[PCB] Design a UHF variant

[VanceVagell]

Design a UHF (e.g. 70cm) PCB for use with a DRA818U chip, the main challenge will be figuring out how to filter spurious emissions (need to see if components with values that low are available and/or if it's even necessary). Part 97 doesn't stricly require filtering at UHF, but I want to make sure it doesn't do something truly egregious with harmonics.

Note that once this enhancement is complete, the Android app AND ESP32 app need to be updated as well. The ESP32 app needs to report (probably in response to GET_VERSION) that it's the UHF or VHF version, so the Android app can use the right frequency ranges.

[nigle]

It might be worth the response including the transmit power options to cater for other RF modules later, such as the SA858 - although I doubt that it can be powered from the phone!

Grimkell

[VegaDeftwing]

The ESP32 app needs to report (probably in response to GET_VERSION)

This is probably worth making a separate issue and doing sooner rather than later...

[SmittyHalibut]

The only change needed here will be parts selection on the LPF between the radio and the antenna. Everything else can be the same.

Though, I will point out that I had a VERY hard time getting the UHF modules to transmit cleanly through a filter. Mine was a BPF (for $reasons), but even when I tried just a LPF, it still had problems.

I'll design up a UHF LPF (instead of the BPF I had used on SOAR) so we can try it out, see if its any better now.

[SmittyHalibut]

When selecting a different BOM for UHF, replace C3 (the tuned bypass cap on v3.3) with this for UHF:

When you get to making a UHF, Murata GCM1885C2A221JA16D (JLCPCB: C388907) 0.1 ohms at 436MHz