[aprssig] RFID antennas

Wes Johnston, AI4PX wes at ai4px.com
Mon Jul 21 00:01:01 EDT 2008

I once read about the people that made VNC used a similar badging system to
track people's movements within a building for transferring phone calls and
such.  They used a motion detector to allow the critter to not transmit all
the time.  I'm thinking that if you are sitting still, it'd beacon once or
twice before entering sleep mode.  It would wake up when you started
walking... I'd hope that your stride would be enough vibration to wake it
up.  We'd just need a RC timer to keep it on for some number of seconds
after movement was detected... like 2 minutes for example.  A piezo element
tied to a diode, cap and resistor would serve as a motion detector directly
into a pin on the pic.  The pic would actually program itself to sleep
between transmissions for seconds at a time.  Power averaged would be on the
order of nanoamps, 10s of milliamps during the transmit cycle... 4800 baud
transmitting 16 to 20 bytes is 200ms at say, 100mA.  As far as battery life
goes... if a tiny trak with a 300mw transmitter can last 100 hours on a 9v
battery, that's pretty reasonable - that's 4 days at the EOC minimum....
more if you go off duty and don't move for a while.

It would beacon randomly every 30 to 45 seconds while active.  Maybe every 5
minutes when not active.  Sorta like smart beaconing but since we can't tell
direction, we just up the beacon rate when the wearer is wiggling the unit.
It certainly shouldn't send an entire aprs packet... heck it doesn't know
where it is, only it's callsign.  So it could only beacon a callsign,
perhaps an icon/symbol and a 3 bit message indicator (optional, but let's
spec it from the start).  I like the idea of a keyfob, and the idea of a
name badge equally.  Heck, even if this thing ended up being the size of a
pager on your belt, that'd be OK.   When you start to approach the size of a
pack of cigs or a cell phone, people might start to not want to wear it...

I'm thinking of using this even around our drills at the SCSG.  What would
be really kinda slick would be to actually put the badge to aprs converter
on one of the official's golf carts.  As he drives around, the badge to aprs
coverter uses a GPS ot keep track of it's location to which it prepends the
received badge IDs.  And official driving past all our groups of troops
would end up marking them on the map for us as a roving "proximity
digipeater".   We could use a few fixed installations such as on the commo
trailer, mess tent and such... and one "rover" on the golf cart.


On Sun, Jul 20, 2008 at 11:13 PM, Bob Bruninga <bruninga at usna.edu> wrote:

> Regarding APRS RFID Name Tags:
> >> Why not use those little 433mhz data transmitters?
> > I hooked an early prototype SMT OpenTracker
> > to one of those ...  Frequency drifted a bit
> > with temperature extremes, but it was standard
> > APRS decoded with a 70cm rig.
> Great to hear of such testing...
> That is the ideal approach, but I just figured the full APRS AX..25 was
> overkill and would take more power.  THe winning application here is the one
> that is either complete passive (meaning it always works) or is a 1 year
> battery replacement (meaning it almost always works mostly)...
> If it takes a battery per full day or weekend of operation, then I think it
> is a non-starter, since it simply will not be a 100% passive detector of the
> wearer's presence... everywhere and always...
> I figured the DATA key-chain FSK transmitrers that have companion receivers
> and are immune to the effects of drift and all they have to send is a
> callsign/SSID (40 bits).  Everything else is added by the RFID sensor.
> Now I would like to know ideas on what additional bits might be needed?.
>  Bout the only additional info I can imagine is an SSID or SYMBOL byte so
> that one ham can wear different name tags to convey slight differences...
> SYMBOL  (can be a very reduced subset)
> But since almost everything that is germane to the event or situation is
> known and can be added by the RFID DOORWAY READER, I still have not come up
> with much other than callsign/ssid is all that is needed.  Ideas?
> in fact, SSID is not really needed since again, the doorway-reader can flag
> it as a RFID and not any other use under that persons call...
> And that makes it work perfectly under the KISS principle!
> Bob, WB4APR
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Where there's silence, there is no Hope.
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