https://en.wikipedia.org/wiki/Casio_F-91W#Usage_in_terrorism

Also, no F-91W thread would be complete without a mention of the sensorwatch project: https://www.sensorwatch.net/

That frankenstein'd chip... no way it can ever be assembled back into a "holder" card without risking everything breaking.

While this is really impressive, putting something in front of the faceplate breaks the great F-91W look and feel for me.

I don't know, every time I see someone paying with their watch it looks super awkward when they have to twist their arm so the watch gets close enough to the reader.

Most of the time they are holding their phone in their other hand which makes the whole thing extra silly and unnecessary.

I still think this is a pretty cool project.

> Therefore, an ideal antenna should consist of a 22.12 metre long wire, but by convention fractions of λ-lambda (λ/2, λ/4, λ/8, λ/16, etc.) are opportunely chosen.

This sentence is confused enough to be incorrect. λ/2 is generally preferred as an antenna length (standard dipole configuration) because it will resonate at the appropriate frequency with desirable standing wave characteristics (current maximum and zero voltage at input, voltage maximum and current minimum at ends). λ/4 can be used as a half-dipole, but it requires a ground plane to resonate properly. There are also arguments to be made for a 5λ/8 antenna, but none that I'm aware of for λ/8 or λ/16.

In practice for small antennas, physical length and electrical length are only tenuously related, so it's a matter of creating a circuit that acts like an antenna of the chosen length.

Forget modifying the watch itself. Instead, build a strap add-on to hold the NFC payment chip and antenna.

Here is a project where I did something similar to carry a microSD memory card.

https://www.thingiverse.com/thing:6784665

https://www.theguardian.com/world/2011/apr/25/guantanamo-fil...