I always advise using a non-resonant length for a multi-band doublet with
tuner combination. There's a magic figure: multiples of 44ft, 88ft... that
Cebik came up with which is a good compromise with impedance matching, ie
not horrendously high or low X and R. I notice no-one has mentioned the
G5RV and its derivatives, yet.
David
G3UNA
----- Original Message -----
From: "Mike Morrow" <[EMAIL PROTECTED]>
To: <elecraft@mailman.qth.net>
Sent: Tuesday, February 20, 2007 11:50 PM
Subject: Re: [Elecraft] wire antennas
Stuart wrote:
Even simpler is a 80m dipole fed with balanced line to a tuner for all
band
use. The window line is less costly than coax. A good quality tuner is
less lossy in multiband use than coax/ tuner balun, etc.. Balanced
antennas
have fewer problems than off center feeds. Balanced line to dipole does
not
need a balun at the antenna.
That's always seemed the ideal approach to me. You can go anywhere with
it with very low resulting losses, which is also very useful for
MARS/CAP/SHARES work on those odd HF military frequencies.
The only real problem seems to be routing the balanced line from the
antenna into the shack without it having much interaction with nearby
materials.
A second problem is the lack of real balanced-line antenna tuners.
Unbalanced tuners with that small output balun are problematic. I bought
an old Johnson Matchbox just because it is one of the few true balanced
tuners can can still be found. I know that MFJ has a couple of non-balun
tuners design for balanced line, but I've never investivated their
technical details, nor read reports on how well they perform. Obviously,
these would not serve the "gotta swap bands in five seconds" contest
crowd, but that's not me.
I never trusted those resistor-terminated folding dipoles. Every analysis
of them that I've ever read over the past 30 years has been basically
unfavorable, as one would expect, with performance at best very much below
that of a simple dipole. It is similar to a broad-band antenna design
using any length of center-fed non-folded dipole fed with coax, with a
hefty 50-ohm resistor across the coax leads at the connection to the
dipole. You'd get good VSWR with that from 1.8 to 30 MHz! Come to think
of it, about 25 years ago some outfit was hawking something just like that
to hams at high cost. Yet, I'm sure you could make some contacts with it,
just like you can with a resistor-terminated folded dipole. What these
types of antennas show is that, no matter how bad an antenna design is,
it'll work sometimes. TANSTAAFL!
Mike / KK5F
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