----- Original Message -----
From: "David Pruett" <k8cc@comcast.net>
> Paul,
>
> I understand that equal SWR does not mean equal load impedance. However,
> as the SWR approaches unity the number of different combinations of R & X
> diminishes. In both examples I quoted, the "equal and low" meant under
1.2:1.
>
> I've used the "tuner on each antenna" approach here at K8CC for many years
> on 80M dipoles using MN-2000s. While this provides a 1:1 match on A
> PARTICULAR FREQUENCY, at least with the MN-2000s the SWR bandwidth through
> the tuner is much, much narrower than that of the dipole by itself. In
the
> past this has been tolerable with my Henry 2K-3 (2 x 3-500Z) but recent
> operating patterns seem to have spread people out (more SS activity in the
> extra bands, and people going higher, particularly on CW) so some
> "tuner tweaking" is necessary.
>
> I would think that different tuner circuit configurations might be better
> or worse for this. However, I would think that the desire for high
> efficiency (and thus hi-Q) would work against the need for broad
bandwidth.
>
High Q doesn't necessarily mean lower loss, Dave. I think
you may be confusing loaded and unloaded Q. Unloaded
Q is the instrinsic quality of the component. You want this
to be as high as possible to minimize losses in an LC
network. Loaded Q on the other hand involves the ratio
of the network reactance to the system load impedance.
For minimum losses and maximum bandwidth you want to
minimize loaded Q and while at the same time maximizing
component unloaded Q. High loaded Q in a tuner is usually
associated with arcing and inductor heating as well as
narrow VSWR bandwidth. Dan Maguire AC6LA's free
XLZIZL program and Dean Straw N6BV's "AAT" tuner
program are a great way to observe and optimize these
tradeoffs. With XLZIZL you can load in the antenna
impedance as a function of frequency and then add different
tuner configurations to see how it effects overall VSWR
bandwith. Dean's program allows you to check antenna tuner
component voltage and power dissipation to see if
you are getting close to the "arc and spark" region.
BTW, Leeson developed an impedance compensation
method for broadbanding 80 meter dipoles which
should be generally applicable to this sort of problem.
It involves placing a resonant network (L/C network
or coaxial stub) in parallel with the load such that the
network compensates for the impedance change
of the load. I believe this is what Yuri was talking about
when he mentioned using a "Bazooka Balun" to improve
VSWR bandwith. You can simulate this with quite easily
XLZIZL.
> Finally, putting a tuner on each antenna gets financially ridiculous if
> quality tuners are purchased.
>
Yeah, no kidding there, Dave. The only way to make it
affordable is to scour the flea markets for old bread
slicers and roller coils.
73 de Mike, W4EF............................................
> 73,
>
> Dave/K8CC
>
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