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[aprssig] RFID antennas

Greg D. ko6th_greg at hotmail.com
Wed Jul 16 04:20:45 UTC 2008


Hi Bob,

"Cheap" will probably be the biggest issue, as they are meant for commercial and enterprise applications.  Tracking nurses and critical equipment in a hospital, that sort of thing.  But, perhaps one of the providers could be persuaded to help out the on-profit Ham community with just the tags.  Judging from the pictures on their website, these things look quite cheap.

As for battery life, looking at the Ekahau T201, they claim "up to several years" battery life, and are rechargeable.  So, it doesn't sound like much of a problem.

>From an RF safety perspective, I wouldn't worry either.  802.11g Wi-Fi packets are sent in microseconds, and I expect these things don't chirp all that often.  It looks like there is a button on them (so you'd need to give it a pat as you walked through the door, Captain Kirk style), but even so, the average RF output is going to be near zero.

But, on your site you mention kits, and while I can see the receiving end being a do-it-yourself project, what do we do about the Tag?  If we can't get a deal on them, perhaps one could be fashioned from one of those cheap USB Wi-Fi sticks, and a PIC processor?

I guess my point in all of this is to not assume we need to use 1200/9600 baud AFSK AX.25 stuff to accomplish this job.  I have no interest in Ekahau or any other tag company, by the way...

Greg  KO6TH


----------------------------------------
> From: bruninga at usna.edu
> To: aprssig at lists.tapr.org
> Date: Tue, 15 Jul 2008 19:04:47 -0400
> Subject: Re: [aprssig] RFID antennas
> 
>> 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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