> This is because on 80m, you have a full wave dipole with
many
> 1000's of ohms drive impedance driven through a half wave
line,
> which puts this same impedance at the tuner. I don't know
of
> any commercial tuners that can deal with this.
Each 1/2 wl antenna half has a driving impedance of about
3500-6000 ohms. There are in series. The result is the tuner
has to match 7000-12000 ohms if the feeder is lossless.
>
> An alternative to changing the line length is to shunt the
> line with a doorknob capacitor of several hundred pf.
This is
> usually easier than changing the line, assuming you can
get
> the capacitor. This feature ought to be built into
tuners,
> but I've never seen it.
That's because adding a shunt C across the line doesn't do
anything unless you are very lucky and have a mismatch
caused by inductive reactance that the C cancels. It will
not extend the range of a network to higher impedances, as a
matter of fact extra shunt C reduces the high Z range of the
network.
The real issue here is the Nye tuner is for all real
purposes an L network. L networks require an exceptionally
wide range of reactance values to match wide impedance
ranges over wide frequency bandwidths. Where a T could
easily handle 3000 ohms and a wide range of reactances on
160 with reasonable components, you'd be hard pressed to
have an L network cover from 30 ohms to a few thousand ohms
without any load reactance. That means the feedline length
would have to be chosen more carefully than with a T
network.
L networks are dandy for trimming in small or modest
mismatches especially in a direction where the load Z is
higher than the source Z. That's about it. The way to fix
them isn't to add a shunt output C. It might work to add a
series reactance and make a T out of the L.
73 Tom
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