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Monday, 27 June 2011

Thoughts on tuned loop variable capacitors

While reading Jim VK5TR's article in July's Amateur Radio Magazine, Jim bemoaned the very narrow useful tuning range of butterfly variable capacitors, with fractions of a degree of rotation greatly altering loop tuning, necessitating the parallel addition of a fixed capacitor to the variable capacitor.

It occurred to me one could use cam or nautilus shaped plates in the variable cap on the loop.

Imagine, if you will, a capacitor with two plates which are circular. Rotating the plates axially 180 degrees relative to one another does not change the capacitance.

Now, add some lobes, like a cam in an engine, or like a nautilus profile in and old
school temperature controller or on nautilus  gym equipment.

Rotation of the cams from 0 through 180 degrees will yield the same capacitance
as the two circles when the cams  are at minimum engagement, and when the
lobes are at maximum overlap, the capacitance will be (the capacitance of the two circles at minimum engagement) + (the capacitance of the the two overlapping lobes).

Ideally, the cam profile should be calculated to allow a linear increase in frequency across the desired portion of the band through which the loop is to tune as the capacitor is rotated from 0 to 180 degrees . If you are really keen on a particular part of the band, the cam could be modified to "stretch" this part of the band, to allow finer tuning.

See attached picture. I have only shown a simple two plate cap, but obviously for
higher Q you would implement it with a rotor betwixt two stators, etc....

Excuse the graphics, the GIMP is not ideal for this sort of thing.

Something for the must try this sometime list.

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