12
$\begingroup$

Recently, I have begun experimenting with JT-65A, mostly on 20 meters. I usually run about 5 watts with that mode, since I know that JT-65 is a weak-signal mode. Is that still too much power? What is generally considered excessive power on JT-65?

$\endgroup$
7
$\begingroup$

5 watts is probably what most JT-65 purists will tell you to use, but in practice, I tend to aim for 20 watts. That gives you a good 6dB boost (every doubling of power is equal to +3dB) which is actually meaningful in JT-65. Any more than that, and you'll start finding certain problems. Note that the same is true for the WSPR family of protocols.

First, that since everyone else is using less power than you are, you'll have many cases of people hearing you, but you can't hear them.

That in turn means that you run the risk of interfering with a conversation that you can't even see, especially if you're running 100W and they're each at 5 or 20.

Finally, many transceivers will cause distortions in the JT-65 waveform when run at above a quarter to a half of their maximum power. Transceivers are meant to work with analog data, not the sharp frequency transitions of the digital signal. Overdriving them can cause clipping, which will cause you to actually waste power and may even make your transmission unreadable.

So to answer the question, the standard maximum is either 20 or 30 watts - and there's really no point in exceeding that. Many people win awards on the mode at 5 watts! Most transceivers will distort your waveform above a certain limit, and there's no point in risking it for another 2 or 3 dB.

$\endgroup$
  • $\begingroup$ Thank you. I didn't even think about clipping as a possible problem. $\endgroup$ – krzysz00 KF5SOQ Oct 30 '13 at 1:35
  • $\begingroup$ I would never use more than 5 watts for JT65 or any other weak signal mode. $\endgroup$ – Pedja YT9TP Jun 25 '17 at 20:54
  • $\begingroup$ @PedjaYT9TP If I wanted to work Australia on 160m, l might run 1000 watts. Having said that, I would place consideration for others above attempting that. $\endgroup$ – Mike Waters Jun 26 '17 at 0:19
  • $\begingroup$ If you use 1KW, you would erase everyone else on that part of the band. You may work Australia on 160 m with 100 w SSB (even less). Using 1 KW for weak signal mode is very strange idea. $\endgroup$ – Pedja YT9TP Jun 27 '17 at 7:11
  • $\begingroup$ @PedjaYT9TP Australia is on the other side of the planet from my QTH in Central USA. Unless band conditions are exceptional, it's rare to work Australia on 160m with 100w, even using CW (arguably also a weak signal mode :-). To that region, it almost always takes an amplifier and Beverage receiving antennas. And Topbanders -with narrow CW filters- regularly use amplifiers, without "erasing" anyone. Those are just facts, my friend. $\endgroup$ – Mike Waters Jul 1 '17 at 17:22
5
$\begingroup$

As you stated, JT-65 is a weak signal mode. This should not be confused with a low power mode. Depending on your antenna and location, the amount of power you need to conduct a successful contact may vary, just as in any other mode.

A good guideline would be to start at the lowest power setting you can, and if you're not able to work the stations you want (and can hear!), gradually increase power. If possible, use some of the online spotting networks (jt65.w6cqz.org or hamspots.net) to see what signal reports you are getting. If you are being received anywhere between -10 dB and -20 dB, you've nailed it! If you are consistently getting numbers between -10 dB and 0 dB, you can reduce power. If you aren't being heard or you get numbers below -20 dB, try increasing power.

I've worked stations using milliwatts who were booming in for me, and I've worked at least one station who claimed to be running 1 kilowatt. The kilowatt guy got a -10 dB report from me, so I guess he wasn't using too much power!

Good luck and 73!

$\endgroup$
  • $\begingroup$ This is a better answer than the accepted one. $\endgroup$ – Mike Waters Jun 26 '17 at 0:38
2
$\begingroup$

Re: "Finally, many transceivers will cause distortions in the JT-65 waveform when run at above a quarter to a half of their maximum power. Transceivers are meant to work with analog data, not the sharp frequency transitions of the digital signal."

K1JT has purposefully tailored the audio output of his software so it has none of the effects mentioned above. The "data" signal is an analogue signal with no such "sharp transitions". As only one tone is present at any time, there can be no intermodulation. In addition, since there is no AM component to the JT65 signal, the linearity of subsequent RF amplifiers is not critical. It is not necessary to de-rate output powers for the sake of linearity as is needed in PSK modes.

Re:"As you stated, JT-65 is a weak signal mode. This should not be confused with a low power mode."

Absolutely agreed!

There are no "low power" modes. Even WSPR will allow me to enter a power of 1kW. Not that I would ever do that but in this "low power mode", its not the low power that is important. It is the fact that the power level is known and can be taken into account when assessing results.

Frank, g0gsr

$\endgroup$
0
$\begingroup$

If you go with the US FCC license rule of 'minimum transmitter power necessary to carry out desired communication' then that should be a good rule of thumb. If you're making contacts with JT65 using using 5 watts then maybe that's 'just right'.

$\endgroup$

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.