> Having said that, what are the benefits of doing all of that work, over
> 450
> ohm ladderline, then using a well made 4:1 current balun outside the
> shack, and a short run of rg-213 into the shack, and using a Dentron
> MT-3000A
> tuner? I've used this setup for awhile here, at 1kw, and an 80 meter
> dipole
> at 50', and have been thrilled with the results. No RF in the shack, no
> heating of the balun, or the tuner, and, everything works great.
> Am I missing out on something?
Not at all if you're happy with the results. On the other hand, you may not
know how much of your line is radiating due to coupling of other objects,
nor the efficiency of the system unless you know the line-to-load mismatch,
the Z seen by the coax at the "remote balun," and loss through the tuner.
For example, assuming that the Dentron tuner is a "T" type, how do you know
you've found the optimum placement of the C and L controls since more than
one combination of settings will yield a 1:1 SWR at the tuner input? Do you
know if you happened to end up with an electrical 1/4-wave multiple of
450-ohm line on 80m just because it was a convenient length between two
points? If so, the coax is terminated into a Zs of roughly 3500-ohms when
using a flat-top 1/2-wave dipole and both tuner and coax losses can escalate
quickly:
ZS = Z02 / ZL
where:
ZS is the input impedance;
Z0 is the characteristic impedance of the 1/4 wave transmission line;
and
ZL is the load impedance.
For a T tuner, experimenting with the W9CF applet can be a real eye opener.
http://fermi.la.asu.edu/w9cf/tuner/tuner.html
Try various combinations of loading and component values and see what
happens to tuner efficiency, especially low-Z termination values below 50
ohms. Then, increase the C size and component Q values in the set-up menu
to see how much efficiency you've gained from the tuner alone. At the end
of the exercise, you will be gutting your T tuner and replacing the standard
500pF output cap with a switched 5000pF vacuum variable for added C and Q --
and using a low-loss inductor. To keep coil Q high across the full range of
bands, it might mean building two tuners: one for 160m-40m, and perhaps
another for 30m-10m to keep efficiency high unless you've mastered layout
skills or you are okay with using plug-in coil assemblies just as all of
those before us used in the 1930s.
There are countless systems like your being used everyday to everyone's
satisfaction. Myself, I like to know just a bit more about what's happening
and try to achieve the most efficiency -- even if that added efficiency has
a negligible effect on the other guy's S meter.
Paul, W9AC
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