MEASURING LOSSES IN COAX

Discussion in 'Ham Radio Discussions' started by WD0EJA, Aug 3, 2019.

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  1. WD0EJA

    WD0EJA Ham Member QRZ Page

    MEASURING LOSSES IN COAX



    Our coaxial lines are out in the elements day after day. Some are buried. Not the best environment for longevity. How can you measure the losses in coax, the length in wavelength and velocity factor?


    Measuring power loss is not difficult. There are several ways to make the measurement. You can use a watt meter (two works better) or you can use your fancy shmancy antenna analyzer.


    With two power meters (calibrated close together) insert one at the transmitter. The other at the other end of the coax that is terminated with a dummy load that is the same resistance as the coax (normally 50 ohms).


    Send a carrier at a known power taking note of the reading at the meter nearest the transmitter. Now dash out to the other meter, have someone key down, then take the reading. The closer meter will have a higher reading than the farther one. Now you can calculate the loss. The formula is:

    P1

    dB = 10 log P2



    P1 is the meter near the transmitter.

    P2 Is the farther meter.


    Simple? Anyone can do it, even I can.


    How about with an analyzer?


    This is even physically easier.


    Put your analyzer on the Z function. Open the end of the coax. What ever length your coax is, it will show an impedance (Z) dip on the first 1/4 wavelength. Therefore, you want to start at the lowest frequency your analyzer is capable of. It will dip in Z value every

    other 1/4 wavelength.


    When you find the first dip, take note of the Z value. Then take note of the frequency. If your analyzer goes low enough in frequency, then you have the 1/4 wavelength of your coax.


    Take the Z value and calculate the dB loss with this formula:



    Loss (50 ohm line) dB = 0.17 x minimum Z



    You can make this measurement every other 1/4 wavelength. This will give you the losses as you increase frequency.


    The 1/4 wave frequency can also be used to confirm velocity factor.


    Calculate the free space wavelength of your minimum 1/4 wave frequency your analyzer is reading. If your coax is too long for your analyzer to measure, then you can do the same measurement by shorting the coax. Now the reading will be your 1/2 wave frequency of the line.





    Wavelength (free space in meters) = 300

    frequency in MHz.



    Then divide by 4 for 1/4 wave or 2 for 1/2 wave.


    Take the actual length of your coax and divide it by the free space wavelength. This percentage is your velocity factor.


    You can use this value to determine actual 1/2 or 1/4 wave lengths of coax at the frequency you desire. This is done by calculating the free space wavelength at your desired frequency, then multiplying it by the velocity factor percentage.


    There are other ways to make these measurements however, these are one of the easiest and fastest.


    73,

    Ralph WD0EJA


    AUG 2019
     

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