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FT8 Power, how much ????

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

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  1. VA3VF

    VA3VF Guest

    Agree, but in this case it's all my fault. I knew better, but did not follow my own procedures, so I deserve it.:(

    Not if you are in a 'dog and cat fight' for the rare one. It was not a single cycle at maximum power. The power supply fan switched on for the first time. I did not know it even had a fan.;)

    It may not be the PA itself, but the temperature control. The radio switches off when it goes into TX, then it switches on again after a few seconds.

    The radio is close to 20 years old, bought used. I got my money's worth.

    Again, coincidence, maybe.
  2. K6BRN

    K6BRN XML Subscriber QRZ Page


    Very good question.

    In an ideal world, with a single pure, fixed-amplitude tone with no side lobes, amplifier non-linearity would not seem to be a problem.

    But that's not where we actually live. So here are my thoughts...

    Imperfections in sound cards and the processing chain of the TX signal path result in TX tones that are neither pure nor constant amplitude. When combined with an additional RF amplifier stage the tiny tone space spreading that results grows larger. It's still very small, compared to the operating frequency. But because the tone space is already very narrow-band (47 Hz for FT-8), this small spreading can become significant, especially on a crowded band where tone space may already be overlapping.

    But that may not be the entire story. Comments are welcome.

    I've seen this spreading effect in experiments I've done on 20M with two amplifiers (one solid-state, one tube) at my own station, using a spectrum analyzer, directional coupler, attenuators and dummy load. It's been confirmed by verbal comments I've solicited on-line (thanks to the JT-Alert texting back channel) regarding waterfall appearance with/without an amp.

    In cases where I used an amp to increase power to the load (or antenna) from about 50 watts (transceiver only) to 150-180 watts (amplifier with VERY low drive), the Quadra showed significant spectral power spreading out to about 90-100 Hz. Spreading decreased with higher drive, up to about 500W out, where testing stopped. SB-200 results were all over the map, from really ugly to slightly better. SB-200 testing was much more difficult because consistent tuning/loading (my skill drove this) was difficult to achieve. And I only took this amp up to about 300W.

    Online reports via JTAlert confirmed that my signal widened slightly with modest (Quadra) amplifier use. And in trials with significant power (500W), there was major "blooming" of the RX waterfall that was not indicated in the dummy load/spectrum analyzer tests. I believe this was driven by RX overload rather than any real widening of the signal. RX overload is probably the chief cause of waterfall signal blooming seen by many users when an operator is too close or running too much power.

    BTW - these "tests" were never intended as laboratory condition tests with detailed setup logs and data sheets. I did them while "puttering around" (as my XYL calls it) to satisfy my own curiosity. From them , I concluded that at low drive levels an external amplifier can contribute to signal bandwidth spreading at very low drive levels and can overload RX stations processing at a significant distance when running high power (500W+). This seems consistent with my own experience when simply operating the JT and FT modes.

    Brian - K6BRN
    KZ4TN likes this.
  3. K5TRI

    K5TRI XML Subscriber QRZ Page

    ya live and learn :)

    Yeah I gave up on those fights. It ain't rare enough for me to spend two hours or more to make the contact. I guess I'm not a hardcore DXer anymore.
  4. VA3VF

    VA3VF Guest

    Good 'education' costs money.:D
    AA0RG and KA9JLM like this.
  5. W4LJ

    W4LJ Premium Subscriber QRZ Page

    Exactly; as I attempted to explain previously. Comparing the short FT8 "QSO" sequence to a Pulse Width Modulation (PWM) duty cycle was what I tried to convey. When considering FT8, the receive time must be considered for duty cycle to even make sense.
    The basic concept of duty cycle must include a time period. I didn't need to look it up. I dealt with PWM duty cycles every day as an electrical engineer. For FT8, duty cycle is really a very basic concept: "On" time versus"Off" time over a 1 minute period.
    Last edited: Jan 1, 2018
  6. W6UV

    W6UV Premium Subscriber QRZ Page

    So how is FT8 any different, say, than a CW pileup? Are all of those CW guys running 1500 watts to stacked yagis "crushing" the other guys unfairly? Should they throw away their amps and take down their yagis and replace them with a low dipole so they're not abusing a shared resource?

    The difference between 35w and 1500w is only 16dB--propagation phenomena can easily result in a 30 dB difference, so don't assume that everyone with a strong signal is running high power.

    DXing, contesting, and similar pursuits are always going to be competitive activities and complaining about guys who run more power and who have better antennas is just pissing in the wind.
    K5TCJ likes this.
  7. K6BRN

    K6BRN XML Subscriber QRZ Page

    Hi Gerard (W6UV):

    Wow! Pretty cynical. "Everything is a contest..." Maybe not. A lot of things are cooperative I win/you win situations. "Out of such idea are nations built."

    So here are a few thoughts... (stand back, you might get wet) :)

    The JT and FT modes were created with a great deal of order and mutual cooperation in mind, which is pretty obvious in their mostly rigid and automated QSO protocols. At the moment the informal JT/FT sub-bands are far more crowded than the CW sub-bands and the built-in JT/FT tools, like the waterfall display, make it S.O.P. for high density/picket fence side-by side stacking of signals just a few Hz (if any) apart. So cooperation is essential to avoid turning mutual enjoyment into mutual interference.

    Run 1500W on CW today and stations that are being interfered with can usually move up or down a few 100 to a few 1000 Hz to avoid it - or even use their own hearing and audio perception skills to distinguish between CW tones. With JT/FT modes, the entire sub-band IS a few KHz wide, and it can be wiped out due to signal overload in the sound card of stations relatively close to the operator running 1500W. Relatively close can mean many miles and encompass hundreds of operators.

    So comparing CW pileups to the JT/FT modes is an apples to oranges argument - the two modes are more different than alike, and they benefit from different operational approaches.

    The JT/FT modes are also a great equalizer - they can be enjoyed with minimal to moderate (1-100W) power and compromise antennas (magnetic loops, wire and whips), or complex antenna systems (real big Yagi arrays at 100ft.), too. So it enables those that are new to the hobby to make a lot of contacts, evolve their equipment and skills and move on to other operating modes as they wish while maintaining enthusiasm because the minimal equipment they start with.... works... even in a very poor sun spot/propagation cycle. Good for the Hobby. Good for us.

    Regarding contesting... FT8 is finally offering short QSOs that enable effective Field Day and contest use. I guess we'll just have to wait and see how that works out. But then, combative contests are pretty infrequent compared to day to day operation. Those who prefer more benign contests against themselves, like WAS, WAC, DXCC ...etc. may just turn off their radios on contest days, as many do today.

    Brian - K6BRN
    K1BQY and KZ4TN like this.
  8. K6BRN

    K6BRN XML Subscriber QRZ Page

    Oooops! Jerry... Sorry about that. I did not read DOWN far enough on your QRZ page. Nice family photo. hope you enjoyed the holidays.
  9. K2WH

    K2WH Premium Subscriber QRZ Page

    Yes it is called "weak signal mode".

    Meaning, you can copy very weak signals on RECEIVE.

    It doesn't mean you are supposed to use low power on XMIT to make your signal intentionally weak !

    In these very bad propagation conditions for the foreseeable future, I generally use at least 500 watts. If my amp could handle the long xmit time, I would use more.

    I don't want to be intentionally weak.

    Following that logic, then we should all go to microwatts or less until we just turn the rig off.
    N9ATV likes this.
  10. K6BRN

    K6BRN XML Subscriber QRZ Page

    Hi Bill (K2WH):

    It looks like you live in a state forest with relatively low population density compared to my CA QTH. In running FT8 at 500W continuously, do you receive complaints from other FT8 operators?

    Brian - K6BRN

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