Made a couple of single-hop Es QSOs on 6m tonight. Can confirm
the calling frequency is 50.318 MHz. It's startling how quickly
the QSOs happen -- only takes about 20 seconds!
Band was wide open, but the vast majority of activity remained on
50.313 FT8. For this reason, I doubt FT4 will get much use in the
CQ VHF contest this weekend, particularly if propagation is poor.
Unlike FT8 and JT65, FT4 is asynchronous or quasi-synchronous, ie. similar to RTTY, PSK31, and Olivia. Just click your mouse and call. Like the other WSJT modes with forward error correction, FT4 still requires very tight station synchronization. This is accomplished by embedding each message with four, 4-tone Costas Arrays, each with a unique 4-tone sequence. This allows the decoder to recognize and align signals with as much as 10% TX/RX time sequence offset. This is not as forgiving as FT8 and appears to be one of the tradeoffs that was made for speed. About 15% of each 5-second message is used for synch (I'm still working through the source code).
Interested in the initial experiences of others.