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I'm trying to understand how my (rfid?) travel card works. It looks like this:

rfid-card

I'd like to know if the two portions of wire shown at the end of the arrows are capacitors and, if it is the case, how does it/they work(s). Could it be for example charges that accumulate on the surface of the wire creating a capacitive effet?

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  • \$\begingroup\$ That looks like a zig-zag shape. It may serve some RF purpose, but it's not a capacitor or inductor. \$\endgroup\$
    – Kevin Reid
    Commented Oct 27, 2012 at 15:28
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    \$\begingroup\$ RF is somewhat of a black art. The large rectangular 3 turn coil around the outside is the main pickup / transmit inductor. I'd guess that the small zigzags are effectively inductors - possibly to assist with matching - and possibly to minimise ringing (which is approximately the same thing). \$\endgroup\$
    – Russell McMahon ♦
    Commented Oct 27, 2012 at 20:01
  • \$\begingroup\$ @RussellMcMahon Do you mean "effectively a capacitor"? If it is an inductor, how does it physically works? \$\endgroup\$
    – cjorssen
    Commented Oct 28, 2012 at 13:00
  • \$\begingroup\$ "... an elephant's faithful, 100%." Small squiggly messes of too and fro wires will have capacitance between the lines BUT I'd expect the majority effect to be inductive. For how inductors get their inductance see wikipedia or many other references on "Inductance". [[Current in wire produces a magnetic field, field couples to adjacent wires and induces a voltage, ...]] \$\endgroup\$
    – Russell McMahon ♦
    Commented Oct 29, 2012 at 0:54

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Yes, the zigzags are to match the RFID antenna.

The basic idea is that the impedance of the antenna and the impedance of the whatever you are hooking the antenna up to need to be matched in order to transfer power from the antenna into the input circuit.

Whoever designed this probably used a simulator to figure out how to make those zigzags and then measured the results on a vector network analyzer.

Read RF Circuit Design by Chris Bowick to really understand (and then get some play time on a network analyzer).

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  • \$\begingroup\$ Thanks for your answer (+1). I'll read your reference as soon as I can go to my library. Can you elaborate on "the zigzags are to match the RFID antenna". AFAIU, this antenna is some kind of LC circuit: the 3 turn coil is the L part but where is the C part? If I understand you correclty, the desinger used a sort of try and error approach rather than a "physical" approach to build the antenna? \$\endgroup\$
    – cjorssen
    Commented Oct 28, 2012 at 13:06
  • \$\begingroup\$ @cjorssen - The book that Mariano recommended is an utterly superb RF circuit design reference - about the best I';ve ever seen BUT for your RFID question you'd probably get more from Microchip ... RFID system design guide AND see the AN's it references at the end AN710 relates directly to RFID antenna design and AN707 is like unto it. \$\endgroup\$
    – Russell McMahon ♦
    Commented Oct 29, 2012 at 1:00
  • \$\begingroup\$ The C part is everything else --- the plastic acts like a diaelectric and so you have a little capacitance between each wire. \$\endgroup\$
    – Mariano Alvira
    Commented Oct 29, 2012 at 22:41
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RF is somewhat of a black art.
The large rectangular 3 turn coil around the outside is the main pickup / transmit inductor.

The small zigzags are effectively inductors (NOT capacitors although they will have some capacitance) - they are possibly to assist with matching and possibly to minimise ringing (which is approximately the same thing).

Mariano recommended Chris Bowick's book RF Circuit Design - that is an utterly superb RF circuit design reference - about the best I've ever seen (I bought myself one years ago plus an extra that I gave to a friend) BUT for your RFID question you'd probably get more from Microchip's very useful and practical AN710 - Antenna circuit design for RFID applications and their excellent Microchip ... RFID system design guide

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