[amsat-bb] Antenna Horizon and Fresnel Zone

[Bob Bruninga <bruninga@xxxxxxxxxxxxx>]

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".

We are on an industrial 200' x 200' flat roof that is 80 feet above
average terrain.. Around the edge is a 4 foot high copper facade.  Anyone
over 4 feet tall can see the horizon in all directions out many miles.

>From a location back about 75 feet from one edge, we used a 5 element 2
meter beam pointed at the Johns Hopkins DX Beacon on 144.285 MHz about 30 
miles away. Here is what we observed:

   Antenna height Smeter Calibration (dBm)
   -----------    ------ -----------------
   3 feet high    S3     -119 to -115
   5 feet high    S5     -114 to -111
   8 feet high    S7     -110 to -107
   12feet high    S9     -106

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!

This was a HORIZONTAL beacon.  As a quick test before coming down off the
roof we did a quick check on the only repreater we could find on the air
and confirmed to the same order of magnitude that the effect was similar
with Vertical polarization as well...  (I would have thought there would
be definate polarization differences in the fresnel zone and knife edge
refraction... but it was not obvious if there was any...

Im sure glad we did the test.  We have been using an OSCAR array about 8
feet above the roof and are designing a second one.  Clearly it will be
much better to go up the added 4 feet!   Oh yes, this is 2 meters.  So
presumably the effect will be lower for the UHF side and higher
frequencies.

Just thought you would like to know...

de Wb4APR, BOb

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