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From: "Peter Martinez" <peter.martinez@btinternet.com>
To: <rsgb_lf_group@blacksheep.org>
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Date: Wed, 5 Jul 2006 08:03:27 -0000
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Subject: LF: Re: Antenna Tuning
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>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