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How do you use an SWR tester ?

Discussion in 'Communication (CB | GPS | HAM)' started by danielzink, Aug 2, 2003.

  1. danielzink

    danielzink Registered Member

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    Hey All,

    I got an SWR tester from work today. It's an old "Micronta" field strength/swr tester #21-525b.
    There's a VU meter on the front, a cal dial and a small toggle switch that says ref and fwd. On the top is a tiny metal port that says fs ant and on the back are two connectors that say ant and trans.

    Any ideas on how to use this to calibrate my antenna ?

    Thanks, Dan

    87 Burban
     
  2. rebelk5frk

    rebelk5frk 1/2 ton status

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    Use a short coax cable to hook it to your radio using the "trans" connector. Then hook your antenna coax to the "ant" connector.

    Most people tune their antenna for the middle of the band so it works about as well through the whole band. Here's how you do that.

    1) Set the radio to ch 1.
    2) Select "FWD" on the swr meter.
    3) Transmit and use the "cal" knob to adjust the meter to line up with the "cal" mark on the panel.
    4) Select "REF" on the meter and make note of the reading on the meter's scale.
    5) Stop transmitting.
    6) Set the radio to ch 40.
    7) repeat steps 2-6.

    If your SWR is less than 1.5:1 on both reading then no tuning is really needed. The lower the reading the better in most cases.

    If one reading is significantly higher than the other then you need to tune the antenna.

    If the ch 1 reading is higher than the ch 40 reading then you need to lengthen the antenna.

    If the ch 1 reading is lower than the ch 40 reading then you need to shorten the antenna.

    Normally you should only adjust the antenna length about 1/4 inch at a time, then repeat steps 1-7.

    For best results you should take these readings with all the doors closed and in an open area away from all obstructions, especially metal buildings.
     
  3. rebelk5frk

    rebelk5frk 1/2 ton status

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    The field strength meter part is totally seperate. Since you don't have the little FS antenna then you can't use that function.

    If you manage a way affix a small wire antenna a few inches long to the "FS ant" port then you will be able to take relative field strength readings also.

    It is usefull for finding out which directions the antenna radiates the most from and comparing how different antennae work from the same mounting location.
     
  4. danielzink

    danielzink Registered Member

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    [ QUOTE ]
    anage a way affix a small wire antenna a few inches long to the "FS ant" port then

    [/ QUOTE ]

    I've got access to several gauges of wire (lots of safety wire / welding rod etc.) So let me know how long an antenna I need to fab up and how the field strength cal works.

    Thanks so much for the post !!!!

    Dan
     
  5. rebelk5frk

    rebelk5frk 1/2 ton status

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    The stock ones were around 5-6 inches. Any reasonable length will work though.

    To use the FS meter part either set it to "FWD" or if it has another switch for "FS" etc then switch it to that. Then have someone transmit the radio or use a rubberband etc to lock the microphone into transmit.

    Then just hold the meter close to the antenna. It should cause the meter to move. Then move the meter variable distances from the antenna and watch the movement of the meter. Once you move far enough away that it is about half scale try different sides of the antenna at the same distance to see if there are any noticable "low spots" as you circle the antenna. It isn't any type of exact measurement but will give you an idea of which directions are stronger than others. Ideally on a mobile you want it to be equally strong field strength from all directions(omnidirectional).

    The meter is also handy for comparing like I mentioned before. Take a measurement at a set distance with one antenna then change to another antenna, take a reading from the same distance and see which one is better.

    A trickier use involves the tuning of the antenna. As I said most of the time the lowest SWR is best but not always. A typical quarter wave antenna should actually show an SWR of around 1.5 when "resonant". Many times though you can get the SWR to "look better" by having a lower SWR such as 1.2:1 or 1:1 even. This is where the field strength meter comes in handy. You tune the antenna as mentioned before then set a "baseline" FS reading at a set distance. Then try adjusting the length of the antenna and see if there is any gain in field strength. Field strength is more important than the SWR as long as the SWR isn't too high. Anything lower than about 3:1 is safe.

    Did I confuse anyone yet? /forums/images/graemlins/confused.gif
     

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