Friday, December 17, 2010

A few issues about JT65-HF

A few issues about JT65-HF.

First and most important is that of time synchronization. You cannot successfully have a JT65 QSO without a close synchronization of your local computer's time value to UTC. Period. End of discussion. I use Dimension 4 for synchronization.

The built in time synchronization software with Windows XP (and beyond) is usually not sufficient to maintain a high degree of accuracy. Trusting those methods will likely leave you routinely out of temporal sync and make your enjoyment (and others as well) of JT65 much lessened.

If you do not have internet access your only option is to try to maintain a manual sync by some broadcast time service. If you can get (and maintain) accuracy within +/- 1 second of UTC you should be able to work other stations. The closer to perfect the better. Do not assume your PC's clock is stable. Many have highly inaccurate clocks that gain/lose time at a substantial rate. Considerations for transmitting JT65.

JT65 is indeed an FSK mode in that it transmits distinct single tones from a tone set of 65 values. JT65 is NOT TRANSMITTED using FSK. It uses AFSK and this is a critical distinction. Avoid ALC action if possible with an AFSK signal. Much like any other audio input to an SSB transmitter JT65 can generate a poor signal if over driven.

JT65 is a weak signal mode. Its primary purpose is to facilitate EME (Earth->Moon->Earth) communications. For use on HF you need to re-think your power levels. Often a few milliwatts will do amazing things. Some situations call for much higher power levels, but in day to day HF usage, 10 to 20 watts is probably enough, if not too much.

[Source: Joe – W4CQZ ex W6CQZ]


  1. I recommend NTP for Windows for time synchronization. I use NTP on all my computers and have done for years. You don't even know it's there, it doesn't show any icon, just install it and forget it.

  2. Hello Paul
    I'm with Julian, NTP sits in the background and works its magic, I was using dimension 4 BUT for some reason I was getting some issues with it under W7 - I have disabled the w7 time sync and all's well here - I noticed this morning some stations were starting their cycle way way out of sync - I like jt65, and I run 15/20w MAX and if the bands are in good cond'x I make DX.

    Rgds Peter

  3. Hello Julian and Peter, I use dimension 4 since I work with digital modes like WSPR and JT65. I works fine here, even with Win 7. Never change a running system. ;-) 73 Paul

  4. Never had any problems with D4. I think I have a look at JT65/HF again. Long time ago that I played with it. 73, Bas


Thank you for visiting my website and leaving a comment. 73 Paul

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