FT8 Power

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I have a general FT8 question. I’ve noticed that on some transmissions (not all) my power will drop just before the end of the transmission. For example, I have it set to 90W on some exchanges just before it’s done the transmit power drops to ~60W. Is this normal behavior?

I also tried cutting the power down to ~20W and have it drive my ACOM 1000 - just as a test. Seeing the same thing, though the numbers are obviously higher.

SWR is a constant 1.2:1 I mentioned this to someone, and they told me that it’s how FT8 behaves, something to do with the way it calculates signal levels. Just did not sound quite right to me.






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WA2SQQ

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Posted 2 months ago

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John - AI4FR

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That doesn't sound right to me either. I do not recall ever seeing a 30% power reduction like that. There will be some back and forth as it generates the tones, but going from 90 watts to 60 watts seems a bit extreme. So let's check some settings.

Are you transmitting near the edges without having fake it or rig on in wsjt? How wide is the transmitted signal settings in SSDR? I run my TX(under the phone tab) at 0 to 3kHz. That helps with the sidebands even though the other station will not see them. Lastly, what does your ALC meter show when you are transmitting? The little yellow marker should be at or near the red but not touching the red at all.

One other thing, I've seen DAX issues cause all sorts of problems including something similar to what you describe.
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Jim KJ7S

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Generally speaking the timing of the actual audio is such that the power will drop to near zero about 1 to two seconds before the system unkeys since the audio ceases, which is normal.
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WA2SQQ

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That’s interesting because that’s just about what I am seeing. I’m watching the power on the Acom’s meter, so this might be a slight lag of some sort. I’m going to hook up,the Bird and see what it says. Thanks
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John - AI4FR

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Give it a shot and good luck with it. The transmission length is around 12.64 seconds.
(Edited)
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WA2SQQ

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The 90-60W reference was w/o an amp.  Cut the power back to ~25W which produced ~300W. It stepped back to ~220W. Again, not on every transmission - seemed it was when the "RR73" was being sent. One other point - I ruled out a problem with the amp or radio. Had them keyed up for 1 minute at both power levels, with no fold back noted. It's very strange, but not causing any problems I can see. BTW, AGC max's out just short of the red area, top scale.
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John - AI4FR

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From your first post above I did not think that it was going to solve your problem but it was worth a try. After all, I am fairly confident that you can tell when the rig/amp/software is transmitting and when it is receiving. You have something else going on.

Ok, if you have checked the other settings I mentioned above and all is correct then this is odd. Unless you are near the band edges on some transmissions with out using the split option in wsjt as you could be cutting off half your signal or more. Hmm, I don't know what other software you are running but I have seen a ton of flex problems that turn out to be aftermarket software as the culprit. I also assume that you have the flex set to DIGU mode, right? On this end, I do not see such a massive drop in power towards the middle/end of a transmission. Sure it fluctuates some but not that much. You did state that the SWR was good, but please watch it. Maybe you are heating something up and the system is folding back some.

Let's get you up and running. Maybe Jim has a few suggestions that I have missed.
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John - AI4FR

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Something else just came to mind. There was an issue with the 64 bit version of wsjt and they recommend we use the 32 bit version even if you have a 64 bit PC. I forget now what the issues were. On this end I am running version 2.1.0.
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WA2SQQ

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Ah ha, and I just installed the 64 bit version .... wonder if I can reinstall it over the 32 bit version? More testing tonight .....
(Edited)
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John - AI4FR

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Good question and I have never tried. If I recall correctly, the wsjt instructions tell us to install over older versions. Then again, the 64bit version might not be the "power" culprit but it might cause you issues at a later date.