CHIRP

Some radios, for example the Yaesu FT-65R, allow the user to configure an duplex offset that would cause the transmit frequency to be out of band (and thus rendering the radio unable to transmit). This configuration can be easily achieved through FPP (see attachment).

If an image is downloaded from the radio, Chirp tolerates this. As soon as those rows are copied/changed, we get validation errors such as "Tx freq X is out of supported range". What I'm proposing is a small feature that will enable a user to configure a memory like this, even though the radio has no capability of transmitting on this frequency.

While it may sound like this is a strange or even irresponsible proposal, I think it has merit for two reasons:
1) It feels odd to be able to configure a radio from the front panel that is not allowed by Chirp. This change would improve Chirp's parity with the FPP experience, and only apply to radios that allow such offsets.
2) This configuration is currently the only way to configure a memory channel as "receive only" on some radios. Because this change wouldn't expand the radio's ability to transmit off-band, this seems like a reasonable option.

The ability to configure "receive only" channels can be important in scenarios where users are configuring banks of channels they want inherently locked from transmit without having to remember (if it even exists as an option) to set a PTT lock. This is even more true for radios like the Yaesu FT-65R because many users are tempted to unlock them (MARS/CAP mod) because no hardware modifications are required. In such cases, users may have local scanner frequencies programmed which the unlocked radio would enable transmit on, unless a feature like this enabled such users to effectively disable transmit.

Now, I would prefer to find a radio-agnostic way to mark a memory channel as "receive only" - much like setting duplex to "off" on Baofeng radios - but I figured a good starting place would be to simply enable Chirp to be able to upload a valid (if a bit nonsensical) memory configuration to a radio if the radio supports and stores offsets in this way.

The only consideration I'm torn on is if this should be a valid behavior by default, or if there should be a toggle in a menu to allow offsets that exceed band limits.

I will creating a PR shortly (presumably https://github.com/kk7ds/chirp/pull/998 ) with the most basic form of this feature, but any and all feedback on the feature idea or implementation is most welcome.