From G3PLX:
JA:
Using the figures you gave in your posting, and the IN3OTD variometer
calculator quoted by Brian, I estimate that your variometer has inductance
between 3.0 and 6.8mH.
To do this, I converted the outer coil diameter from 10.75 inches to 0.275m,
the inner one from 6 inches to 0.15m, and the length of both from 7.5 inches
to 0.185m. I had to guess that both the inner and outer coils have 122 turns
(both halves) and I calculated on the basis that these turns were uniformly
spaced along the whole length. In other words I had to ignore the 1-inch gap
in the middle and imagine that the turns were spread evenly. This is the
best I can do without trying to calculate the total inductance of the two
(coupled) halves of each gapped coil.
From this it looks like you could never resonate your antenna at 136kHz with
that variometer. It's too big and it's inductance cannot be swung down to
the 1mH you would need to tune it, if it's capacitance is about 1200pF as
Alan has estimated.
Before you tear it apart and rebuild it, I would echo what Scott has
suggested, and use a general-purpose LF generator to..
(a) Check the inductance of your present variometer using a known-value
capacitor (e.g. 1200pF).
(b) Measure the capacitance of the antenna using the signal generator and
the variometer.
(c) Calculate the inductance required to resonate that.
(d) Finally build a new variometer to give an inductance range spanning the
expected value.
73
Peter
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