I plan on terminating the coax with simple bullet connectors at the radio and below the mast
step so that it wouldn't even look like coax to anyone, and not be confused with the VHF antenna feed ever. I'd just be a thick round-profile cable. Also, I don't want to mess around sourcing and putting a coax connector on the tiny pigtail from the radio.
I was contemplating using regular cable up to the horn those last few feet just to simplify things but I think at least It should be shielded cable, or perhaps more coax. I have some used RG-68 from WestMarine that I might use for that last bit since the Belden 8267 is pretty stiff/inflexible stuff compared to that lighter-weight cheap
antenna wire. I certainly don't want to shift between 75 and 50-ohm coax as that would likely cause problems too with standing waves building up on the pickup.
I am a commercial
electrician although we don't do a lot of structured wiring
so my experience with PA systems is limited to pulling the wire, terminating the wire, and hooking up to equipment
that was all spec'ed out by someone else. That's the engineer's job for the most part to tell me what wire to source and pull. If they call for coax we pull coax of whatever type they specify. But B&G gives NADA information to me here in the installation
manual. Most of the hailer horns out there just have "bare wire" connections at the other end too. So the hookup up top will probably be butt splices under shrink tube.
But now I need to fall back on the theory I was taught in school
decades ago and my own experience hooking stuff up that didn't work
so well. Speaker wire typically doesn't need
to be shielded but this new "listen-back" feature seems to be so new that best practices are not yet quite firmed up here. It is really a mic cable when "listening" and mic cables
are always shielded, yet rarely are they single-conductors with a shield used as the return in my experience. I've wired a lot of large auditoriums and pulled many thousands of feet of audio cable in my career. I've never installed a microphone cable that wasn't shielded.