RFID tag reader coil size, how small can it be?

vanmunch

Hey Everyone,

I was wondering if anyone has had any experience with making their own RFID tag readers? I was wondering it was possible to make a reader that is smaller than the “standard” credit card size?

Thanks for your time,

Dave

Beau Schwabe

As long as your LC combination has a resonant frequency of 125kHz, then different coil sizes will work. You must also take into consideration that the detectable distance is roughly going to be equal to the diameter of the coil, meaning that if your coil is 3 inches in diameter then you should be able to "see" a signal that is 3 inches away from the face of the coil. That said, if your coil is to large then the tag won't be strong enough to properly modulate the signal. The best solution is to have a coil that closely approximates the physical size of the coil used on your tag.

Here is a link to a RFID project I did awhile back. The design uses a similar "Double D" approach to metal detecting, but the concept for creating your own "single" coil will be the same.

http://wiki.ohmspace.org/User:Bschwabe#OHM_Space_.28EM4100.29_RFID_reader

vanmunch

Hey Beau, that is really amazing! I really like the videos. Do you have any more information/code on it? Would it be possible to etch the coil on a PCB? One coil for on each side? I've been trying to see if I could get Micah's RFID reader to work, but no luck so far.

Thanks again for your help!

link:
http://forums.parallax.com/showthread.php/105889-World-s-simplest-RFID-reader

Beau Schwabe

vanmunch,

Etching coils on a PCB is possible, but you almost always need to do empirical testing first and take measurements. i.e. Design some test coils that are binary weighted in their values (number of turns) and note the differences in inductance between each coil. Then based on your results, and using the SAME manufacturing process, create a proper value coil, and then test again... :-)

I do have some code I will try to dig up and post later.

Mark_T

Bear in mind a smaller coil will need a more turns for the same sort of inductance value, leading to higher
field strengths unless you reduce the current levels too. Of course you can vary the capacitance value too
to allow for different inductance, but the Q of the resonant circuit depends on losses in the various components,
so there are several variables that all go together and can't be considered in isolation. The current level in
the LC circuit depends on the Q as well as the driver current, for instance.

Beau Schwabe

vanmunch,

Mark_T is correct a larger coil and/or a larger capacitor tends to decrease the Q (Quality factor) ... in some cases where a tag might not be exactly 125kHz (due to damage) or the detector circuit might not be properly tuned, a lower Q could be beneficial because of an increased bandwidth... The caveat is a lower signal strength, so the benefit may be a moot point.

Attached is the program I used for the RFID reader you see in the video.

vanmunch

Mark_T, Beau, thanks for the info. What's the best way to "see" the inductance value? I found this link with 3 different methods. Do either of you use one of these?

Thanks!

Dave

Link:
http://www.wikihow.com/Measure-Inductance

Beau Schwabe

I use this method ... It would fall under the RC method described in your post above. The method I use compensates for temperature and tolerance variations in components.

http://wiki.ohmspace.org/Determining_an_unknown_inductor_value_using_a_%27known%27_inductor_as_a_reference_value