Comments on: Adding a remote antenna to Parallax’s RFID reader

By: Matthew Potter

Matthew Potter — Sat, 28 May 2011 20:20:25 +0000

@Matthew Hall – how many turns did your final antenna have? I don’t have an original antenna to work with, so I’m flying in the dark, trying to match up the 1.08mH my RFID reader needs (ID-2). It’s a shot in the dark, I know, but a manufactured an antenna or a reader with a built in antenna will be cheaper than an oscilloscope if I can’t get mine to work… :)

By: todbot

todbot — Mon, 31 Jan 2011 06:38:34 +0000

This is great, Matthew! I’ve always wanted a way to do this that didn’t use a inductance meter. (I told those who asked to just do 100 winds of 30 gauge magnet wire, a sloppy solution that usually works to some degree)

By: Matthew Hall

Matthew Hall — Mon, 31 Jan 2011 05:48:22 +0000

So, in case anyone is wondering how best to build a coil to match the one on the reader, when one has no access to an inductance meter (though having access to either a scope or a frequency meter), here’s how I did it:

I did the same thing as in these instructions, but I also soldered two extra wires onto the PCB antenna (i.e. on the other side of the cut trace). I then built a simple Colpitts oscillator, which is an oscillator whose frequency depends on the inductance of a coil. Its a single NPN transistor, a couple of caps, and a couple of resistors. The schematic I used can be found here:

http://www.falstad.com/circuit/e-colpitts.html

There are more robust Colpitts circuits out there, but that one worked well (plus the little Java animation is edumacational!). Once you have the circuit built, plug in the coil from the PCB card, and measure the frequency with either a scope or frequency counter (I got about 50 kHz on mine).

Then, to build your coil, use a formula, or a calculator like the one at the bottom of this page:

http://www.zen22142.zen.co.uk/Theory/tunedcct.htm

I built mine on an 8″x8″ frame, since I wanted it to be something my cat could walk through. I used the inductance formula to come up with an initial estimate of 40 turns, and then (burning off a bit of enamel on the magnet wire), tested it in the same Colpitts circuit, and found that I was a fair bit slow (only about 25kHz!). So I took off about 8 coils, and tried again. I continued in this fashion until I got something within spitting distance of same frequency that the PCB gave me, and then attached it to the RFID circuit. It worked very well right off the bat, reading the card starting at about 6 inches away at a variety of card/coil alignments.

Anyway, beyond the scary bit where I had to scrape off the insulation from the tiny traces without destroying the board, it wasn’t that hard to add an external antenna. Thanks, Todbot!

By: rajeev

rajeev — Thu, 26 Aug 2010 05:15:42 +0000

How long can the connecting wire to the antenna be – I am looking for maybe 18 feet?

By: Adrian

Adrian — Sat, 22 May 2010 00:55:38 +0000

Hi everyone! for the parallax reader, can i create multiple antenna after i disconnect the built in antenna?

By: Clark

Clark — Sat, 17 Oct 2009 04:00:50 +0000

Sorry a 3′ by 2′ area.
Thanks

By: Clark

Clark — Sat, 17 Oct 2009 03:59:52 +0000

Looking for a reader that I can extend the antenna to read a 3″ by 2″ area. Any suggestions?

By: todbot

todbot — Tue, 25 Aug 2009 20:55:11 +0000

Hi Gush,
My understanding is that at 13.56MHz it’s best to make using a PCB, where the traces on the PCB form one or more inductors and a couple of capacitors are placed next to the traces to form the resonant LC circuit needed. It’s a lot more complex than the rather simple single loop + single capacitor of the 125kHz antenna, but there are many application notes on how to design the PCBs.

Also, an L/C meter just measures either an isolated inductance (L) or capacitance (C), and doesn’t really have a frequency component. It cannot produce meaningful measurements on a resonant circuit like the complex antennas used on 13.56MHz RFID readers. The L/C meter I use is this one:
http://www.aade.com/lcmeter.htm

By: gush

gush — Tue, 25 Aug 2009 20:40:08 +0000

Hi Tod,

Would home-brewing a remote 13.56MHz antenna for something like the Skyetek readers require a LC meter that could sample at 13.56MHz? Also, were you just matching the inductance of the antenna on the PCB, or did you calculate what you were going for? Any detuning because of the leads to the remote loop?

Thanks!

By: todbot

todbot — Tue, 19 May 2009 22:06:41 +0000

ioan, that’s pretty hard. The antenna is embedded in the PCB the reader circuit is on. It would be pretty easy to slip with the cutting tool and destroy the circuit.

By: ioan

ioan — Tue, 19 May 2009 21:37:27 +0000

What about cutting off the old antenna and use the old antenna (that you just cut off) as a remote antenna?

By: Sha

Sha — Wed, 21 Jan 2009 02:33:31 +0000

So did the hack of ASPX RW210, 13.56 ever happen? If so any details would be appreciated. Thanks!

By: todbot

todbot — Tue, 16 Dec 2008 03:22:22 +0000

Hi Alain,
Unfortunately, no. The Parallax RFID reader (like most low-frequency readers) are designed for a specific frequency and protocol. In the Parallax reader’s case, it’s designed for the EM410x protocol which is at 125kHz.

By: alain

alain — Tue, 16 Dec 2008 02:31:52 +0000

Hellow i need to use this rfid the parallax to read the glass rfid for animal uses and the frecc
is like 134,2khz how i make or is posible to modifi the software or the hardware for read this freccuencie thans
A Durot

By: todbot

todbot — Wed, 19 Nov 2008 23:36:14 +0000

Hi Koyote,
Not a dumb question at all. Unfortunately, the hack as presented effectively replaces the original antenna. You can use the original antenna by sticking a jumper across the two-pin jack that was added. So in theory you could put a relay in that that lets you programmatically switch between one antenna or the other. But having both at the same time I think would be a no-no.