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[aprssig] Reading APRS becons w/Dopplers?

Curt Mills archer at eskimo.com
Tue Mar 14 13:48:43 UTC 2006


On Mon, 13 Mar 2006, Steve Noskowicz wrote:

>   I'm interested in how well folks have seen Dopplers read the
>   direction of APRS "squawks".

APRS usually has a position in it, so you must mean a misbehaving
APRS station?

I've read a bunch on various RDF devices in the past.  The type I've
always wanted to build is a doppler scanning antenna.  These can
lock on quite quickly, plus they can also show you if there are
reflections/multipath in the area as you can get multiple directions
lit up on the LED's (old design) at once.  You can either manually
integrate the LED's with your eyes to come up with the strongest
signal direction, or with some of the newer designs they'll
integrate multiple readings for you.

I also understand that they're not as sensitive as a beam antenna,
or even a single whip due to the scanning noise, so if you're
tracking a weak signal you may need a directional non-scanning
antenna first to be able to sniff it, then as you get closer you can
switch to the doppler array.

When hooked into APRS, you can get automatic vectors appearing
either on your map alone or on everybody's maps if you transmit
them.  Have 2 or 3 doppler arrays running around and you should be
able to close in on a particular area very quickly if you're all
playing together.  If it's a foxhunt then the "playing together"
part goes away and it becomes more difficult/time-consuming to find
the transmitter.

Does anyone on here have any direct experience with an RDF unit
hooked to a receiver on 2 meters or 220, plus an APRS TX/RX system
running in the same vehicle to coordinate with others?  I've heard
that its easy to get false readings and/or blow out the RDF array by
transmitting from the same vehicle.  I have an application where I'd
like to do this (transmit, not blow things up!).

-- 
Curt, WE7U.				archer at eskimo dot com
http://www.eskimo.com/~archer
  Lotto:  A tax on people who are bad at math. - unknown
Windows:  Microsoft's tax on computer illiterates. - WE7U.
The world DOES revolve around me:  I picked the coordinate system!"




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