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Adjusting SWR with amp

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

  1. T-bone

    T-bone Member

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    Do you adjust the antenna SWR with just the radio and no amp in line or with the amp in line? I've done it without the amp inline but then when I hook the amp up the SWR's increase. With the amp inline are you getting an actual reading of the SWR?

    One other thing. I thought of using different radios in my truck. Would a correctly tuned antenna for one radio be the same for another radio (SWR) or would you have to keep a stock of antennas for different radios?



    Thanks
     

  2. Switch Kit

    Switch Kit Active Member

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    different power readings from different radios will change it very slightly (the SWR), if the antenna is grounded well the match should stay fairly the same with other radios of the basic same caliber . When you put your amp in line and check the SWR with it on high it should read slightly higher , no more then 1.5 and you should be OK. Lets say you have a 1.1 match and it goes up to 1.3 to 5 , I'd say you were OK , if it goes up past 2 , I'd say you had a problem , as I said , could be ground concerning the antenna ? or it could even be circuitry in the amplifier that has lost it's values in parts that help keep the SWR down in the first place. They have antenna analizers at shops that can be put on your system that help to ween out any problem you might have. IM sure others will have ideas for you on this. Good luck to you.
     
  3. W5LZ

    W5LZ Crotchety Old Bastard

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    SWR doesn't change with power levels. The accuracy of the meter reading the SWR can and does change with different power levels, that's why you calibrate them.
    If the SWR changes when the amplifier is in/out of line, when using a correctly calibrated meter, then it means that the output impedances of the amplifier and radio are not the same. Which one's right? Good question, beats me! Odds say that the radio's output impedance is 'right'er than the typical amplifier's. There are 'rules' about making radios, not about "CB" amplifiers.
    So, which should you adjust for? Depends on how much you use that amplifier. If you use it all the time, then make the proper adjustments. If the difference isn't all that much, and if you don't use the amplifier that much, leave it alone.
    Where you make those 'adjustments' means a lot. If the amplifier's output isn't at least close to the radio's (hopefully around 50 ohms), then the problem is in the amplifier. Fix the problem -where- the problem exists.
    All that assumes you're taking the SWR measurements in the proper place. If you aren't, then they don't mean anything anyway. Right?
    - 'Doc
     
  4. Chainsawgang

    Chainsawgang Active Member

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    Where is the "proper place" for those measurements ,WL5?
     
  5. dudmuck

    dudmuck Active Member

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    The true authoritative device to measure antenna performance is the antenna analyzer.

    One item to be concerned with when measuring antenna VSWR from a high-powered amplifier is harmonics. Most (if not all) CB antennas tend to reflect harmonics back to the source, which will show up has a higher VSWR. So its a good idea to use a low-pass filter on the output of your amplifier when you check antenna VSWR on the output of your amplifier.

    I can see amplifier output impedance as also effecting VSWR reading, but i see harmonics has having a bigger effect. This is why its good to have a dummy load, because it absorbs your signal harmonics and all, so you can make sure your VSWR meter is ok.

    A fancy word used in place of VSWR is "return loss". You can sound like a real RF nerd if you ask somebody what the input return loss is on their antenna.
     
  6. i didnt know harmonics affected SWR . now i really really really want a fatworldforcemade 2x4 amp :shock: :p :twisted: :LOL: :bounce
     
  7. lords

    lords Active Member

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    I wouldn't say harmonics so much affecting VSWR but it does affect the places around you and other channels...
    its more like a Feedback into the radio not matching the amp, from not being 1.5 or lower on a SWR meter ( linear not biased or capped right...or a bad or to small of a ground...or just bad or cheap coax...

    I had 2000 Watt carriers with mega harmonics on a lot of amps (splattering) and 1.5 on any SWR meter with or without a dummy load:)

    hell, years ago you just tune the radio to the antenna make sure the linears patch cord is up to par...and fire it up
    and this is years later its still working...so who's right or whats right? most won't never know will they unless you try...
    but being years into it and nothing blowing ...that is a better answer then nothing and its got to be better then wrong, right?
     
  8. W5LZ

    W5LZ Crotchety Old Bastard

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    In a typical radio+amplifier set up, after the amplifier and before the feed line to the antenna is the 'proper' place to measure SWR between radio and antenna system -IF- the amplifier is OFF. If the amplifier is ON, then you are measuring the SWR between the amplifier and the antenna system. That SWR meter measures the difference in impedances between what's in front of it and what's in back of it. If the amplifier's 'by-pass' circuitry is done right, turning the amplifier OFF is the same as taking it out of line. If there's more than a little bit of difference between the SWR when the amplifier's off and when it's out of line, then there's a problem in that amplifier's 'by-pass' circuitry. If that SWR difference is more than something like one or two SWR 'tenths', something's not right.
    Just 'cuz it's supposed to be right definitely doesn't mean that it is right. That goes for those 'by-pass' circuits and my advice. Try it and see.
    - 'Doc
     
    GTA61 likes this.
  9. T-bone

    T-bone Member

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    You mention antenna ground. I'm in the process of replacing my Wilson 5000 mag mount with a Monkey Made that will be mag mounted. The Monkey Made coax is hard wired at the antenna but has a PL-259 at the other end which I assume is grounded the the radio and or amp. Do you ground it another way or a better way? I do have the amp, radio, batteries and alternator grounded well with either 4 gauge ot 1/0. Or at least I think those items grounded well.
     
  10. T-bone

    T-bone Member

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    Thanks Doc! I'm unloading my 10 meter Galaxy which currently runs my 6 pill. I bought a Cobra 29 LTD and a Fat Boy 2 pill to use in it's place. So I thought since I went to the lower powered CB radio I would pretty much run the 2 pill continuosly. Then I was wondering if I did that if I should adjust the SWR with the 2 pill in line. And that Cobra/2 pill operation will now operate the 6 pill when needed.

    But I'm also interested in trying various radio/antenna setups. I'd like to get the new Galaxy coming out soon and I'm ordeing a 10K. My hopes were to be able to use both radios with both antennas. Or even 4 radios and 4 different antennas.
     
  11. T-bone

    T-bone Member

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    I've seen and heard the term harmonics before. What is it? Feedback, distortion or what? Would I hear it in my "talkback"?
    Thanks
     
  12. T-bone

    T-bone Member

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    Turned "off" or have the coax disconnected which is the same as no amp.
     
  13. W5LZ

    W5LZ Crotchety Old Bastard

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    Turned off.
    - 'Doc
     

  14. I've seen and heard the term harmonics before. What is it? Feedback, distortion or what? Would I hear it in my "talkback"?
    Thanks[/quote] http://www.dxing.com/radterms.htm
     
  15. Beetle

    Beetle Sr. Member

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    Harmonics - Integral multiples of the original (or "fundamental")frequency. The fundamental is also the first harmonic.
    Second harmonic of 27 MHz would be 2 x 27, or 54 MHz. Third harmonic would be 3 x 27, or 81 MHz.

    When a transmitter generates harmonics and these harmonics aren't filtered at the output, they represent a signal which an antenna tuned to the first harmonic will reject. Lots of harmonics = lots of reflected power. This explains many cases of "my antenna does just fine until I install a different radio, and then my 'swrs' shoot off the scale." It isn't the antenna, it's a dirty transmitter.

    Other problems: harmonics mixing with other RF signals in a poorly designed transmitter, producing not only the harmonics, but the sum and difference of the harmonics plus the other signals. This is called IMD, or Intermodulation Distortion.

    About the only reliable way to detect harmonics and IMD is with a spectrum analyzer. You might otherwise see and hear indications that something is wrong, but you won't know what or how bad until you take this step.
     
    Tallman likes this.

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