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[aprssig] APRS RFID reader?

Wes Johnston, AI4PX wes at ai4px.com
Tue Feb 2 09:21:04 UTC 2010

The runners in jogging races wear a cylindrical tag tied to their shoe
laces.  Looks a lot like a speedpass tag.

I gotta say, I sure wish we could change the tag ID numbers to the ascii of
my callsign.

God help those who do not help themselves.

On Mon, Feb 1, 2010 at 16:14, Patrick <winston at winston1.net> wrote:

> The issue is you run into is with the magnetic field generated.  Passive
> rfid tags are being powered by the magnetic inductance of the field they are
> moved through, so you run out of power the further away from the reader you
> are.  This is sorta on purpose because they also typically do not contain
> collision handling ability so you can't have more then one tag in the read
> area at the same time.
> There are commercial grade reader / chip systems which don't have the same
> constraints, but the costs go up accordingly.
> That said you really don't need more range.. Instead of putting the chips
> on their hats, put them on their feet..  if people are walking through the
> door with both their feet 10" off the floor, then they are purposely making
> themselves unreadable.  This is the way most RFID based timing systems work
> for distance races.
> p
> Quoting Robert Bruninga <bruninga at usna.edu>:
> I still think APRS-RFID... Is a next thing for APRS
>> experimenting
>> Every ham hat could have a $2 chip in it.
>> Then we can tell who enters the clubhouse or EOC... (it gets
>> converted to APRS...)
>> See http://www.aprs.org/aprs-rfid.html
>> Problem is, the maximum range reader I can find is only good for
>> 10 inches max and it costs about $24:
>> http://www.sparkfun.com/datasheets/Sensors/ID-12-Datasheet.pdf
>> It has provisions for a  wind your own antenna but still can
>> only get to about 10 " (25cm).  But since hams are RF
>> experimenters, it would seem that we could improve on this.  One
>> sentence in the limited docs says that there has to be enough
>> energy to activate the chip.  This implies to me that the limit
>> is on the energy transmitted to the chip, not necessarily the
>> read range.  Frequency is 125 KHz.
>> To get reliable coverage for people walking through a door, I
>> think we need about 48" range...
>> Does anyone want to fill us in on the details?  I assume a 125
>> KHz carrier in the antenna coil provides the energy for the RFID
>> chip (this can be scaled up... Just needs more power)...  But
>> then I guess it has some off cycles so the chip can send back
>> the 32 bit code?
>> Bob, WB4APR
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