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SWR Question.

Discussion in 'General Technical Questions and Answers' started by KD8GFC, Jul 17, 2008.

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  1. KD8GFC

    KD8GFC Ham Member QRZ Page

    Can anyone tell me why when i check my SWR with 5 watts it's 1.2 and then when i check it with 200 watts it's 1.9. I thought the reading should be the same no matter what power level is being used. And yes i have changed nothing as fare as whats in line at the time of both tests. Thanks Jim !!
     
  2. W0EA

    W0EA Ham Member QRZ Page

    It depends on how accurate your meter is. It's likely that it is not calibrated at 5 watts. If you're trying to tune, most tuners suggest using 10-20 watts, which even at a bad match, most rigs can do just fine for (very) short periods. I know my FT890 tunes differently depending on how much power I apply sometimes, so I always tune with about 20 watts then crank it up to 100 and hit the tune button again, that way its usually close enough that it only takes a couple more seconds to get it good at 100w.
     
  3. KD8GFC

    KD8GFC Ham Member QRZ Page

    Ok should have mentioned Im not useing a tuner. Just radio into amp and from amp to swr watt meter and out to antenna.
     
  4. W0EA

    W0EA Ham Member QRZ Page

    All the same, probably just not calibrated, try using an outboard meter, it might be more accurate at both ends of the scale.
     
  5. WB2WIK

    WB2WIK Platinum Subscriber Platinum Subscriber QRZ Page

    ::Nobody could possibly answer this question without knowing exactly what meter you're using. What is it?
     
  6. WA4ILH

    WA4ILH Subscriber QRZ Page

    Your original assumption was entirely correct. SWR is not based on power. (unless your power is so high that something starts arcing)
    Tom WA4ILH
     
  7. KD8GFC

    KD8GFC Ham Member QRZ Page

    It's just an old dosy test center left over from the cb days. I know they are on the cheap side but the swr portion of the meter seems to work good just get the 2 different swr readings with the different output levels
     
  8. KI6NNO

    KI6NNO Ham Member QRZ Page

    The *current* state of affairs

    While the antenna (system) is resonant at a given frequency, even if power were micro-watts, there are other factors in play, one of which is I^2*R loss in the feedline.

    W2DU has an article on this:
    http://www.w2du.com/Chapter05.pdf

    As power increases, so does the voltage and current in the coax (I'm assuming that you're using coax here) and thus I^2*R losses increase as power is increased. While this seems like the reflected I^2*R losses would cancel out against the forward I^2*R losses, it simply doesn't happen that way.

    What occurs is that power reflected from the antenna is re-reflected at the transmitter output and becomes part of the forward power - which is therefore greater than the power actually being output by the transmitter power amp stage. Therefore, the forward I^2*R losses are somewhat greater than they otherwise would be (if there was not reflected power)*.

    Because increasing power increases the forward power loss more than the reflected loss, the ratio of forward vs. reflected power loss increases as power increases. Thus an SWR meter reads differently at higher power than at lower power (all other things being equal).



    *The increase in forward power causes an additional (small) increase in reflected power and so-on... The system quickly reaches an equilibrium state. Power doesn't "cascade" into a runaway condition.


    Hope this helps.

    73 de KI6NNO
     
    Last edited: Jul 17, 2008
  9. W4HAY

    W4HAY Ham Member QRZ Page

    Often the shielding within meters is rather haphazard, causing stray fields to couple into the rectifier/meter circuit. The non-linearity of the diodes exacerbates the problem.
     
  10. WB2WIK

    WB2WIK Platinum Subscriber Platinum Subscriber QRZ Page

    ::I have no idea what that is. But some of the old "CB" SWR bridges I've used cannot really handle 200W, they were never intended for that. They're designed to support 5-10W and above that the can be "overcoupled" (too closely coupled) and the detector diodes start generating harmonics, overheating and doing bad stuff.

    WB2WIK/6
     
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