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[aprssig] APRS RFID reader?

Scott Miller scott at opentrac.org
Mon Feb 1 21:21:39 UTC 2010

Passive RFID tags don't do long range very well... returned signal 
decreases with the 4th power of the distance.

With the active tags I've got on my bench (somewhere in the $12 to $20 
range, I think) I can get a reading from across the warehouse, maybe 70 
feet, and with some work you can get a position estimate using omni DF 
from multiple receivers - the tags are set up to send their ID at 
several different power levels, since the receiver doesn't have any sort 
of useful RSSI output.

Even the active tags don't have much penetrating power, though.  You can 
put your hand around one and block off the signal when the receiver's 
only a foot away.  I was hoping to use them to track people and 
equipment in SAR vehicles, but I'm still not sure it's reliable enough.


Robert Bruninga wrote:
> 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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