[aprssig] RFID antennas

Wes Johnston, AI4PX wes at ai4px.com
Sun Jul 13 16:18:57 EDT 2008

The RFID tags that are cheap are pre-programmed with unique serial numbers.
We'd need some way (ie central server) to register your unique serial number
and map it to a callsign.  There are RFID tags that are programmable, and
they would be slick for this application...  I suppose you'd want a RFID
reader to send data to a PIC processor that would combine a canned GPS
location with the serial number of the RFID tag.  Perhaps you'd want the PIC
processor to randomize the position it used with the RFID tag serial

Hmm... many aprs clients will display a tactical callsign, so if the RFID
pic processor were to construct a 3rd party APRS packet using the RFID
serial number as the "callsign" we could manually assign it a tactical call
of the user's actual callsign on our aprs client (like xastir).  But aren't
we limited to 9 digits for a callsign in the aprs spec?  I think the RFID
serial numbers are longer than that.  Of course the serial numbers would be
serial in a sequence, so perhaps we could truncate the RFID number and just
use the last 9 digits?

I've thought about indoor tracking.... have a tiny track or open track (what
have you) and program it so that it will transmit w/o a GPS fix.  Have a pic
processor above the doorway into a room sending via IR either a fixed GPS
position string (ie fake GPRMC), or a fake GPRMC that chooses random coords
with set boundaries.  If you randomize the coordinates it will prevent
everyone in the room from appearing to "stack up".  your tracker is able to
receive serial data from a 38khz IR photo transistor or from your personal
GPS unit.

It goes like this....   When you walk indoors, the GPS will loose lock and
begin to transmit (serially) that it is out of lock.  The pic processor in
the opentracker should ignore this data.  The GPS data is fed to the open
tracker via a 10k series resistor.  The data from the IR photo transistor is
fed via no resistor and is easily able to "over power" the serial signal
from the real GPS unit.
If it were me, I'd place one of the fake GPRMC sentence generators over a
doorway shining down at a 45 degree angle into the room.  One on each side
of a door header to catch you entering the room and leaving the room.
On Sun, Jul 13, 2008 at 11:05 AM, Bob Bruninga <bruninga at usna.edu> wrote:

> > "The Texas Instrument HF Antenna Cookbook."
> > The designs are for RFID at 13.56 MHz.
> I seriously want to see the ARRL/Ham radio invest in RFID for ham radio
> ops.  Embed the chip in the ARRL name tag.
> Then we can have gateways to APRS...
> Put one at each venu or checkpoint or clubhouse, or whatever.
> Range should be about 100 feet...
> Bob,WB4APR
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Where there's silence, there is no Hope.
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