Owen Duffy wrote:
Bruce in Alaska wrote in
:
Bruce,

An interesting post overall, but ...


System. All these tuners NEED a Low Impedance RF Ground to work
against, as well as a Longwire who's length is SPECIFICALLY set up to
put the 1/2 Wavelength Point in a non used portion of the Spectrum.
They will NOT tune within 2% of the Natural 1/2 Wavelenth point of the
Longwire connected, where Antenna Impedances near Infinity.


The "rules" above are offered without definition or explanation, so they
don't really raise the art.

Manufacturer's "rules" and explanations are often inconsistent. For
example, some state a minimum length that can be tuned on say 3.6MHz, and
it is often around 3m, yet they rabbit on about avoiding half wave
resonances in the wire to avoid high voltage at the feed point. The feed
point voltage on a 3m whip at 3.6MHz is likely to be much higher than a 10m
whip at its first parallel resonance.


there's a goodly amount of empiricism in these rules of thumb from the
mfr, too.

If you start to analyze it, there's all sorts of issues that crop up in
the analysis: the heating and/or breakdown of the Ls and Cs inside the
box, for instance (and, because it's a binary sequence with components
with 20% tolerance, the voltages (for L) or currents (for C) don't
divide evenly... the big Cs take more of the current than the small ones).

And then, there's the whole thermal management issue, and "real ratings"
of the component vs catalog vs derating. The tesla coil folks regularly
run low ESR extended foil polypropylene capacitors (typical 0.15 uF at
2kV) at 2 or 3 times rated voltage at a few hundred kHz, as long as the
rms current is within bounds. But they benefit from a fair amount of
destructive testing and failure analysis by a coiling hobbyist with
access to the appropriate gear.


Owen