I know it's hard to believe that any parallel wire feedline works when
you first install it. There's something counter intuitive about it
compared to the way coax is constructed. I think the same psychology
plays a role in why some hams resist putting down a lot of radials on
or in the ground for a vertical counterpoise. Something about
blowing a lot of money on wire, then putting it on the ground instead
of in the air that doesn't make sense, but that's another topic. Back
to open / window / ladder line. To me, the trick is in how you use
it. First, I don't know what a Cobra antenna is, but from my
experience, if it is some kind of fixed wire antenna and it is
balanced, then chuck the balun and bring the open wire feed to the
building housing your shack. For a balanced feedline, a balanced
antenna such as a loop or center fed dipole, is best. The next part
is getting the parallel wire impedance transformed to 50 ohms
unbalanced without that balun, so you can use it as an acceptable load
to your transmitter. I bring my feedline in through a basement
window. The center pane is plexiglass with two holes drilled in the
center for banana jacks. This serves as a quick disconnect for the
feedline. One of the great things about two wire feed is what a
breeze it is to splice it, or put on connectors! Right on the other
side of the window pane, which is about 9 inches on a side, I have a
balanced transmatch. Then I have a 1:1 unun or common mode choke on
the 50 ohm side between the matching network and the transmitter,
where the Z is almost always 47 to 52 ohms with no or an ohm or two
reactance. this is essentally a 1:1 vswr and the choke is happy.
Because of this, the length of the ladder line doesn't matter. This
is one way of avoiding all that business of lengthening or shortening
the feedline. There always seems to be at least one band where the
length is a problem when you go that route. I use PVC poles with T
tops to hold my line up off the ground. I know PVC is supposed to
degrade in sunlight but mine have lasted 5 or 6 years now. Yes, it
takes some care to route it but once you have it set up, it's great
stuff. Wild impedances and high vswr don't matter much in terms of
line loss. This is why it's still used by shortwave bc stations.
73
Rob / K5UJ
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