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:
Dave Richards AA7EE 
Dissimilar metals, when submerged in an electrolyte, produce electricity. With the capacitor shielded in a watertight housing, your QRP transmitter would be self-powering. Think, man – think! 😉
Brilliant minds must think alike, John. I’ve used several of these over the years and never could figure out why they used different metals. Perhaps you should make a YouTube video of a receiver in operation with the capacitor submerged in electrolyte and list it under “Free Energy.” You would get dozens of “Likes” and positive comments from the free energy and perpetual motion crowd. 🙂
The smallest plate of a capacitor determines area A (where electric lines of force are perpendicular to the plates). Even if aluminium vanes become larger due to thermal expansion the effective area of a capacitor is that of the brass vanes. You can observe that pivot determining distance d is brass. Therefore the net temperature coefficient is 2 alpha – alpha = alpha, where alpha is the linear thermal expansion coefficient of brass. This Al-brass capacitor is simply cheaper than a full brass.