Thanks Dave KH6AQ for the reference to Tom W8JI's article on loading coil
currents. I just re-read it and it makes interesting reading. As I thought,
he showed that current out of a loading coil is essentially the same as the
current into it. However the results shown in the QEX article pointed out
by Rick N6RK don't seem to agree with Tom's, maybe because Tom constructed
a pair of all-plastic RF ammeters which didn't affect the loading.
Quote by W8JI: <*Measuring the current into and out of the loading coil
with a small thermocouple RF meter, I detect no difference This is in close
agreement with the model*.>
Of course as has been pointed out, there is bound to be some loss through
the coil.
The original question was how to match a sloper which is shorter than a
quarter wave length. If possible I believe it is better not to use
inductive loading at all, but to extend the top of the antenna making it a
full quarter wave sloping inverted 'L', which is what I have. Better still,
two top wires as a 'T'.
My inverted 'L' has its 90 ft vertical section supported by a tall tree.
It slopes away from the tree in order to minimise absorption by it. Also it
is slightly longer than a quarter wave making it slightly inductive and is
matched to the 50 ohm coax by a parallel fixed capacitor acting as an
L-match. This also has the advantage that the point of maximum current is
some way up the 'vertical' section.
All good stuff.
73,
Tom G3OLB
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