Variable capacitors may be useful, but the air gap that provides their capacitance is their greatest weakness. Rather than deal with the poor dielectric properties of air, some high-end variable capacitors replace it with a vacuum, which presents some obvious mechanical difficulties, but does give the resulting capacitor a remarkable quality factor, high-voltage performance, and higher capacitance for plate area than their air-gapped brethren. [Shahriar] of [The Signal Path] managed to acquire a pair of these and took a detailed look at their construction and performance in a recent video.The vacuum capacitors don’t use quite the same parallel plate design as other variable capacitors. They instead make the plates out of interlaced concentric metal rings mounted in a vacuum tube. Both sets of rings are connected to terminals, one fixed and one capable of being pulled in or out on a threaded rod surrounded by an accordion-pleated copper seal. A nut on the outside pulls the rod out, and the interior vacuum pulls it in toward the other set of plates. Unfortunately, since the mobile terminal needs to be mechanically connected to some adjustment mechanism (such as someone’s hand), it can’t really be at a floating voltage. The mobile terminal needs to be grounded for safety. Alternatively, for automatic control, one of the capacitors had a chassis with a motor, gearing, and a positional encoder.[Shahriar] also tested the capacitors with an impedance analyzer and lock-in amplifier. They had fairly low capacitance (for the one he tested, 36 pF at maximum and 16 pF at minimum), but the dissipation factor was so low and the DC impedance so high that they couldn’t be meaningfully measured. He also tested one at 5000 volts and found almost no dissipation.We recently saw another video going over a lesser-known feature of normal air-gap variable capacitors and another new non-standard variable capacitor design. On the opposite end of the fanciness spectrum might be this variable capacitor built out of aluminium cans.