Rob Gill wrote:
...Historically, developers have used their own preferences to determine key parameters such as operating frequency and mode with the result that many systems are incompatible.
I suggest that we shape the next generation of cave radios by producing a set of recommendations describing preferred modes, frequencies, beacon centre frequencies etc., to bring about compatibility of future radio systems to support inter-operability for rescues, expeditions and general use....
Whilst it's a bit of a pain, 87kHz seems to have been a fairly standard frequency for a good few decades. It was the choice for molephone, and for backwards compatibility was adopted by HeyPhone. Nicola1, Nicola2 and Nicola3 are slightly off 87k, but are close enough to allow compatible operation with only a very slight shift in pitch. A search online shows a few other homemade radios based around the HeyPhone design have adopted frequencies that are basically compatible with 87kHz as well. In a similar way it seems upper side band has become the default standard, even though in practice testing suggests lower sideband offers slightly better Loran-C rejection. I think there's at least one rescue team which skipped HeyPhone and still have Molephone for voice comms. If the aim is to settle on a standard frequency and mode for interoperability, then for backwards compatibility with a lot of radios already in existence, I'd suggest 87kHz USB needs to at least be a fall-back option, even if something else is chosen as a preferred channel when compatibility with others isn't required.
Ian.