RFID tags have as long a range as you are willing to make an antenna (and how
sensitive your receiver is, I suppose).

One of microchip's RFID publications (available as PDF on their web site) has
the formula you need - put in feet from RFID tag, and it will give you coil size
in diameter.

I'm not sure what is going on in your case, though.  Is the person carrying the
RFID tag, or is the RFID tag stationary, and the device the person is carrying
detects its presence?

At any rate, the size of the coil is exponentially related to the distance from
the tag.  A coil about 6" in diameter will successfully scan a tag within 6-10
inches.

-Adam

"Dr. Chris Kirtley" wrote:
>
> Dear Chris,
>
> I suppose by "a few feet" I mean at least one and preferably 5-10.
>
> The problem with ultrasound is that the transducers are quite thick - I
> have to keep the thickness down to about 3-4 mm.
>
> I have a telemetry transmitter on there already, which is working fine,
> but I need to tell it when to transmit. If it is out of range the data
> will be lost (not to mention wasting power). So it needs to know that it
> is within range. At the moment I simply press a switch, but it would be
> nice to automate this. I don't want to incorporate an RF receiver
> because that would require be a big increase in size and power -
> basically it's not an option. I was wondering about an RFID tag, and I
> still think that's probably the best option. Do you know what sort of
> range you can get with them?
>
> Chris
> --
> Dr. Chris Kirtley MD PhD
> Associate Professor
> HomeCare Technologies for the 21st Century (Whitaker Foundation)
> NIDRR Rehabilitation Engineering Research Center on TeleRehabilitation
> Dept. of Biomedical Engineering, Pangborn 105B
> Catholic University of America
> 620 Michigan Ave NE
> Washington, DC 20064
> Tel. 202-319-6247,  fax 202-319-4287
> Email: kirtley@cua.edu
> http://engineering.cua.edu/biomedical
>
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