Dave Richards AA7EE

July 3, 2012

An Air-Spaced Variable Capacitor With Brass Vanes

I think I might be closing in on the ideal variable capacitor for a VFO.  This arrived in the mail yesterday from an eBay seller:

The capacitance swing is 9-50pF with wide gaps between the vanes.  Rotation is fairly smooth, though the bearings could use a re-lube. Brass is a good metal for the vanes, as it has a lower temperature coefficient than aluminum.  However, this particular capacitor interested me because the fixed vanes are brass, while the moving vanes are aluminum. Why not all brass?  This would mean that as the vanes are engaged and the capacitance increases, more of the aluminum vanes will be meshed with the brass vanes, meaning that the temperature coefficient of the capacitor will increase.

Why would you need a variable capacitor that has a (positive) temperature coefficient that increases with decreasing frequency?

According to the seller this was a National Radio part, so I’m sure there was a good reason.  I need to put my thinking cap on. I should have cleaned it up a little more for it’s photo op, but it’s still quite a looker:


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