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[aprssig] RFID antennas

Robert Bruninga bruninga at usna.edu
Tue Jul 15 23:04:47 UTC 2008


> Sounds like what you need are the RFID tags 
> that run the Wi-Fi protocol. ... They can 
> be worn by people, or attached to equipment.

But I doubt they are cheap and more importantly, can run for
hours or days on tiny battery cell that would fit on a  nametag?
A passive device with 10 foot range would be even better...  

I cant resist!  I started a WEB page:
http://www.ew.usna.edu/~bruninga/aprs-rfid.html

Lets see.  A $89 microwave oven with the door off....  That
could beam plenty of power.... (JOKE!)  but seriously, what is
the lowest power microprocessor we have readilly available?

Lets see.  The ARRL exposure limits are about 1 mW/sqcm at 900
MHz so a 3 x 10 cm nametag could conceivably capture about 30 mW
... Lets go to 10% of that amount to be safe..  Store up 3mw
over one second and then tx at 30 mW for 100 mS at the end...
Hummh... To get that much power into the name tag, through a 6x6
foot door would need 100 W though!  Maybe I'm doing something
wrong...

A safer approach would be a few sqcm of solar cells would give
over 100mw in the sun, you'd get only 10mw indoors.  But if you
charge up a cap over a few minutes in the light, you could power
a burst?

Remember, from a CALLSIGN, we would get all of the following
when that station walked through the door:

CALL, DATE, TIME, LOCATION

In addition, the door system would add these:
FREQUENCY in use in that arena
FUNCTION being performed there...

Of course the location is offset-and plotted as a list... See
the web page...

And that is pretty much what APRS is all about.

Bob, WB4APR
> 
> 
> ----------------------------------------
> > From: bruninga at usna.edu
> > To: aprssig at lists.tapr.org
> > Date: Mon, 14 Jul 2008 23:43:32 -0400
> > Subject: Re: [aprssig] RFID antennas
> > 
> >> I would question the need for such a device. 
> >> I can see it being used at something like Dayton...
> >> but I wonder if there is a significant enough 
> >> collection of Hams in other avenues to justify the cost.
> >> Where do you see this system used?
> > 
> > First thoughts that come to mind are at the door to the 
> clubhouse, the EOC (we have 3, county, city and state).  At 
> the mobile comm vans, the HQ and other gathering points for 
> emergency comms.  At each checkpoint in any ham supported 
> event.  At hamfests..
> > 
> > Then there are applications where we loan our badges to 
> things or people that need to be tracked.  Not just hams.  
> > 
> > By rolling our own systems, my guess is we could get the 
> readers down to the price of a tracker or so.  The point is, 
> that it would be a new capability that we have never had 
> before, so we dont really know what all the applications 
> would be.  Kinda like barcodes invented for grocerystores, 
> but then they became to be used everywhere for all kinds of 
> applications...
> > 
> > Bob, WB4APR
> > -- 
> > 
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> 
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