[aprssig] PSAT & igating and Optimum antenna

[Max Harper]

Another problem (if you ask me) is that when you send a beacon, and that beacon gets digi'ed, and your own station recieves it back and igates it, the station that digi'ed it gets stripped out of the packet.This beacon is set to go out via 'RF only' but looks like is was sent direct to the APRS-IS. Also note the time stamp at the beginning of the payload as well as the timestamp from findu showing 5 seconds difference. There was one or more digi's in there
 
20150607070734,KG4PID-14>APRX28,TCPIP*,qAC,T2TEXAS:/070729h3417.45N/08742.32W#PHG7250 Bear Creek, Al
Here is the packet as it was recieved back by my igate (aprx) and there were 2 digi's in there.
2015-06-07 07:07:33.789 KG4PID-14 R KG4PID-14>APRX28,W4UNA-7*,N4IDX-7*,WIDE2*:/070729h3417.45N/08742.32W#PHG7250 Bear Creek, Al
Here is the same beacon sent 20 minutes later that made it to an igate before the digie'ed packet could be recieved back by my station.
20150607072701,KG4PID-14>APRX28,WIDE2-2,qAR,N4XWC:/072659h3417.45N/08742.32W#PHG7250 Bear Creek, Al 
So for satilite work, if you hear your own digi'ed packet, it will just look like it was sent direct to the APRS-IS.I've questioned this behavior before and was told that it is normal. But why would the path get changed just because you heard your own packet?
Max KG4PID
       From: Robert Bruninga via aprssig <aprssig at tapr.org>
 To: TAPR APRS Mailing List <aprssig at tapr.org> 
 Sent: Saturday, June 6, 2015 10:03 PM
 Subject: [aprssig] PSAT & igating and Optimum antenna
   
Steve's insight is very revealing...

> The APRS-IS filters duplicates based on payload, not on path,
> Suppose I have an IGate station... [and I use it to also send a packet to
> the ISS]
> [when it is digipeated by the ISS, it never makes it to the APRS-IS!]
> [because the transmitted copy made it to the APRS-IS first
> [and so the digipeated copy gets ignored as a dupe
> [BUT, since the original one did not get tagged with RS0ISS*
> {Then the FINDU page looking for successful RS0ISS* packets wont see it!

This also means then that although an IGate provides a WINDOW to downlinks
over millsions of square miles, it also creates a DEAD ZONE to every station
it can hear direct!

So, what we need for OMNI IGates are antennas that hear great at high angles
and poorly at low angles (exactly the opposite of what terrestrial antennas
are designed to do.

But the good news is that the 3/4 Wave vertical I always tout is ideal  in
this case.  It provides almost 7 dBi gain above 30 degrees and more like - -
4 dBi on the horizon.  This means it favors Satellite signals by more than
10 dB.

PLUS, if you mount it low in your yard, so that it sees WELL above 30
degrees but is blocked to the horizon in all directions by trees and houses
and other stuff, then you can probably reduce the terrestrial gain by
another 10 dB or more.

SO again, a 3/4 wave vertical mounted low is an IDEAL no-moving-parts IGate
antenna for PSAT and ISS.  SO make it 59" tall with a ground plane.  Don't
even have to climb to the roof.  Done.

Thanks Steve!

The antenns is described about 85% down this page I have had for more than
15 years:
http://aprs.org/astars.html

Though it is talking more about using a 19" vertical as a 3/4 wave for UHF.
But the same goes for a 59" whip for 145.825!

Bob, Wb4APR
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