Confused about smart card wavelengths and antennas

Question

The tap-to-pay cards. These are also called contactless cards, right? And also smart cards?

The interrogator frequency is 13.56 MHz, correct? The reply (subcarrier) frequency is .969 MHz to 3.013 MHz, correct? That's what ISO/IEc 18000-3 says.

Using \$\nu\lambda = c \$, we can calculate the wavelength.

Frequency (MHz)	wavelength (m)
.969	310.
3.013	99.
13.56	22.12

Per Wikipedia, the picture of a chip on a Canadian penny makes them look like they're about 3mm square.

Now, various articles and web sites indicate that for a dipole antenna, the antenna has to be about 1/2 the wavelength. But that means the chip is waaaay too small, by a factor of around 33000.

What am I missing? Yeah, I know that the dipole isn't the only antenna configuration, and I get that other configs respond differently, but 4.5 orders of magnitude????

I guess what I'm asking is about how the near field works vs. "normal" antennas.

Answer 1

I guess what I'm asking is about how the near field works vs. "normal" antennas.

Near-field communications works with either electric or magnetic fields. Dipole antennas work in the far-field where wavelength matching to the frequency is a relevant thing to consider. These types of card (or tag) use magnetic fields for powering the tag and for performing communications. Hence pretty ordinary techniques like coupled inductors (aka transformers) work just fine.

These are also called contactless cards, right? And also smart cards? The interrogator frequency is 13.56 MHz, correct? The reply (subcarrier) frequency is .969 MHz to 3.013 MHz, correct?

They may also be called tags I guess but what's in a name?

The interrogation frequency is quite often 13.56 MHz (coinciding with the frequency that is used to power the tags. I cannot be sure if all tags use sub-carriers or just straight remodulation of the incident magnetic field.