[aprssig] APRS Satgate Antenna page

Robert Bruninga bruninga at usna.edu
Fri Jun 19 15:57:05 EDT 2015

Subject: APRS Satgate Antenna page

Since the ideal APRS Satellite IGate OMNI antenna is exactly the opposite of
the typical terrestrial IGaate antenna, I prepared the following WEB page:


It shows how a vertical ¼ or ¾ wave VHF omni equals the performance of a
full OSCAR class array (over half the sky) but does it with no moving parts.
It makes up for the weak-signal horizon part of the sky by there simply
being more omni-IGates.  The APRS-IS cloud with all of its IGates is
probably one of the largest spatially distributed satellite receiver system
in the world (?).

But with people used to the 10W transmitter on the ISS, just a few IGates
can capture just about every packet from the ISS horizon to horizon on a
whip.  But with the 14 dB weaker signal from PSAT, we need more SatGates to
make up for their smaller skyprint.  For the USA we need more than a dozen
such Omni-SATgates.

To see the significance of the weaker downlink from PSAT, look at the
successful IGates on the http://pcsat.aprs.org page compared to the IGates
that hear the ISS packets on http://ariss.net   Both are listening on
145.825 and passing along every packet they hear.  But only TRACKING IGates
or good vertical gain satgates hear PSAT. And since we want these running
24/7/365, we do NOT expect people wearing out motors when an omni will do
fine (if we have enough).

The page also shows how every SATgate with a HIGH and terrestrial type
antenna actually creates a DEADZONE around it, effectively blocking any
nearby user heaerd direct from appearing on any of the APRS-IS cloud
satellite web pages.  Hence, Omni-satgate antennas should be low to
everything surrounding it w hile still seeing the sky above 30 degrees.

If your TH-D7 HT is just sitting there, not in use 99% of the time, then
just hook it to a vertical whip and let it be an IGate.  With the low
antenna it will also probably be safe from all weather lightening too.  Put
the UHF side on 435.350 and turn up the speaker.  When PSAT PSK31 comes into
view, you can watch the PSK31 activity as a bonus!


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