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[aprssig] APRS RFID reader?

Patrick winston at winston1.net
Mon Feb 1 21:14:10 UTC 2010

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.


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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