Shorted turns, turns left floating, coils within coils are not the way
to proceed,
althought this is a convenient method of resonating an antenna system.
Having experimented with the above methods to obtain resonance, measure
the inductance, then proceed to make a single layer coil that is exactly
resonant at the desired frequency of operation.
If it is found necessary to make a fine adjustment for a very precise
frequency then use a small variable coil in series with the main coil at
the cold end, and with a minimum of turns to achieve the object.
If you have a vertical antenna around 100 feet plus the above exercise
is not difficult, however with smaller vertical antennas where a lot of
L is required obtaining resonance and especially fine tuning could be
very critical, therefore one would possibly have to resort to the more
lossy approach of having coils with shorted turns, variometers or
shorted copper variable loops within the main inductor. One other
approach is to use a suitable ferrite core material, that does not
saturate, to fine tune, this is probably the most efficient method.
The inductance required at my station to resonate the antenna is less
than 450 microH so I do not have any real problems. With a little effort
given time I could probably do away with the coil, by adding another
couple of inv L's at 32 m high and over 100 m horiz.
G3KEV
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