!
>

Hi Bob,

Several years ago, I built an omni-directional antenna for 435 MHz 
that was designed to allow horizon-to-horizon coverage of FO-20 at a 
height of 40 feet. (You might remember it from the AMSAT Symposium
in 1998.)

When I initially tested it, I mounted it on a photo tripod at a height
of 30 feet (roof level) rather than to its mast so that I could easily 
change things if needed. But, when I tested it, the performance was 
quite a bit worse than I expected and I was not able to hear the beacon 
with the satellite at the horizon. 

After checking everything and finding nothing wrong, I mounted 
it to the mast anyway and put it up at 40 feet. Guess what? At 40 feet, 
it worked as expected, a good 3-6 dB better and I was in fact able to 
get the full horizon-to-horizon coverage. There were several possible 
reasons for this so I never investigated further. But I would say that 
those last 10 feet made a dramatic difference, even at 435 MHz.

Tony


Tony AA2TX@amsat.org
North Andover, MA

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