The advice I would give anyone is pretty simple. Don't design or use an
antenna that has extremely low or reactive feed impedances, unless you
have no other choice. If you must use one, plan on low efficiency and
other problems.
My conclusion as well but adding more switched C will certainly help in most
common situtaions. The coil Q problem is more difficult to manage with
tapped L and roller type L. Coil Q can be managed very easily with just a
plug-in air coil, but then we've just lost much of our tuning flexibility
across multiple bands.
Further, it may be tempting to look at changes in line length to manage the
problem with say...a 1/4 wave section to raise the low feed Z to a higher
value at the input end of the line, but then that's fraught with matched and
unmatched line loss. For example, I just ran a calculation with a 5+j0
antenna feed Z and a 1/4 wave section of RG-213 (90 feet owing to velocity
factor) at 1.8 MHz. The total line loss is over 1 dB and is in the same
league as my T tuner example with a 500 pF output C. So, after modeling
with the 1/4 wave section, there was little to gain from total loss (tuner +
line) although the tuner's components will be less stressed with the added
line length and tuner loss would be reduced as the line approaches an
electrical 1/4 wave in length. With the use of the line, heat dissipation
due to loss is also spread across 90 feet of line instead of being
concentrated in the tuner's components.
It really would require a book of text and drawings to cover this
properly, just like it would with many one page or one line "always do
this" or "it works this way" answers.
A lot of examples are needed that independently move a lot of variables.
There's just no way of adequately explaining these concepts without the aid
of some visual and graphical help. The same is true when trying to explain
transmission line mechanics. You can end up expressing a point in a
thousand words that can be accomplished with a few visual charts.
Paul, W9AC
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Topband Reflector
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