Fw: [Antennas] Twin lead 450 Ohm
Sandy and Kees Talen
[email protected]
2002年12月11日 09:55:35 -0600
Relative to the changing characteristics of ladder line when it
becomes wet .... what is the major cause (other than rain) ? Is
it due to the web between the lines becoming wet (sounds most
likely) and/or is it due to the line itself becoming wet, or ? Since
most ladder line has a pretty thick web, has anyone tried removing
some of it (cut, punch, etc) to see what happens during wet weather ?
73 Kees K5BCQ
-------- Forwarded message ----------
From: Sandy and Kees Talen <[email protected]>
To: [email protected]
Date: 2002年12月11日 09:00:31 -0600
Subject: [Antennas] Twin lead 450 Ohm
Message-ID: <[email protected]>
Great discussion on ladder line. I use the stuff exclusively (it's still
tied directly to my beam) and I'm always looking for ways to
improve it ...like how best to keep moisture off or bury it using PVC.
I've always looked at feeding an antenna as having two basic problems.
One is impedance matching and the other is any balanced to unbalanced
conversion. Beam antennas and the like are balanced loads and ladder
line is balanced so no problem there. Since good ladder line is extremely
low loss you don't have to worry about the impedance mismatch except
at the transmitter end and you solve that with a good tuner. Two parallel
"balanced" conductors do not radiate as long as the parallel paths are
not altered by running next to metal, etc. Coax, on the other hand, has
loss and will require impedance matching and some sort of balun, coil
choke (that's what that coil of coax the manufacturers tell you to add at
the feed point is), or ferrite bead choke for the transition between the
unbalanced shielded coax and the balanced antenna. If you don't do this,
the outside shield of the coax will tend to radiate. You want the signal
on the inside of the coax.
73 Kees K5BCQ
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