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  1. #1
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    how to measure vswr

    HAI,

    HOW TO MEASURE VSWR , FIRST WE ARE SETTING MAXIMUM AMPLITUDE IN CRO THEN WE PLUG COAXIAL CABLE TO THE VSWR METER? HOW TO FIND VSWR MEASUREMENT

    ITS ANNA UNIVERSITY SYLLABUS FOR FINAL YR ECE STUDENTS CAN ANYONE EXPLAIN HOW TO set GAIN CONTROL NOB AND dB NOB , etc., and how to predict vswr.


    thanking you
    venkatesh

    •   Alt5th August 2008, 14:11

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  2. #2
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    vswr meter

    Hmmmm. Sounds like you do not understand what VSWR actually is. It sounds like you have one of those old style SWR meters.

    VSWR is Voltage Standing Wave Ratio. To actually see a VSWR you need four things:
    1) an RF signal source
    2) a coaxial cable with some sort of mechanism to allow access to the center conductor at various lengths
    3) a load impedance for the other end of the coaxial cable
    4) some sort of meter to measure the total RF voltage as some sort of probe various points along the coaxial cable


    So you set your signal source to a fixed frequency, and tune it up for a good output voltage.

    In the olden days, your "leaky" coaxial line would involve something called a "slotted line". It is a length of coaxial line that had a long slot cut in the ground plane of the coax, along its length. An electric field probe would be inserted into the slot, and the probe position along the slot could be varied precisely.

    Assuming your coaxial line was 50 ohm, you would hook a mismatch to the other end, like a 25 ohm load. This insured that power coming from your source would hit the load (forward wave), and since it is impedance mismatched some of that power would reflect off of the load (reverse wave). Now, at any position that you set the EField probe, the overall voltage measured would be a linear superposition of the forward and travelling waves. At some positions their phases would be the same, and you would have an EField maximum there. At other positions you would have the forward and travelling waves be 180 degrees out of phase, and you would have an Efield minimum there. By recordeing the Min and Max Efield, you could get the ratio of the two.

    This is where the SWR meter comes into use. The efield probe was typically connected directly to a RF Diode detector. The Diode detector turned the RF energy into a detected DC voltage. So typically you would have a slotted coaxial like, a diode detector mounted on the moveable portion of the slotted line, and a BNC cable running from the diode detector to the SWR meter. The SWR meter was reading DC volts, NOT RF energy.

    Does this sound like the setup that you have? If it is, I will write some more on how to use it.

    More recently, VWSR had been measured using two directional couplers and two RF Power meters. This is a simpler measurement scheme, but it only measures forward and reverse power independently, it does not give any indication WHERE a max or min efield is along the line, so you can not measure the phase information of your load.

    And of course today we would simply use a network analyzer to do all this. But it is useful for a student to really learn how it used to be done in the old days--you really have to understand transmission line theory and smith charts to get the experiment done.


    3 members found this post helpful.

    •   Alt5th August 2008, 19:29

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    Re: vswr meter

    yes its the type of meter we r using...pls continue



    •   Alt5th January 2011, 03:28

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  4. #4
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    Re: HOW TO MEASURE VSWR IN VSWR METER IN LAB EXPERIMENT

    its best to not use a vswr meter

    but to work out the geomerty of the antenna

    and measure its mean node distance
    and use a standing wave meter

    or a traveling wave tube via a rf sniffer/meter

    swr isnt a good mean to test any antenna

    and is only really usefull is a perfect enviroment

    tuning antanns is a matter of square mean distance from a known measured antenna to trim it

    do it .5mm at a time on both planes

    or you will chase your tail in antenna design

    vswr is a small measure a bit like voltage is in the grand scale of a circuit

    and for a 'lab experiment' is pretty useless




    too measure vswr
    you tune the meter to max scale for foward power scaler max

    then switch to reflected

    max/min ratio of 1.1 or more is 50.50

    or 50% loss line

    so the meter should fall bellow 1.5 - 1

    1.5 foward cycles to 1 refected for 75% efficient transfer - isimus

    1-1.5 is also known as the golden circle ratio of the sun

    as helium is 1.5% less efficient as hydrogen
    so the sun is also a dipole .. 1.5/E = C
    Last edited by VSMVDD; 5th January 2011 at 04:02.



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    Re: HOW TO MEASURE VSWR IN VSWR METER IN LAB EXPERIMENT

    Hi venkatesh kumar,
    In what frequency range are you doing the measurement. Is this the instrument that you are using to measure SWR ?



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    Re: HOW TO MEASURE VSWR IN VSWR METER IN LAB EXPERIMENT

    CAn you please help me implementing VSWR test in matlab...???


    2 members found this post helpful.

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