Chris -

I notice you're an MD.  I hope this isn't an in-body or life support
application, because if so you'll have many regulatory/approval hoops to
jump through.  I wouldn't even want to touch the job if you paid me, and
I do this stuff for a living.

Assuming it's not for implantation or life support, you're still asking
for a lot.  You want data transmission over several feet without using
hardwire, rf, or light.  Since no one has yet been able to harness
gravitational fields for data transmission, that leaves magnet and
electric ( but not E-M) fields.

Motorola has been working on a new ID tag technology they call
Bi-Statics, which uses electric fields to communicate, as well as to
power up the tag, which consists (I think) of a tiny chip mounted  to two
capacitive plates which are side by side instead of face to face.  They
are being very tight-lipped about technical data.  We tried to get specs
so we could build a reader, and they won't even talk to us except to say
"call back later".

I do, however have some experience in magnetically coupled RFID devices
at 125 kHz and 13.56 MHz, as well as a power-plus-data system operating
at 70 kHz.  Transmission of data over several feet is possible.
Unfortunately, magnetic antennas tend to be pretty directional.  Is this
OK?    Also, I'd never even attempt this in a system like RFID where the
data is in an unpowered tag, and the reader has to send enough power to
energize the tag, then the tag sends data back by applying a tiny
modulation of the RF field generated by the reader.  I might attempt this
if the data were in a powered system that could generate a field that can
be sensed by the receiver.  Is that what you have?

As far as trying to charge the battery, it can be done, but requires
fairly close proximity of the two coils--say 1 or 2 inches max.  Forget
about getting to several feet.

If you're still interested, give me some more information and I'll
respond.  The kind of info that's important is:  size limitations, cost
limitations, approvals required, hobby or commercial, power available at
each side, reliability requirements,

By the way, I do have the M'Chip RFID developer's kit, and have played
with it.  It's not bad, but it's probably not for your application.  It's
really aimed at a skilled developer who wants to develop his own system.
If you have specific questions, ask away.

Don

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