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RFID coil antenna design question

Discussion in 'Electronic Basics' started by Ben, Oct 23, 2003.

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  1. Ben

    Ben Guest

    Hi all,

    I am going to use some RFID (radio frequency identification) tags to
    track some birds as they move around our aviary. If you don't know how
    these work, basically the idea is that you have a coil antenna which
    creates a field which is somehow modulated in a unique manner by any tag
    which might come into its range, in a way which allows the antenna
    driver circuitry to identify the tag.

    You can only put small tags onto birds, which means read range can be a
    problem. Most easily available antenna are loop shaped, and the area
    where the tag can be read is mostly inside the loop. This works well
    with animals if you can get the animal to pass inside the loop, but I
    would like to try something different and I wonder if anyone can offer
    me some advice. I want to build a perch antenna, but I'm not certain how
    to go about it. Even if you don't know much about how RFID works, but
    you know the shape of a coil determines its field you might be able to
    help me because I know nothing!

    Assume a perch would be a cylinder around 10cm long and 4cm diameter. I
    have two ideas. One would be to just wind the coil around the cylinder
    with each turn going around the cylinder like a solenoid or something.
    But I have no idea if that will produce much field outside the coil? My
    other idea is to wind it with each turn going around the whole perch
    from end to end, to produce a coil shaped like a very flattened
    rectangle. Would either of these ideas work? By the way I do know how to
    use a formula to tell me how many turns I need to tune the antenna to
    the right frequency. I need 1600 LuH so for the first idea around 366
    turns.

    Thanks very much for any advice.

    Cheers,

    Ben Kenward
     
  2. Terry

    Terry Guest


    I vaguely recall; Inductance (L henries etc.) to be something
    about 4pi x Number of turns (squared) divided by the inductive
    constant of the core (air being 1) and the area within the coil
    or summat?
    So with that in mind;
    Could you wind a big coil/s around the whole cage, or a
    section/zone of it? Thereby identifying bird is in a certain
    'Zone' at a certain point in time etc.? Idea.
    Regarding winding around the perch; I guess that would give you a
    field with greatest intensity at right angles to the perch? With
    least intensity off the ends of the coil/perch, just like a ferro
    resonant antenna in a portable radio etc., which can be used for
    direction finding the signal being strongest when coming in from
    broadside to the coil antenna? What frequency are you using?
    This make any sense?
    Terry.
    PS. Four (4) cm. diameter perch! That's a big bird? I guess you
    are not looking at budgerigars?
     
  3. Ben

    Ben Guest

    Crossposting this now to sci.electronics.design to see if anyone there
    has anything to add!

    Thanks:

    Yeah, I could do that. Although, to get a system like that working you
    either need a more powerful reader than I have, or bigger tags than you
    can put on these birds.
    The frequency is 125kHz.
    Yeah, kind of. So you are saying with winding the coil with turns going
    round the perch, the strongest field would be broadside to it. That's
    exactly what I need for the tags to be read when the bird lands on the
    perch. Looks like I should give this method a try.
    Well, they're bigger than budgies, but not huge... About the size of a
    pigeon. Here's our research page:

    http://users.ox.ac.uk/~kgroup/tools/tools_main.html

    Thanks for your advice!

    Ben
     
  4. Ben wrote:
    (snip)
    The strongest coupling will happen when the two coils are on the same
    axis, with the small one inside the larger one. It may be helpful to
    extract the pickup coil from an RFID tag, and connect it to an
    oscilloscope, while waving it around a few different energized perch
    designs, to make sure your assumptions are real.
     
  5. Wim Ton

    Wim Ton Guest

    The strongest coupling will happen when the two coils are on the same
    Temic (ex Telefunken) has an application note about antenna design for their
    U2270B rfid receiver

    Wim
     
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