How does rewinding a smart card antenna into a smaller coil leave it operational?

Question

Here is a description of how one guy extracted the electronics from a smart card and then embedded it into a ring wearable on a finger.

The card in question is more or less similar to the card in another question, the minor difference is that the card in the current question uses a thin (presumably copper) wire as antenna and the card in the linked to question uses wide thin flat traces. The wires run along the card perimeter. The card is credit card size, so the antenna is more or less an ellipsis.

So that guy uses solvent to soften the card plastic, strips the card and extracts the electronics. Then he rewinds the antenna wire so that the antenna coil can be embedded into the ring.

The resulting coil has several times smaller diameter yet the electronics work.

How does it happen that rewinding the antenna into a smaller loop does not break the card operation?

Answer 1

Without a link to the ring related page it's hard to be very specific BUT

• There is much rubbish and many false claims published on the internet.This has a good chance of being less veracious than may appear.

• The system may still function but with reduced range.

• It is exceedingly unlikely that it will work anything like as well as it did in its original form.

Patch antennas are based on a good grasp of the technology involved, a certain degree of magic and variable amounts of luck and experimentation. An antenna that worked in a card will have different area, different inductance and capacitances, different relationships of portions to other portions in terms of fractions of a wavelength and relative phase, and more.

A look at Microchip's AN710 RFID Antenna design application note description page here and PDF here will demonstrate the differences that varying geometry makes - and will also show you what may work best in a ring.

Answer 2

The "antenna" used in an RFID tag is a coil of wire (an inductor), and it interacts with only the magnetic portion of the RF field emitted by the reader. It may also be part of a tuned circuit that helps the tag extract power from the signal.

Therefore, any coil that has the same value of inductance, regardless of its geometry, will work correctly with the rest of the tag circuitry, and the sensitivity (range) of the tag will be a function of the "loop area" (basically diameter) of the coil.

Answer 3

How does it happen that rewinding the antenna into a smaller loop does not break the card operation?

Because your not really changing anything. The antenna still is an antenna of the same length, however it now just takes up less space. The range at which it works may be altered, however there isn't anything preventing it from working.

The wire that was used looks like winding wire (like in magnets, motors, and transformers,) and is insulated. You could cause damage in the process that would break the unit. Or if the antenna wire gets shorted it could cause it to not work or at least not work at any distance.

The second link you gave shows a wound antenna too, it's mostly done to save space but still give it the surface area it needs to work.