RFID antenna

Discussion in 'General Electronics Chat' started by geoffers, Jun 16, 2012.

  1. geoffers

    Thread Starter Active Member

    Oct 25, 2010
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    Hi all,
    Hope someone can help/point me in the right direction, I've been working on a rfid reader for some time now based on iso 11784/85, the reader needs to read both hdx and fdx-b transponders. My prototype works and will read both but the range on the fdx-b is poor. (fdx is when the the transponder loads the activating field, but you probably knew that!) I've spent some time playing around with different values for the coil (I want to keep the same dia so it fits in a bit of plastic 35mm pipe) the tranponders are the 3mm dia glass type so have a very small coil. I'm using the atmell u2270b front end ic (the data sheet should be attached) with diode feedback. Changing the value of L from 700uH up to 2.2mH (which is where I started) seems to make little difference to range, peak to peak voltage goes up as L goes down but I guess the current the chip can supply is the limiting factor? Would making a antenna with a ferrite core help? Or should I add a H bridge or similar to drive the coil at a higher voltage and current.
    Thanks Geoff
     
  2. kubeek

    AAC Fanatic!

    Sep 20, 2005
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    I think the H bridge should help, but 200mA seems like it should be plenty. Have you measured the actual current in the coil?
     
  3. geoffers

    Thread Starter Active Member

    Oct 25, 2010
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    I havn't, it could be I'm close to the limit of whats possible with that size coil? A transponder will read upto 10mm or so, I will see if I can get a reading for the current and post again.
    Thanks
     
  4. geoffers

    Thread Starter Active Member

    Oct 25, 2010
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    I dont think my multimeter is fancy enough, put it in series with the coil set on ac amps but got no reading, I suppose it could only be designed for 50hz ac not 130khz?! However the whole reader is only drawing 50mA max! This is running a 18f252, bluetooth modem and a 16x2 lcd as well as the u2270b. Read range is a little bit more than 10mm, could be closer to 20. Also the coil is energised for 50ms the turned off for 50ms.
     
  5. kubeek

    AAC Fanatic!

    Sep 20, 2005
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    I guess you really would have to use an oscilloscope to see what´s going on. The current consumption doesn´t say much about the resonant LC circuit though.

    I am currently making a universal tag for 13.56mhz and wihtout oscilloscope you can´t get anywhere. Anyway I don´t think the inductance of the coil matters much, the only important thing is that the circuit is tuned to the proper frequency. Also the Q factor of the LC circuit has some importance for the data transfer, see this pdf.
     
  6. geoffers

    Thread Starter Active Member

    Oct 25, 2010
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    Thanks for reply, I've got a Velleman handheld scope I use, my coil is tuned to be resonant. I guess I sort of knew the answer before I posted, I was hoping there was a 'dodge' or technique I could use. Am I right in assuming if I increase the wire diameter of the coil, until the Q factor is too high I will get a increase in read range due to the increased current? Also if I could increase the voltage driving the coil this would also increase current and therfore read range? The micro chip data sheet on rfid antenna (hopefully attached?) says coil radius is roughly 1.2x read range so I suppose I'm not to far away from that. I'm not sure how the diode feedback works though? I'm self taught so there are big gaps in my knowledge I'm trying to fill in!
    Thanks Geoff
     
  7. kubeek

    AAC Fanatic!

    Sep 20, 2005
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    put some small resistor like 0.1ohm in series with the coil and see what power it consumes, try the same with the driver outputs.
     
  8. geoffers

    Thread Starter Active Member

    Oct 25, 2010
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    I've been fiddling about with this again! I went up in the diameter of wire I'm using from about 32gauge to 28, this has much improved things, my reader now works up to about 40mm from the coil which was about what I was aiming for. Thank you for your advise and the data sheet you posted, I think it applys to lf rfid aswell to a degree. Does anyone know how the diode feedback works? I just know it does so far!
    Cheers Geoff
     
  9. geoffers

    Thread Starter Active Member

    Oct 25, 2010
    239
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    Hi again,
    I got some 20 gauge wire and wound a 300mm coil, when I turn it on first I get realy good range, if I leave it running for 20mins or so the range drops off! I guess I'm drawing to mucch current from the drivers in the chip now. I want to make a even bigger coil, however according to the microchip rfid design guide, amp turns are the deciding factor! As you make the coil bigger for the same inductance the number of turns goes down, so I assume I need to drive the coil with a higher voltage and therfore current? Any one got any good designs fo H bridges that will work at 125kHz? Or is the a chip I can use? (which would be prefrable)
    Thanks Geoff
     
  10. geoffers

    Thread Starter Active Member

    Oct 25, 2010
    239
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    Hi again, found these two so far, one on here. A simpler one elsewhere. I was going to substiute my coil for the motor in the designs. Could I do without the diodes in the more complicated design? The four transistor one needs buffers? Which I guess losses its simplicty? Any thoughts
    Thanks Geoff
     
  11. geoffers

    Thread Starter Active Member

    Oct 25, 2010
    239
    6
    [​IMG]
    Oops here it is (I think!)
     
  12. geoffers

    Thread Starter Active Member

    Oct 25, 2010
    239
    6
  13. geoffers

    Thread Starter Active Member

    Oct 25, 2010
    239
    6
    Hi all,
    Been at it again, I built a H bridge (6 transistor design) as pinched from some elses posts on here! With the input to the h bridge connected to the u2270b's driver output, and the h bridge driving the coil at around 13volts (which is un-regulated), (as I write I think I may answer my own question?!) I get just over 170v peak to peak on my coil. (without h bridge it was 80ish volts). Unfortunatly it wont read now! Could this be because my supply isn't as stable as it needs to be? I'm using diode feed back, I'm still unclear how that works and may also be a factor. The h bridge and u2270b share a common ground line.
    Any help would be welcome.
    Thanks Geoff.
     
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