[amsat-bb] Re: J-Pole Antenna
bruninga at usna.edu
Mon May 13 10:12:46 PDT 2013
> The antenna, that I want to build, is described in a paper (probably 20
> by Dick, WD4FAB, titled 'Antennas for microsat ground stations', and the
> describes the large time, a LEO remains at low elevations - about 76 %
below 20 degrees -
> and then concludes, that this is fine for a J-Pole.
Absolutely true with respect to the*antenna pattern* but the current batch
of LEO satelltes even though they are in the main beam of the omni antenna
at the horizon, are -too-far-away- to be heard with only 2.1 dB of omni
dipole antenna gain.
So yes, those are good "satellite antennas" for omni coverage, but they
wont hear anything that low because the satellites are 3000km away and the
current crop of satliltes mostly operate in the 1/2 watt or less area.
The only thing you will hear down to the horizon with these antetnnas is
the ISS that is operating at 10 watts or more.
You are better off simply giving up on the horizon (for an omni) and
increasing your gain higher up. And a 1/4 wave whip over a ground screen
will give you 5.1 dBi instead of 2.1 dBi. And then you may begin to hear
things above about 15 degrees or so (and 3 dB better than you would hear
on any antenna optimized for the horizon..
And it is even better to go to a 3/4 wave vertical over a ground screen
and then you get almost 7 dBi gain starting around 30 degrees. You wont
hear the low satellites (you can't anyway on an omni) but you will hear
them much better when they do get above about 25 degrees... (but notice,
this is less than 1/4th of all passes).
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