This began as a microphone unit purchased via ebay, which was originally part of an intercom system. It included a dynamic microphone which may or may not have worked, but certainly didn't after I tried to open it up. No problem - electret inserts can be bought for less than £1.
I replaced the PCB with a veroboard PCB containing the microphone interface and a microcontroller. The microcontroller (Atmel ATTINY84) controls a latching action for the PTT pushbutton, and generates up and down tune pulses from a rotary encoder. The microphone interface is largely copied from the M0MTJ design and includes a low pass filter.
The completed unit works well. It had its first use at the Jamboree on the Air event for Clanfield scouts in October 2013.
To be on the safe side, I've kept the metal body and the gooseneck grounded.... it is after all an RF hot environment. That's what the green wires with crimp connectors are.