[BC] Open Wire Feeds

[Charles Lewis]

    Open Wire Lines


    Our 600 kW MW transmitting facility in Northeastern Greece uses the
    longest transmission line I have ever seen.  It is an unbalanced
    open wire line, otherwise known as a quasi coaxial line, nearly 2
    miles long.  If I recall correctly, the nominal impedance is 200
    ohms.  The center conductor is copper tubing, and the outer
    conductor is an octagonal cage of parallel copper wires.  One reason
    for using open wire for such long runs is to keep the losses down. 

    The above is a four tower array with a square configuration.  It is
    remotely switchable between several patterns.  If I recall
    correctly, when I was there we were using omnidirectional for
    daytime Hellenic Radio broadcasts to the immediate region and two
    (or was it three?) directional patterns for VOA broadcasts in the
    early morning and evening periods.  The switching/power
    division/phasing equipment is contained in a fairly large sheet
    copper lined building in the center of the square of towers. 

    The quasi coaxial line is put to good use in the feeds to the four
    individual towers also.  One great advantage of the large quasi
    coaxial open wire line they use in that setup is that it is more
    forgiving of gross mismatches at high power than any reasonably
    sized conventional coaxial line would be.  By using the quasi
    coaxial line  to feed the individual towers of this multi pattern
    system, they get around having to use complex switched impedance
    matching systems at the tower bases. 

    The relatively new (about 10 years) IBB (VOA) station here on Sao
    Tome Island, West Africa uses 300 ohm open wire feeders to its 15 HF
    dipole curtain antennas.  The RF output of the 100 kW shortwave
    transmitters is 75 ohms unbalanced.  That is fed to a coaxial switch
    matrix  that allows us to connect any one transmitter to any one of
    antennas.  (We don't used diplexers here, so we can only connect one
    for one.)  The RF leaves the switch matrix on 7" - 75 ohm hard
    line.  Outside the building, each line has a wide band balun for
    conversion to 300 ohms balanced for the remainder of the long run to
    the antennas.  The antennas themselves are a balanced design with a
    nominal 300 ohms input impedance.

    Again in this case, one good reason for using open wire line is to
    keep power loss low.   Some of these lines are nearly half a mile
    long, and they operate as high as nearly 22 mHz.  The loss is less
    than it would be with coaxial line, especially at the upper
    frequencies. 

    Another reason we use open wire has again to do with its being more
    forgiving of mismatch than reasonably sized coax.  Our antennas must
    work over a very wide band of frequencies.   Our dipole curtain
    antennas are mostly arranged in high/low band pairs.  Typically,  a
    low band antenna might be specified for the range of 5.6 to 13 mHz
    and it's high band companion might be specified to cover 13 to 22
    mHz.  There is no way one is going to get (passively) a perfect
    match to the line of 300 ohms across such a wide range of
    frequencies.  We have to accept much higher levels of VSWR than
    would be considered acceptable for fixed frequency operations. 
    Properly designed open wire line can handle fairly large mismatches
    without breaking down and without the losses due to increased VSWR
    becoming extreme.  If we used reasonably sized coaxial line for
    these feeds, we would have to employ matching networks at the
    antennas that would have to be retuned each time we changed
    frequency significantly.  (In large shortwave broadcasting
    facilities, a transmitter typically changes to widely separated
    frequencies many times over the course of a broadcasting day.)

    I have digital photos of our HF antennas and their feeder systems
    here on Sao Tome Island.  Anyone who cares to see them can find them
    in a photo album at http://groups.msn.com/s9ss    .   

    BTW.  We continue to have temperatures in the lower to mid 70's F
    here on the Equator.  This is unusual; the cool season usually means
    temperatures in the mid 80's.


    Charles Lewis  - S9SS
    (CPBE)