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Are there limits on what can be done in a wristwatch? I mean in terms of RF reception? Like, is it possible in such small sizes to receive any frequency, or is there certain limits, say based on heat, or necessary component footprint or other constraints?

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I think the primary constraint would be the antenna. Generally speaking, the longer the wavelength, the longer the antenna needs to be to efficiently receive it (or transmit it). You can still receive/transmit with a too-short antenna, but efficiency drops off very quickly.

Thanks! This makes me wonder if it might be possible to build a wristwatch SDR, essentially the same as this project but with a very wide band into the Ghz, as with some small USB SDR's. It'd be so cool to have something so versatile in Casio form-factor.

This watch uses a wire in the wristband. Can the antenna length be increased by simply using multiple coils?

Antenna design is wildly complicated, but once you start coiling you get more problems as it becomes more inductive and you get weird phasing. I think the short answer is no, but this is out of my element (unintentional antenna pun)

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