FT8 Power, how much ????

Discussion in 'Working Different Modes' started by K4EM, Nov 12, 2017.

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  1. WA6MHZ

    WA6MHZ Subscriber QRZ Page

    OOPS duplicate post
     
  2. VA3VF

    VA3VF Guest

    Use as much power as needed for the QSO, up to the maximum allowed. Just make sure it's clean.

    As for split: click on the waterfall to set the receive frequency (green bracket). Shift click on the waterfall to set the transmit frequency (red bracket). Control click to set both to the same frequency.

    Don't forget to check (enable) the Auto Seq and Call 1st.

    Have fun and good luck.
     
    W4RAV likes this.
  3. N4NYK

    N4NYK Ham Member QRZ Page

    If you consider the transmit duty cycle of the FT8 mode over a period of 1 minute, which is the full length of a normal contact, the duty cycle is less than 50%. Transmit time is actually 12.6 seconds not 15 seconds. The term duty cycle has to have a defined period. Otherwise it makes no sense. If the transmission time and the period are equal, it's a 100% duty cycle. Of course, if the continous transmit time is long enough, the transmitter temperature becomes excessive and duty cycle becomes irrelevant; not the case with FT8. (Yeah, there is no dash).
    By what you are saying, morse code is a 100% duty cycle because it is full transmit output.
    However, if 1 minute is defined as the period, it's duty cycle is much less than 100%.
     
  4. W6UV

    W6UV Premium Subscriber QRZ Page

    By what you're saying RTTY is not a 100% duty cycle because the ops alternate sending and receiving.

    What I'm saying when I say that FT8 is 100% duty cycle is that during the 12.6 second transmit period, the transmitter is on 100% of that time.
     
  5. N4NYK

    N4NYK Ham Member QRZ Page

    For that 12.6 second period, if you defined the period as 12.6 seconds, yes. You could define the period as 1 microsecond with a 1 microsecond transmit time and it would be 100% too. However, over a normal FT8 period, it's less than a 50% duty cycle.
    RTTY with the same 12.6 second transmit 15 second receive would have the same duty cycle as an FT8 contact. Of course a typical RTTY duty cycle is nothing like FT8 and the duty cycle and can vary widely over some randomly defined time period.
    What I am getting at is that a 100% duty cycle is not an accurate description of the FT8 mode because the receive time must be included in the period considered.
     
  6. W6UV

    W6UV Premium Subscriber QRZ Page

    The JT and FT8 modes are unique in that they have a fixed transmit/receive cycle baked right into the mode. No other modes have this, and as a result, the way you're defining duty cycle is not the way duty cycle is defined by anyone else in the radio biz.

    I stand by my statement that FT8 is 100% duty cycle by the conventional definition of duty cycle.
     
  7. N4NYK

    N4NYK Ham Member QRZ Page

    OK. You define the period as 12.6 seconds (and ignore the receive time) and I'll define a 1 minute period which is the standard FT8 contact period. Cool :)
     
  8. G0GSR

    G0GSR Ham Member QRZ Page

    You need to measure duty cycle with reference to the thermal inertia of the equipment and its cooling system. This is what really matters. How quickly can the system remove heat.

    I'm sure that in most amateur equipment, 12.6 seconds is far too short short compared to this and any duty cycle measurement over such a short period is meaningless.
    Frank.
     
  9. W0AEW

    W0AEW Ham Member QRZ Page

    A duty cycle can be defined as that period of time between signal initiation and PA destruction.

    If an operator modulates an SSB signal with his voice and another operator modulates his with FT8 tones, which of the two trains will reach the station first? Or will it make a sound in the forest?
     
    AF4RK likes this.
  10. K6BRN

    K6BRN XML Subscriber QRZ Page

    I really have to chuckle about the "weak signal mode" branding of JT9, JT65 and FT8, even by its creators. These modes are designed for improved performance in RECEIVING weak signals, which has little to do with how much power (ERP or TX output) is transmitted.

    FCC rules simply state that U.S. hams are allowed to use whatever power is require to complete the QSO, up to the limit for that band and class of license. "Weak signal mode" or not. And with the highly variable band conditions and noise we've had for some time now, that is often more than 25 watts. As always, some courtesy, common sense and patience needs to be exercised for all of us to get along on the bands, especially in the FT8 segment (which is defined by convention rather than rule) because this mode has grown so explosively since its very recent inception.

    For example - a gentlemen on FT8/80M last night at a distance of ~250 miles was hitting my S-meter at 35 db over S9. Hmmm. Then he turned the power UP and hit me with 45 db over S9. You can imaging what my FT9 waterfall display looked like. His signal appeared about 5x the normal FT8 signal width. And I had already thrown on a high-Q preselector, 20 db of attenuation plus a notch filter to try and suppress his interference. I'd guess he was running at least 500W and bumped it up to 1000W+, as many SSB 80M operators do in the evening.

    That same night I was "free-message" cursed on 40M as a LID for not acknowledging a QSO with "73". I had sent "73" about 9-10 times with little luck, and even bumped TX power to 85W to try and close the QSO, but no dice. The other OP clearly had some interference on his end, or something like it. I could hear him just fine, at the end. He just could NOT hear me. So he blew up. Maybe he should try knitting as a hobby. Conditions are less variable.

    And then there is the new WSJT-X "feature" that unlocks TX and RX frequencies by default. What this little gem has caused is a huge rash of contacts where the operator calling CQ has their frequency taken over completely by the responder after QSO completion, with the responder not realizing that even though they clicked on a new frequency to start their own "CQ", unless they held down the "Control" key while switching frequency, they are still transmitting on the old QSO frequency. BRAIN FART! So I just have patience and move frequency myself. We all just need to get along, after all.

    Regarding the tolerance of new HF rigs to high power at 50/50 duty cycles - they are remarkably robust. 25, 50, 100 watts - all seem to work fine on the FTDX-3000, FTDX-1200, FT-991, IC-7300, etc. I've seen many other OPs run this all day, 60 sec on ./ 60 sec off without any problems. I've no doubt that SOME rigs will NOT survive this. And driving 100W into a poorly matched load through a built-in tuner in a modern HF rig WILL dramatically increase thermal dissipation (and can trigger a failure). So there is no hard and fast rule on this. "You takes your chances and pays the price." Personally, I start with lower power (20-25W into a wire) and work my way up as needed. If I cant close the contact at 85W, then I switch bands and try again, or just go get a cup of coffee and do other things until the band improves.

    Soooo... hope you are all enjoying this great hobby. Aren't we lucky that we can?

    73 DE Brian - K6BRN
     

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