In looking over the characteristics of the new weak
signal FT8 digital mode, I noticed it has a sustained TX
time of only 15 seconds (versus nearly 50 seconds for JT9
and JT65). Presumably this is followed by a minimum 15
second RX period... probably a bit longer.
Wouldn't this new FT8 digital mode be a lot easier on the
IRF510 final amplifier found in our KN-7QA and CS-series
transceivers compared to JT9 and JT65? It should cause a
lot less heat buildup right?
The following excerpt is from the version 1.8.0 release
candidate WJST-X software:
Brief Description of the FT8 Protocol
WSJT-X Version 1.8.0 includes a new mode called FT8, developed by K9AN
and K1JT. The mode name "FT8" stands for "Franke and Taylor, 8-FSK
modulation". FT8 uses 15-second T/R sequences, provides 50% or
better decoding probability down to -20 dB on an AWGN channel, and
maintains good performance on Doppler-spread fading channels. An
auto-sequencing facility includes an option to respond automatically
to the first decoded reply to your CQ. FT8 QSOs are 4 times faster
than those made with JT65 or JT9. FT8 is an excellent mode for HF
DXing and for situations like multi-hop E_s on 6 meters, where deep
QSB may make fast and reliable completion of QSOs desirable.
Some important characteristics of FT8:
- T/R sequence length: 15 s
- Message length: 75 bits + 12-bit CRC
- FEC code: LDPC(174,87)
- Modulation: 8-FSK, tone spacing 6.25 Hz
- Constant-envelope waveform
- Occupied bandwidth: 50 Hz
- Synchronization: 7x7 Costas arrays at start, middle, and end
- Transmission duration: 79*1920/12000 = 12.64 s
- Decoding threshold: -20 dB; several dB lower with AP decoding
- Multi-decoder finds and decodes all FT8 signals in passband
- Optional auto-sequencing and auto-reply to a CQ response
- Operational behavior similar to JT9, JT65
We plan to implement signal subtraction, two-pass decoding, and use of
a priori (AP) information in the decoder. These features are not yet
activated in v1.8.0.
We haven't yet finalized what the three extra bits in the message
payload will be used for. Suggestions are welcome!
-- Joe, K1JT, for the WSJT Development Team