Tonight I was sitting at the station fiddling around with things (like I usually do), and thought I'd fire up WSJT and give JT65A a spin on 30m. For those of you unfamiliar with JT65A it is the ultimate weak signal mode. It was originally designed for EME and troposcatter operations, but also works very well on HF for long haul operations in less than ideal band conditions.
JT65A QSO's are unique in that each QSO takes exactly six minutes to complete. Being an EME mode, JT65A follows the EME convention of each station transmitting for just under a minute, then listening for a minute. The computer being used for this digital mode must have a system clock that is synced to an external reference time standard. My computer utilizes NTP (Network Time Protocol) to connect to one of a multitude of time servers available on the Internet which are all referenced to an atomic clock. Synchronizing the computer's system time is of critical importance because all stations working JT65A must transmit/listen in synchronized windows for a successful QSO.
The exchange in a JT65A QSO is in a very standardized format. This is due in part to the fact that each 1 minute transmission only transmits a maximum of 13 characters. This mode is not for ragchewing! It's purpose is to exchange the basic information required to complete a legitimate QSO reliably over extremely weak signal paths. The JT65A software can often decode signals below the noise floor.
A basic QSO begins with a standard CQ transmission.
1) Station A sends: CQ KE7KUS DM33
2) Station B sends: KE7KUS W1AW FN31
3) Station A sends: W1AW KE7KUS -15
4) Station B sends: KE7KUS W1AW R -12
5) Station A sends: W1AW KE7KUS RRR
6) Station B sends: KE7KUS W1AW 73
7) Station A sends: W1AW KE7KUS 73
At this point, the JT65A QSO is complete and KE7KUS begins transmitting CQ again. Let's dig into what's going on during this QSO a little deeper.
1) The CQ call is fairly explanatory. The sending station grid follows the callsign - remember...this mode was originally designed for EME.
2) The responding station also sends their grid following the station callsign. The astute observer will notice these exchanges are more than 13 characters (spaces count as a character). If calls fit the standard format above, JT65A software has a special algorithm which encodes/decodes more than 13 characters. If the transmitted message is non-standard in format, the 13 character limit is strictly applied.
3) This transmission is the first of the two exchanged signal reports. Unlike a traditional RST signal report, the JT65A signal report is given in dB. In this case, KE7KUS is receiving W1AW at -15dB.
4) W1AW responds with his signal report. R -12 mean "Roger, your signal is -12dB."
5) If the signal report is received successfully by KE7KUS, he responds RRR, which means "Roger, Roger, Roger." Successful receipt of transmissions is where many JT65A newcomers get confused. After the other station responds to your CQ call, the exchange should proceed exactly as outlined above. You may find times where a static crash or QSB takes out part of the signal you are receiving which may cause it not to be decoded properly. Any time an expected transmission does not decode properly, you should resend your previous transmission again. This lets the other station know that you did not receive his last transmission and indicates to him that he should resend. Example:
In the above exchange, say I did not receive the transmission KE7KUS W1AW R -12, for whatever reason. Since that was what I was expecting to get, if I don't receive it I would retransmit W1AW KE7KUS -15. W1AW would get this transmission and say to himself "Hmm...he already sent me that...it must mean he didn't get my last transmission...I will send it again." W1AW would then send me KE7KUS W1AW R -12 again and the QSO would proceed to the next step (assuming it decoded properly the second time around.)
6/7) The 73 exchange lets both stations know the QSO was successful. Instead of sending the standard 73 message, some stations use this transmission to send their transmit power and antenna type. In our example, such an exchange might look something like this:
Station B sends: 20W DP 73
Station A sends: 10W VERT 73
This would indicate that Station B is running 20W into a dipole and Station A is using 10W into a vertical antenna. If you choose to do this, you should be aware of two things: a) since the transmission will be nonstandard, the 13 character limit is strictly enforced, and b) that usually prevents you from sending your callsign to close the QSO (a Part 97 requirement). Some JT65A software programs get around this by quickly sending the station's callsign in CW after the JT65A part is sent. If you opt to send your power/antenna, please ensure this is set up correctly.
Much like PSK-31, JT65A QSO's by convention are usually limited to 50W. Even with that accepted convention, many stations seldom operate above 25W due to the efficiency of the mode. Unlike PSK-31, linearity of the transmitted signal is not as critical as with PSK-31; however, I seem to have the best results when running low/no ALC with JT65A.
Detailed instructions on how to operate the WSJT software, and more details on how to conduct a JT65A QSO are available in the WSJT operating manual: http://physics.princeton.edu/pulsar/...T_User_600.pdf
During my session on 30m tonight I made two contacts into the South Pacific from my home here in Phoenix. The first was ZL1ATB in Auckland. New Zealand (6763.4 miles), the second was VK3AMA near Melbourne, Australia (8200.2 miles). Both contacts were made running 10W into my S9 Antennas 43' vertical. The attached pictures show screenshots of both the WSJT software and a snapshot of PSKReporter, which shows the stations which are receiving my signal (each green bubble is a station that has received my 10W signal with the time the last signal was received inside the bubble.) As you can see, the ability of this mode to get out is quite remarkable!
You will find JT65A around the .076's for most bands (7.076, 14.076, and 21.076 MHz). On 17m, JT65'ers seems to hang out just above 18.100 and on 30m they seem to hang out just below 10.140. In general, if you've worked PSK in the past, JT65'ers won't be too far away from those frequencies. Just listen for the very distinctive, slow, musical tones of JT65A and you've hit the jackpot. Although JT65A isn't for everyone, if QRP DX or racking up DX entities with modest station/antenna equipment interests you, this mode might just be what you're looking for. Last year after several weeks of JT65A ops I decided to take a break and go back PSK-31 for a couple of days, only to find myself quite disappointed that I had to ratchet up the power to 50W and still had less success with DX QSO's. This mode will spoil you when it comes to getting out with low power or poor band conditions. Nothing else on the air quite compares. Have fun with JT65A and hope to see you on the air!