FT8 Power, how much ????

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

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  1. K6BRN

    K6BRN XML Subscriber QRZ Page

    Peter:

    I use a spectrum analyzer and an oscilloscope connected to an attenuating (-45 db) RF coupler at the in-house feeds to my external antennas. And I know how to use and interpret the data presented by these instruments because communications is my professional field. Most hams do not have these options nor background, so some simpler, slightly less accurate and more holistic approaches can be used, very effectively....

    The FT8 waterfall effectively IS a spectrum analyzer, with a running history and you should pay careful attention to it. It has many uses, one of which can be used to adjust your TX settings as listed below. Learn how to interpret it.

    1. Avoid running excessive power. If you have a 50% success rate on FT8 at 25 watts, that's actually pretty good. If your success rate is 100%, that means you ARE crushing a variety of other users as they try to make the contact also. So start low and ramp power up as needed. Its not about beating competing operators to death - its about co-existing in a community with shared resources. And what goes around usually comes around, in courtesy.

    2. Set the FT8 (JT-65, JT-9, etc.) audio input level to your rig so that ALC action is appropriate. This generally means minimal ALC indication on older rigs, but can mean 50% of the "safe zone" ALC indication on more modern rigs, like the FTDX-3000, -1200, etc. One way to know how your ALC behaves without instruments is to set TX audio level to drive ALC into its mid--range and then have a nearby ham friend monitor your signal width on his or her waterfall. Decrease TX audio/ALC indication and discover where the signal bandwidth shrinks to a minimum or abruptly transitions from a broad "bloom" to a normal waterfall indication. Chances are, TX audio/ALC levels should be held below this point for a good quality signal that will not annoy other users.

    3. Avoid using an amplifier attached to your rig for FT8. It WILL broaden the bandwidth of your signal, even at relatively low AMPLIFIED output levels like 120 or 200 watts. Many amps have poorer linearity if the input drive is too low (or too high). Any non-linearity broadens your signal. And since FT8 signals are VERY close together, interference increases very rapidly with just a few tens of Hz spread.

    4. If you feel you MUST run an amplifier to get that last elusive DX contact, try to do it on a relatively sparse band. For example, as evening appears, 20M users drift on down to 30M and 40M (80M, etc.) as 20M fades (actually, they usually go BEFORE the band really fades). At the middle to end of this exodus, you may be able to make some excellent grey-line contacts (even without the amp) on a relatively empty band (there will still be die-hards like me taking advantage of the realtive quiet of this period - including DX). As the band fades and you use more power to close the link, you will likely interfere with fewer other operators. But if you run 500 watts on 20M on a weekend during the day, it's pretty certain you will blot out most of the users within (at least) a 5 mile radius, and cause significant interference at longer distances. And be prepared for a lot of nasty feedback from crushed operators. So choose when you run amplifier power VERY carefully and do it infrequently, if at all.

    5. Rather than run an amplifier, put in an antenna with gain and a good front-to-back ratio (3-element Yagi's work very well). While it will be more work to turn to point at possible contacts, it will provide more "clean" EIRP (effective radiated power) with much less interference than an amplified signal, because it does not broaden the bandwidth of your signal and places the extra power derived from antenna gain where you need it most, rather than EVERYWHERE.

    In the end, you are trying to minimize interference to others while allowing them to also make contacts you may be competing for. Interference will STILL happen, bad comments may still be made, but you'll be making a best effort to be courteous and reasonable. And THAT will make a positive difference to the ham community.

    I'm sure others will have more suggestions and some criticism of mine. But these are some ideas for you to consider. Much of this may be elementary to you and a bit "preachy" (sorry about that), but I'm sure many others who are newer to the hobby may read this post and benefit from it.

    Best Regards,

    Brian - K6BRN
     
    K4HAS, KD2NOM and N4UP like this.
  2. N4UP

    N4UP Premium Subscriber QRZ Page

    Brian. Thank you. That was very helpful. I will heed your advice.
     
  3. KD2NOM

    KD2NOM XML Subscriber QRZ Page

    Brian,

    Excellent write-up - thank you.
     
  4. KJ5T

    KJ5T Premium Subscriber QRZ Page

    It is interesting, I see a lot of hits for my call on PSK reporter bust most are like -21db, but on FT-8 I haven't worked anyone yet (just started tonight). I have the EARC End-Fed on the balcony so not much of an antenna and running 5 watts. Mostly just doing this to torture myself. I have had two stations answer me but have been unable to complete a full contact.
     
    N2SUB likes this.
  5. G0GSR

    G0GSR Ham Member QRZ Page

    Brian.
    But aren't JT modes and FT8 designed so that amplifier linearity is of no consequence?
    As only one tone is present at any instant in time, there can be no intermodulation.

    Frank
     
  6. K5TRI

    K5TRI XML Subscriber QRZ Page

    So many experts here yet nobody cared to look up some facts.

    https://en.wikipedia.org/wiki/Duty_cycle (references at the bottom)
     
    N2SUB likes this.
  7. VA3VF

    VA3VF Guest

    All I can say is that using full power killed my IC-718 this past week. I should have known better, but trying harder for that ATNO overrode my good judgement.

    It's an old radio, it may have been a coincidence, YMMV.
     
  8. N4UP

    N4UP Premium Subscriber QRZ Page

    Closing in on 400 FT8 contacts this week ( with 4 FT8 contacts last week and none before that ).

    Today on 20m it is a zoo ( or wild west ? ). I choose to be gracious and assume that many are new at this, but it is a bit crazy. Not entirely sure how to interpret some of the behavior.

    Still fun, though. But when it gets really crazy I switch to another band. 65 countries on FT8 in one week and none are ATNOs.

    Power levels? With so much activity on 20m a lot of people are getting covered up. I am getting reports from euros everywhere from -10 to +05 ( running modest power ). For these conditions, not really much point in adjusting the power.
     
  9. G0JUR

    G0JUR Ham Member QRZ Page

    I have seen you on the waterfall several times this weekend good signal strength.
    20m ft8 is a bit of a wild west frontier type experience, and there are certainly some questionable operating habits developing.
     
    N4UP likes this.
  10. K5TRI

    K5TRI XML Subscriber QRZ Page

    The manufacturer of the radio should specify max power at certain duty cycles. For example Icom specifies in the manual that my IC-746pro is rated at 100W at 100% duty cycle. Now what they don't say
    is if I can run that for two hours straight, but it should shut off prior to failure.

    12.6 seconds shouldn't kill your PA though.
     

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