On 10/11/2013 9:36 PM, Mike Bryce wrote:
I have a 160 meter doublet feed with 600 ohm line. 130ish feet on each side.
Please define "doublet." Do you mean a single wire dipole, or do you
mean a folded dipole? How high is the antenna? How long is the feedline?
A single wire dipole has a feedpoint Z in the range of 40-80 ohms,
depending on height and the nature of your ground. A "folded" dipole
tends to look more like 300 ohms if it is high enough. Neither of these
is a good match to 600 ohm line, so the transmission line will transform
the antenna Z to some value that depends on the antenna Z and the length
of the line.
You say "1:1 balun." What is it, really? Is it a common mode choke?
How is it made? Is it a string of ferrite beads, or something better?
Most things called 1:1 "baluns" are a string of beads wound onto 50 ohm
coax.
If I had a single wire 1/2 wave 160M dipole (and I did until about a
year ago), I would feed it with 75 ohm coax if it was higher than about
100 ft (mine was), and 50 ohm coax if it was lower. If I had a folded
dipole, I would feed it with 300 ohm line, put a 4:1 transformer near
the shack, and run either 50 ohm or 75 ohm coax to the rig. These
solutions make the antenna a fairly good match to the line, so the
transmission line doesn't transform the antenna to some wild value that
the tuner can't match.
The only good reason I can think of for a folded dipole is slightly
greater bandwidth, and we can get the same benefit from using spaced
parallel conductors from the common feedpoint, which keeps the feedpoint
Z in the 50-75 ohm range, which means we can (and should) use coax to
feed it with a serious ferrite common mode choke at the feedpoint (up in
the air).
73, Jim K9YC
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