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Thread: Using The ImmersionRC Power Meter To Measure SWR.

  1. #1
    Navigator Channel 1's Avatar
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    Using The ImmersionRC Power Meter To Measure SWR.

    A common problem found when trouble shooting range problems with an RF link is a high SWR (standing wave ratio) on the transmit side, what this means is instead of the transmitters power being radiated out the antenna, it is reflected back to the transmitter where it is converted to heat commonly damaging the transmitter in the process.

    This is even more important when working with in-expensive transmitters that lack SWR output power fold-back protection.

    Now understanding SWR and being able to measure it are two different worlds, you can buy a SWR meter, but getting a decent one for frequencies over 900 MHz can be a tad bit pricey, however if you own an ImmersionRC power meter you have already purchased the more expensive part of the system, all you need to do is add is a couple of SMA jumpers and a couple of SMA RP adapters, and a directional coupler that is rated for the frequency range you wish to work with.

    The procedure to make the SWR measurement is as follows.

    Connect the transmitter to the input of the directional coupler IN port, connect the antenna to the OUT port, and with an attenuator in-line connect the power meter to the CPL port.

    Turn on the transmitter and note the reading on the power meter, if it is too low to be read power down and remove the attenuator and re-measure.


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    Note this reading.

    Reverse the transmitter with the antenna and repeat.


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    Note this reading then run the numbers here.

    http://ac6v.com/swrmeter.html

    In this case the SWR is 1.570365510734799:1


    What you are seeking is a SWR as close as possible to 1 to 1 (1:1) a little higher is forgivable, but with linear begin to suspect anything over 1.5 or higher and also expect a little higher with CP antennas.

    For this example I will swap the antenna for one I know is a hunk of crap, looks just like the good one, same e-bay seller just a lot lower in price, but total junk none the less.

    Foward measurement.


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    Reverse measurement.




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    As you can see the junk antenna is reflecting back to the transmitter almost as much power as it is receiving, this is not good and if left powered up wold probably smoke the transmitter.


    It's calculated SWR is 14.853374154642669:1, no antenna would probably work just as well.


    It is also a good idea to run this test when you first put your system into service so you have an as-built baseline to fall back on when trouble shooting a problem down the road.

    Now as to what is being displayed on the meter, the band and power readings are self explanatory, but from reading the other thread the dB reading has thrown a few people a curve, and I will try to explain how I got to that 41 dB number.

    My directional coupler outputs to the CPL port a signal that is 11 dB’s down from the power through the coupler, and I used a 30 dB pad, so 11 + 30 = 41 and by entering that number into the meter it calculated the loss ahead of it so the power reading it displays is correct. That saves a whole lot of head scratching trying to recalculate an indicated power reading when using an attenuator.

    BTW this test should work for most linear receive antennas as well.

    Any questions just ask.

    Wayne
    Last edited by Channel 1; 8th January 2014 at 06:16 PM.

  2. #2
    His Royal Sassiness ssassen's Avatar
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    Thanks Wayne, that's a great how-to, much appreciated!
    Purveyor of the FPV secret sauce at ImmersionRC

  3. #3
    ImmersionRC Support Guru swillhide's Avatar
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    What mw tX are you feeding the coupler with?
    AMA # 1015480 HAM Callsign # Procrastinating..
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    ObsessedFreak PeteDee's Avatar
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    Thanks Wayne, I see that the coupler you are using is only rated to 1.7Ghz, how do you think this impacts on the results for 2.4Ghz and 5.8Ghz?

    Cheers

    Pete
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  5. #5
    ImmersionRC Support Guru swillhide's Avatar
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    Not sure where you got the value of the coupler. The manufacture lists it up to 6 girlyhurtz but only down to 2.4

    http://www.minicircuits.com/pdfs/ZADC-10-63+.pdf
    AMA # 1015480 HAM Callsign # Procrastinating..
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    Retired: Disco(Sold), Skywalker 1900(Dirt Nap), Qav540g/Nazav2/Iosd mk2

  6. #6
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    Quote Originally Posted by swillhide View Post
    Not sure where you got the value of the coupler. The manufacture lists it up to 6 girlyhurtz but only down to 2.4

    http://www.minicircuits.com/pdfs/ZADC-10-63+.pdf
    Look under the specs, "Coupling Typical" which at 6GHz equals 10.7, which I rounded up to 11, as such the CPL port power will be 11 dB down from the mainline, add in a 30 dB attenuator and 11 plus 30 matches the display on the power meter.

    Wayne

  7. #7
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    Quote Originally Posted by swillhide View Post
    What mw tX are you feeding the coupler with?
    I think it is 29 dBm or about .8 watt.

    Wayne

  8. #8
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    The one I used go's up to 6GHz.

    http://www.minicircuits.com/pdfs/ZADC-10-63+.pdf

    Wayne

  9. #9
    ImmersionRC Support Guru swillhide's Avatar
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    I mean what brand and such?
    AMA # 1015480 HAM Callsign # Procrastinating..
    Futaba 9c w/Ezuhf /Skywalker 1880 or anything else that I can Lawn Dart.
    Retired: Disco(Sold), Skywalker 1900(Dirt Nap), Qav540g/Nazav2/Iosd mk2

  10. #10
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    Your welcome.

    Wayne

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