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Adjusting SWR with amp

Discussion in 'CB Antennas' started by T-bone, Nov 28, 2007.

  1. stevejrfan

    stevejrfan Member

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    It a d&a pdx400 ive tryed everything i know to do even what oldtech03 said seems like its all working fine fan works tubes glow bright relays all work when keyed only for a very quick key and release could be something internal i guess


     

  2. stevejrfan

    stevejrfan Member

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    Ok guys heres my problem my swr jumped up out the roof when i hook up my amp no power on same thing with it on out the roof i changed jumpers still the same i can take amp out of line just radio and meter under 1.5 ive tryed just amp and radio no meter swr out the roof new antenna and new coax plus 2 more new jumpers 6 ft from the 3 ft cant figure out why my swrs are high only when i put amp in line took amp to a freind of mine hooked amp up to his set up and amp works fine ive tryed everything i know other than moving it to another room any ideas thanks
     
  3. GrandNational

    GrandNational New Member

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    Technically it does, with an amp you’re magnifying a mismatch that may not be seen with low power scenarios. Even with an analyzer you can only go so far then you tune from there to get it closer.
     
  4. 338_MtRushmore

    338_MtRushmore Sr. Member

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    Yes, it can change if your amp is producing trash that the antenna can't get rid of.
     
    Tallman and The DB like this.
  5. Tallman

    Tallman W9WDX Amateur Radio Member, KW4YJ EXTRA class

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    Nope! The antenna tuning does not change with power levels. If you have the antenna tuned to the primary frequency it is good from 1 milliwatt to 10 Kilowatts.
    The only reason it changes is because of dirty drive signals ie: a dirty cb radio that has been hacked and tuned up for ghost watts.
    When the Harmonic Suppression filters are damaged in the "Tuning" process the dirty radio puts out a trashy signal and your problems begin.
     
    #80 Tallman, Mar 1, 2019
    Last edited: Mar 1, 2019
    Shadetree Mechanic likes this.
  6. GrandNational

    GrandNational New Member

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    So you’re saying if I take a clean radio and clean amp and use only a carrier and no audio for testing purposes that the swr will not change?
     
  7. GrandNational

    GrandNational New Member

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    So let’s use a mobile setup for instance. So when I setup a pair of antennas and I’m tuning the hot with an analyzer and do a test key and I have say 10w reflect with carrier only and I go back and tune for lowest reflect and I get around 2w are you saying that I tuned the antenna for trash instead of getting a better match? Sure some people turn every screw and knock traps out of whack and yes an amp will reproduce what’s been stuck in the input but with the right mod levels you can get pretty clean signals no matter the amp.
     
  8. 338_MtRushmore

    338_MtRushmore Sr. Member

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    Is this a mobile setup, or a keydown setup?
     
  9. Tallman

    Tallman W9WDX Amateur Radio Member, KW4YJ EXTRA class

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    That is correct.
     
  10. Tallman

    Tallman W9WDX Amateur Radio Member, KW4YJ EXTRA class

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    You should have less reflected power than ten watts. If your system is clean and you put a 100 watts in you should have less than 1 watt reflected.
    If your radio has been "Tuned" you are throwing a bunch of harmonics at the antenna and your SWR will fluctuate with power levels. The antenna will reflect all of the harmonics back regardless of the power levels.

    I always recommend tuning with the amp out of the circuit altogether. I use just the antenna analyzer.
    [​IMG]

    http://www.mfjenterprises.com/Product.php?productid=MFJ-259C
     
    #85 Tallman, Mar 1, 2019
    Last edited: Mar 1, 2019
    Redbeard U812 likes this.
  11. Dmans

    Dmans Sr. Member

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    stevejrfan,

    The above statement sounds to me like an internal (amplifier) connection problem.

    The next statement (above) sounds like an internal (amplifier) connection that got moved, jostled or pure dumb luck with moving the unit.

    A very careful inspection of all solder joints from the input SO-239 of the amplifier to the output SO-239 is in order. Don't forget relay contact points and the Hi/Low switch or any connection with coax cable attached.

    Good Luck!

    73's
    David
     
  12. 338_MtRushmore

    338_MtRushmore Sr. Member

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    I could see this being reasonable. Matching the antenna with an analyser for 1.1, then dead keying "tuned" cobra 29 into a davemade at 500 watts for a higher vswr.

    Without details, it is hard to say
     
  13. OLJEFF

    OLJEFF Member

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    Mine is three or higher. The match with the imput impedence of the amplifier must be off. How is that adjusted?
     
  14. OLJEFF

    OLJEFF Member

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    Oh. Come on. Is there is an adjustment to match the impedence of the radio to the amplifier. Surely all should be considered and the match should be optimal. The match between the amplifier and the antenna should also be optimal. Is there an answer and not a rationalization? Truely we are looking for a knowledgable response. Or is that too much to expect from this forum?
     
  15. Shadetree Mechanic

    Shadetree Mechanic 808 On The North Side of Dover

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    Yes
     
    2NC995 likes this.

: vswr

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