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More on Reflected power on antenna feed lines

Discussion in 'Boat Electronics' started by Gary Schafer, Nov 24, 2003.

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  1. Gary Schafer

    Gary Schafer Guest

    When you have a mismatch on one end of a line you must introduce an
    equal mismatch on the other end of the line.

    If there is a mismatch at the antenna end of the feed line that
    mismatch is reflected back to the transmitter end of the line. You
    must adjust your transmitter matching device or if it is fixed at 50
    ohms as most are, you must use a tuner to match the 50 ohm transmitter
    to the new impedance that is showing up on the feed line end. That
    becomes a mismatch to the 50 ohm line. (The line still maintains it's
    50 ohm characteristic impedance.) You now have the same degree of
    mismatch on both ends of the feed line.

    Look up "conjugate match" in your handbook. Interesting stuff.

    Whoooa you say! I am just matching the transmitter to the line. I am
    not providing a mismatch to it. Well, you are providing a mismatch to
    it because the transmission line impedance you are matching your
    transmitter to is not the 50 ohm characteristic impedance of the line.
    It is the new reflected impedance caused by the mismatch at the other
    end of the line.

    To prove it replace the mismatched load (antenna) with a 50 ohm load
    so that there will be no reflected impedance at the other end. It will
    be 50 ohms at the other end of the line. But now you will have a
    mismatch at the transmitter end because the tuner will be tuned to an
    impedance other than 50 ohms.

    This is easiest to see is when the feed line is a 1/2 wave length
    Whatever load impedance you place on one end of A 1/2 wave length line
    you will see exactly the the same impedance on the other end of the
    line. This is regardless of what the characteristic impedance of the
    line is.

    If you have a 50 ohm 1/2 wave length line and place a 100 ohm load on
    one end you will see that same 100 ohms on the other end. The 100 ohm
    load represents a 2:1 impedance mismatch to the 50 ohm line. In order
    to feed that line with a 50 ohm transmitter you must place a 2:1
    mismatch at the transmitter end of the line. In this case we call it a
    matching device. Either way, now both ends of the line see the same
    2:1 mismatch to the 50 ohm line.

    The transmitter is happy because it sees a 50 ohm load through the
    matching device. There are standing waves and reflected power on the
    line because of the mismatch at the load end.

    The reflected power bounces back and forth between the two mismatches
    on the line dumping all of the power into the load except for the
    small amount that the line absorbs.

    These are the same principles that apply when using sections of line
    for impedance transformation.

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