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High SWR after amp install

Discussion in 'General CB Services Discussion' started by Dx717, Dec 1, 2018.

  1. Dx717

    Dx717 New Member

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    Hello all, I hope I’ve posted this question in the correct area.
    I’m running a Stryker 447hpc2 set at 1 watt dead key, Stryker SR-a10 single antenna thru rg8X coax. My SWR was at 1.2.
    A few weeks ago I added a new 2 pill amp with 9’ of 8X coax jumper. With the amp connected but OFF (meter between radio and amp) I key mike and still show 1.2 SWR.
    I turn amp ON and key mike it will jump to 3.8 SWR. So I’ve never used the amp since I installed it. All power leads are connected directly to battery with additional ground straps on each component chassis
    My question is I’m not sure what to do or try to lower the SWR so I can use this setup?


     

  2. TheRealPorkchop

    TheRealPorkchop Certified Sith Pimp

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    Meter between the radio and amp, you’re reading the input tune of the amp with it powered on. Amp off and the relay is bypassing the amp so to speak.
     
    rabbiporkchop likes this.
  3. 543_Dallas

    543_Dallas Sr. Member

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    Yep, youre looking at the input match. It should be low. A quick and dirty fix for it can be to find a different length cowaxe jumper that makes it play nice. Without some tests there is no easy answer as to why this is happening.

    I'm curious what the swr between the amp and antenna is. You could have a bad input match or parasitic oscillation. A scope is the best tool to see oscillation but If you have an average and pep reading watt meter that will help. Put the meter between the amp and antenna. If the swr goes through the roof with the amp on (don't forget to recalibrate)try this. Turn the mic gain down and measure your AM carrier power on pep and then average. They should be equal but if there not the amp is oscillating.

    Different coax lengths can also be a work around for an amp with an oscillation problem. 6 or 12 feet of rg8x seem to work more often than not.
     
    Shadetree Mechanic likes this.
  4. Tallman

    Tallman W9WDX Amateur Radio Member, KW4YJ EXTRA class

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    What type of amp? Some have a compensation network for input tuning.
     
  5. dave457

    dave457 Well-Known Member

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    Probably the amp is not tuned correctly
     
    sp5it likes this.
  6. 338_MtRushmore

    338_MtRushmore Well-Known Member

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    Probably not your problem, but excessive voltage can do that too.
     
  7. nomadradio

    nomadradio Analog Retentive

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    IIRC that radio is capable of around 70 Watt peaks.

    A 2-transistor amplifier will probably be overwhelmed with that drive level.

    Sounds like you need a bigger amplifier or a smaller radio.

    73
     
    rabbiporkchop likes this.
  8. midnight special

    midnight special Well-Known Member

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    harmonics gives false readings,,,,,,,,
     
    rabbiporkchop and 357magnum like this.
  9. sp5it

    sp5it Master of puppets

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    Poorly designed/manufactured amp.
    Mike
     
    midnight special likes this.
  10. Shadetree Mechanic

    Shadetree Mechanic 808 On The North Side of Dover

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    The input might be tuned for 10 meters. This is the only legal way to make it. Some modifications to the amp input will be needed. As mentioned earlier, a longer piece of coax coiled up between the radio and amp might help.
     
  11. SHACKMASTER SAM

    SHACKMASTER SAM New Member

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    The amp has an input impedance problem or is oscillating. Using various lengths of coax is simply using coax as a conjugate matcher but does not solve the root problem. You need to find the root cause of the problem. But unless you are good at working on amps (more than turning screws or pots), you need to look for a good tech. I have had amps that had bad input matches and I used a cheap matchbox between the amp and radio to temporarily solve the problem. But that should only be used as a temporary fix, not a permanent solution. You could be doing damage to either the radio or amp running them in a mis-matched condition. Good luck!
     
  12. TheRealPorkchop

    TheRealPorkchop Certified Sith Pimp

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    Shack Master - I like that name.
     
    Rwb likes this.
  13. Rwb

    Rwb Well-Known Member

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    Lot better name ,shack master than mr golden screwdriver
     
    357magnum likes this.

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