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twin lead

Discussion in 'VHF/UHF - 50Mhz and Beyond' started by KK4YWN, Nov 28, 2014.

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  1. KK4YWN

    KK4YWN Ham Member QRZ Page

    Where can I find the rules? I'd like to learn more.
  2. WB2WIK

    WB2WIK Platinum Subscriber Platinum Subscriber QRZ Page

    I don't know if they're compiled well and published anywhere as a compilation. But if you read enough books about such lines, you make your own rules and they're fairly obvious.

    -Don't run them up against other dielectrics
    -Don't make sharp bends like 90 degree corners with them
    -Don't run them parallel and close to conductors
    -Avoid using them for long horizontal runs if you're anyplace where it snows or the line can be ice coated
    -Real "open wire" line with small diameter spacers are better than solid dielectric or window lines, especially in the rain, snow or ice
    -If you get any substantial wind and use solid dielectric or window line, twist the line 180 degrees every few feet

    And stuff like that. Balanced line can be lower loss than coax -- sometimes. Not always. It depends heavily on the operating frequency, environment, and the way the line is fabricated.

    In "free space" with nothing around it in all directions, and no possibility of it being ice coated, it's really good at HF and even can be at VHF.
  3. W2VW

    W2VW XML Subscriber QRZ Page

    Not sure where the are published all in one place.

    Some rules with balanced feedline are:

    Keep the feeders equal and opposite in mechanical design.

    Axial twisting is handy for reducing coupling into nearby ferrous metal objects. Added benefit is more stability in a breeze for crummy window line.

    Surface area of insulating material should be minimized and weighed against mechanical integrity. Open wire line with plastic spreaders easily out performs window line and twinlead.

    Impedance of balanced line itself is not highly important when the line is operated at high SWR. It is supposed to be something like the geometric mean of the impedances it is matching. I do not remember this one exactly and it is not easy to use as a rule of thumb in non resonant antenna load situations due to complex impedances.

    Balanced line can be normally spliced to dissimilar balanced line of different design and impedance without too much trouble when operated with a conjugate match above flat. Flat meaning the line is operated at its impedance, low SWR. Example: 300 ohm folded dipole fed with 300 ohm twinlead.

    Balanced line should be keps 5 times its spreader length away from metal objects along the run. This one is lifted from old F.E. Handy edited ARRL literature and refers to H.F. use.

    There are more. Hopefully some constructive posters may add.

    One thing I tried and have seen from other new users of balanced line..... A resonant antenna fed by 600 or so ohm line near a quarter wavelength long to replace coax originally used. This is extremely difficult to match and will make one return to coax with plenty of sour grapes. My first homebrew antenna tuner caught on fire. Bess The Cow has nothing on me.
  4. W2VW

    W2VW XML Subscriber QRZ Page

    No fair. I had a rock in my shoe.
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