a transmitter that draws it's power off the detectors power supply(through the headphone socket maybe)I would not bother.
Having to deal with a battery or some sort of separate power supply to either end of a headphone's system SUCKS big time.
Especially if you forget to recharge or bring replacement batteries.
In my honest opinion and gained through my own frustrating and costly experiences, when it comes to using headphones with a metal detector, the concept of wireless is absolutely fantastic and not having a cord is sooo good, but to me it has also been a source of frustration, when things just stop working for whatever reason that could have been avoided by the ever reliable and simple headphone "plug and wire" type system
I 'd like to be able to use my headphone of choice as well so I've tried FM,IR,Bluetooth and UHF BOTH self contained headphone and transceiver units and they're ok........ BUT good quality hard wired headphones( that is plugged into or wired into the detectors audio, is, to me the most reliable and the best way to ensure a reduction in missing out on a find that you would have made if all was working closer one 100%.
Metal detector manufacturers need to incorporate transmitter in with detector electronics MAYBE.
Hard Nosed Dave