Safe power output for FT8 on FT 991A

Discussion in 'General Technical Questions and Answers' started by 2E0RLG, Mar 27, 2019.

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  1. 2E0RLG

    2E0RLG Ham Member QRZ Page

    I am new to Digital Modes and have just started using ft8 from my ft-991a and was wondering what the max safe output power would be on HF ? i have been using 35w with ALC set low on 20/40m as i have a compromised antenna but would like to go up to 50w when trying to work a DX station or when propagation is fading. Is 50w likely to damage my radio as i am aware ft8 is heavy duty cycle altho only for 13 sec tx ?

    Thanks in advance Jack
  2. W2JKT

    W2JKT Ham Member QRZ Page

    Generally speaking, for high duty HF digital modes, you probably should to limit your time-average output power to whatever the rated AM output power is for your radio.
    WA9SVD, KU3X and 2E0RLG like this.
  3. G0JUR

    G0JUR Ham Member QRZ Page

    and the extra 15 watts wont make that much of a difference.
    Improving your antenna will .
    K8PG, KU3X, 2E0RLG and 1 other person like this.
  4. G0GSR

    G0GSR Ham Member QRZ Page

    Does the manufacturer consider 100W FM to be "safe"?

    50W on FT8 is not going to break anything, nor is 100W.
    Over a 15 minute period, the duty cycle of FT8 is still very low and well within the cooling capabilities of any radio.

    AG6UK, KQ0J, KQ8B and 1 other person like this.
  5. W2JKT

    W2JKT Ham Member QRZ Page

    FT8 uses MFSK8 modulation which is a 100% duty cycle mode. So, the overall duty cycle is governed by the on/off time. 13 seconds on and 17 seconds off comes to ~43%.

    So, as long as 100% for 13 seconds doesn't do any harm (and it shouldn't), full power output shouldn't hurt anything, as it would be equivalent to key-down at ~40W continuously, which is also about what the AM rating of most 100W radios comes to.

    I can't imagine a 991 would be incapable of full power output on FT8, but as G0JUR mentioned, improving the antenna will make a bigger difference.
    2E0RLG likes this.
  6. K6BRN

    K6BRN XML Subscriber QRZ Page


    I have two FT-991s (the original, not "A" versions), one of which I travel with. For travel, I take an EFHW-8010 or -4010 eirw antenna with me and string it up as needed, in a variety of non-optimal places. So I sometimes push the 991 to 90 watts out on FT8, but more often operate it at 50-75 watts. Never had a problem. Only reason I don't push the radio to full power (100W, more or less) is that I like to keep some headroom between operating and max power for signal integrity reasons.

    Also, the FT-991 works best on FT8 if set so that the audio input through the built-in USB sound card drives ALC to about halfway into the safe zone indication of the ALC meter. Even 3/4 is OK. The TX signal WILL NOT degrade at these settings on FT8. ALC level and power output will generally drop as the radio warms up, normally stabilizing within 15 min. of being on. So if you set the TX audio level to halfway into the safe ALC zone when the radio is first turned on, you will need to "turn it up" to stay at that level after a few minutes of operation. From that point on, it's quite stable. My two FTDX-3000s behave exactly the same way.

    The FT-991 is a great little radio and does the digital modes very well. And VHF/UHF. And AM (need to turn mic gain way up) Have fun!

    Best Regards,

    Brian - K6BRN
    2E0RLG and W2JKT like this.
  7. W2JKT

    W2JKT Ham Member QRZ Page

    Brian - our club has a 991A and a 3000 as well so I'll give your settings a try the next time we open the shack. Thanks for the pointers!
    2E0RLG likes this.
  8. 2E0RLG

    2E0RLG Ham Member QRZ Page

    Thanks everyone for the informative info its really appreciated ! my next step will be upgrading my antenna in the near future.
    W2JKT likes this.
  9. VK4DX

    VK4DX Ham Member QRZ Page

    Go the full 100w all the time. Limiting yourself to 50 or 70, 80 or whatever is just a nonsense. Your radio is built for this, we're not talking about the tube transceiver from 1970s with a bad SWR, we're talking about the solid state radio capable of running SSB at full power for as long as you want it.

    Another thing - ALC. You set your radio to go full power and set the power slider in WSJT-X to maximum value. Click on Tune in WSJT-X and reduce the Power slider in to the point where the ALC reads zero and your power output is still 100w. Yes, zero, not half , not quarter, not anything else. Zero.

    If at the point when your power reading on the radio starts dropping below 100W, but the ALC is still showing some value, leave the slider in the position that gets your radio output to 100w but reduce your sound card output on your PC until the ALC hits the zero.

    Happy FT8 DXing.
    N6BFG, N5AF and KC8QVO like this.
  10. K6BRN

    K6BRN XML Subscriber QRZ Page

    Hi Doug (VK4DX):

    It's not about finals failure but rather about signal quality. FT8 has evolved some envelope (amplitude) shaping between symbols, which means it benefits (has a cleaner, narrower bandwidth signal) from better TX amp linearity and suffers if the amp becomes non-linear.

    With zero backoff from maximum output, the FT-991 will generate a significantly wider, lower quality signal than running with just 10 or 20 watts (-1 dB) backoff. This is SOP for commercial transponders, even when running constant envelope signals like QPSK, for that reason.

    So, if you don't give a hoot about other operators and buy into poorer TX performance due to amplifier saturation - "ALL KNOBS TO THE RIGHT". YAY! CRY HAVOK AND LET LOOSE THE DOGS OF WAR!

    But if you DO care, back off just a little bit to 80 or even 90 watts and your signal should be cleaner. With the FT-991, you CAN run the (audio) ALC close to the top of the "SAFE" zone on FT8. Linearity suffers a little bit, but it's not hard clipping and I've looked at the spectrum - it's OK IMHO. Running at max output with hard clipping - not so good.

    Brian - K6BRN
    K8PG likes this.

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