Jan-Martin, Jim and others,
I also agree with comments that a series inductor (the traditional "loading
coil") is the best approach for compensating capacitive reactance in amateur
top-loaded verticals, and then either an autotransformer or L match to
convert the R value to being reasonably close to the desired load
resistance.
All this begs the question "why do people use T networks at HF?" The
answer
is, I suppose, that at HF the degree of mismatch is usually smaller,
requiring a much lower loaded-Q network which is more practical to
realise.
Also, being able to twiddle all 3 component values gives you a reasonable
chance of getting a good match, without having the faintest idea what the
antenna impedance actually is!
At HF the antennas can present the full range of capacitive to inductive
reactance, in combination with high to low resistance. This includes the
transformer action of transmission lines that can "invert impedances" every
quarter of a wavelength, so a multiband antenna and feeder can actually have
a wide range of impedance presented at the shack end of a feeder. A high
pass T tuner can address such a wide range of incidental impedances, in an
effective manner. At LF, a top-loaded vertical has capacitance and low to
medium series resistance, and transmission lines are of negligible length,
the net result being that impedance is fairly restricted in limits, so the
versalitity of a T network is not needed. Jim's calculations show why it
should be avoided.
73, Bob ZL2CA
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