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, it has been done. Jasik's "Antenna Engineering Handbook"
discusses tapered lines in section 31.4. You can also find a
discussion of tapered-line matching in Laport's "Radio Antenna
Engineering" in section 4.4.8 (this book is available as a free PDF
download).

In your application, though, I'm not sure why you would want to
bother. Traditionally, tapered lines are used as part of a matching
network, making a transition between two known resistive impedances
(e.g. transmitter and antenna).

That's not really the situation you seem to be setting up. You are (I
presume) setting up a single doublet, to be used at multiple
frequencies, with wildly varying impedances at the antenna feedpoint.
You're then coming back to the shack with a high-impedance feedline
(which has relatively low losses even at the high SWRs it'll be
operating at), and then matching the impedance in the shack with a
good transmatch-and-balun.

In a setup like this, the characteristic impedance of the open-wire
line isn't particularly critical.. because it'll almost never be a
close match to what the antenna is presenting at any given frequency.
600 ohms is a common feedline impedance... but no matter what
impedance you choose here, it's unlikely to be a close match to the
antenna under actual operating conditions. As a result, the impedance
you actually "see" looking up into the lower end of the open-wire line
won't be 600 ohms (if untapered) or 100 ohms (if tapered down close
together). Rather, it'll be the feedpoint impedance of the antenna at
the frequency in question, transformed by the feedline (tapered or
not). You could calculate it, knowing the antenna's feedpoint
impedance (e.g. from modeling it in NEC2) and the length and Z of the
feedline... but it isn't likely to be close to a convenient pure
resistance except by lucky chance!

Hence, there's no real need to "match" the lower Z of the parallel-
coax segment (100 ohms nominal) to Z of the open-wire line, since the
latter value isn't actually what you'd have to match to!

Now, if you're planning to operate at a specific single frequency, you
could perhaps utilize a tapered open-wire line as part of a tuned
matching setup... in effect, designing the taper for that one antenna
and frequency, in order to present a more convenient impedance to your
transmitter's matching network (e.g. your external wide-range
transmatch). However, optimizing the system in this way, for one
specific frequency, might well make the antenna-and-feedline a
*harder* load for you to match in the shack at other frequencies.

--
Dave Platt AE6EO
Friends of Jade Warrior home page: http://www.radagast.org/jade-warrior
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