Inductance of co-located antenna coils

Discussion in 'Wireless & RF Design' started by kender, Apr 1, 2007.

  1. kender

    Thread Starter Senior Member

    Jan 17, 2007
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    0
    Colleagues,

    I’m new to electromagnetic design, so my questions are probably basic. The antenna is for an rfid reader, but my questions are not specific to rfid.

    I have a 125kHz RFID reader, which is based on the EM4095, and it has an coil antenna with an air core on the PCB (CCS RFID kit). Unfortunately, it doesn’t generate enough field for the tag to turn on. I want to add my own wire-wound antenna with bigger diameter and more turns. However, I don’t know what to do with the existing antenna. Since it’s made with a PCB trace, it’s difficult to remove. So, I can see 2 options:

    - Connect the new wire-wound antenna (with inductance L2) in series with the PCB antenna (as shown in the attached picture). The inductance of the PCB antenna L1 is not specified, but it can be calculated from the working frequency and parallel resonant capacitance. Will the cumulative inductance L of the new combined antenna equal (L1 + L2) ? I guess "yes", because the air core doesn't saturate. Is tis correct?
    - Cut the traces going to the PCB antenna coil. Will the separated unpowered PCB antenna interfere?

    Answers to these questions as well as any insights or advice are greatly appreciated!

    Sincerely,
    Nick

    EDIT: here's a picture of the original antenna on PCB (lower board):
    [​IMG]
     
  2. Papabravo

    Expert

    Feb 24, 2006
    10,137
    1,786
    What makes you think any of this fooling around will help you with your problem? Generally speaking changing the geometry or impedance of an antenna chnages its resonant frequency or its directionality. To change the power level you need to look at the circuitry in the final amplifier stage.

    To do this kind of stuff requires an instument that probably costs as much as small house. It's called a Network Analyzer.
     
  3. beenthere

    Retired Moderator

    Apr 20, 2004
    15,815
    282
    You can change the radiation pattern with another antenna, but the one that came with the reader is probably well-designed.

    If you need more power out, then it's like papabravo said, you need more RF into the antenna. Be a bit cautious before buying a 100 watt linear, though. The FCC probably has specs for this stuff that limit the maximum RF output to some distressingly low level. They probably don't want you reading the stock id's on the store next door - or rfidsnarfing id's carried by passing pedestrians.
     
  4. kender

    Thread Starter Senior Member

    Jan 17, 2007
    263
    0
    Proof by existence. I have made an antenna with more turns and I were able to develop higher voltage at the receive side. Still the voltage isn't high enough, though.

    Here in the Silicon Valley a new network analyzer costs about 10% if a small house. By the way, I'm yet to see a network analyzer that works at frequencies as low as 100kHz.
     
  5. Gadget

    Distinguished Member

    Jan 10, 2006
    613
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    I've done a little fooling around with RFID, and according to the data, the RF field the the tags will work within is actually proportional to the Antenna coil size (feild size normally approximately the same as the coil diameter).
    Obviously if rewinding a larger diameter coil, it must be tested for resonance.
    I'm guessing the best option would be to cut the traces to the original antenna and connect the new one at those points.
     
  6. Papabravo

    Expert

    Feb 24, 2006
    10,137
    1,786
    So what convinces you that a small loop on a PC board will effectively radiate a signal with a wavelength of 3000 meters. Assuming it did radiate effectively any conceivable receiver would certaintly be in the near field. Maybe that's the clue, is that this is entirely a near field phenomena.

    BTW if you choose to experiment, cutting the traces to the existing antenna would be the way to go. The logical conclusion of more turns equals greater signal levels must have a limit at some point since we know that other factors will limit the ability to achieve infinite signal levels.
     
  7. kender

    Thread Starter Senior Member

    Jan 17, 2007
    263
    0
    I agree - it's a near field. In my mind the transmit and receive antenna are merely windings of an air-core transformer. I need to communicate only as far as 4 inches.

    Speaking of infinite signals. Resonance in my new winding produces 200 Vpp from a 5Vdc power supply. That makes me a little uneasy, although everything else seems to work as designed.
     
  8. Papabravo

    Expert

    Feb 24, 2006
    10,137
    1,786
    So this really isn't an RF phenomena as much as transformer phenomena. First point is that when you step up a voltage you step down the current. You need both voltage and current to drive various kinds of loads and by changing the number of turns you've also transformed the impedance of the load on the secondary back into the primary and vice versa.

    I think this will lead in circles of ever increasing radius without giving a clue as to why these devices won't work together. Since they were not originally designed to do that I'm not hopeful that a solution can be infered.
     
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