As an academic exercise, for example using a tapered
line to match an OCD with a known FP impedance of 200 ohms
down to a 1:1 balun at the 50 ohm coax line transition point.
What is the recommended minimum length of cable needed to
perform a four to one characteristic impedance change in the
open wire for relating to the wavelength?

Pete


"Jim Lux" wrote in message
...
Pete Bertini wrote:
Just curious if this has ever been done? I'm thinking about
putting up a 90 foot dipole feed with homemade open wire
line. I'd like to bring it into the shack using paralled runs
of LMR-400 cable, since the final 25 feet is via 3" electrical
conduit that also has rotor and other cables.

I believe the parallel cables with give me a 100 ohm impedance.
The open wire will be using #10 with homemade spreaders,
I'm going to try for 600 ohms at the feedpoint. I was wondering
if tapering the spacing on the feedline would give me a smoother
impedance where the open wire, arrestor, and twin coax arrange-
meet?


Yes..

Tapered transmission lines have been around for decades, and work just
like you'd expect.

For the ultimate in performance, you actually want to use an exponential
taper (spacing increases by the same percentage per unit distance), but a
linear taper (spacing increases by the same distance per unit distance)
works pretty much the same, especially if the taper is "long" (10x)
relative to the spacing. that is, going from 1" to 3" spacing over a foot
or two..


Pete k1zjh