[Antennas] Twin lead 450 Ohm
Sandy and Kees Talen
[email protected]
2002年12月11日 09:00:31 -0600
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