Why are variable capacitor plates made from aluminium?

Question

This question comes from doing some research into small magnetic loop antennas for HF.

For transmitting, these antennas are often designed with a big, variable, air (or vacuum) tuning capacitor (e.g. butterfly-style).

I understand that the currents and voltages in a loop when transmitting are fairly big and every bit of loss in the loop would quickly add to a less efficient antenna system.

To increase the efficiency of these loop antennas for transmitting, wouldn't it be better to reduce the internal resistance of the tuning capacitor and have the plates made from silver plated material or copper instead of (what I most often see) aluminium?

Answer 1

Copper indeed would be better. Silver would be better yet. These materials, however, are heavier and less rigid than aluminum. So there is that. To aluminum's advantage, it can be laser cut at very high accuracy. Copper has to be water-jet cut, at lower accuracy and higher cost. Beryllium copper alloy would be stiffer, but it's a health hazard and no shop will cut it.

Copper, in time, will also corrode. And as RF travels mostly on the OUTSIDE of things, that would be a big problem in time.

Conductivity in a Mag-Loop antenna's capacitor IS a concern, however, despite the low currents involved. You want a very high-Q capacitor for best effect. And ANY resistance will drag down the Q. So copper, again, if it can be used.

One solution which I'm presently contemplating, is use of silicone transformer oil as the electrolytic (versus vacuum or air). This gives a K of 2.71 and an anti-arc insulation of > 350 V per mil. Trade off would be the (still very low) loss tangent of 0.0001 up in the HF bands. That by itself would lower Q by a tad. But counter to that, you could have copper or copper-plated plates. Their being immersed in silicone oil would prevent corrosion.

As for the (usually Russian surplus) vacuum variable capacitors, those are truly excellent ... with one proviso. You will be constantly tuning your Mag-Loop antenna (since Q is high) and that would mean VERY frequent re-adjustment on your surplus (already used) VVC. Look at any picture of those. There is a copper (or copper plate) bellows inside. Constant adjustment would mean fatigue. Copper has LOUSY fatigue resistance. So then while it might last a good while, don't go thinking that means forever. It will wear out, mechanically, from constant adjustment. And where, down the road, will you find an exact replacement? So then, maybe buy two?

One final thing. Do NOT simply press-fit your plates together the way I've seen done on YouTube and elsewhere. All those inter-plate contact connections. What must be the ESR accumulation from that? Braze them together for electrical unity. Use AlumiWeld or something like. It's only 25% the conductivity of Cu, but better than a press-fit, for certain. Or maybe use 112 Cu (hand cut from McMaster-Carr stock) for the spacers. That 112 Cu comes in telescoping sizes, so you could have Cu spacers on Cu tube (which runs through the plate-holes, plus inside of all a length of stainless steel threaded rod to hold things together until it's all brazed.

I don't have a final answer to all these questions as yet. I'm only working out my design now. A project for next year. I'll write an article when it's done. Can't say when that will be.

Answer 2

I'm not familiar with the practical design of such antennas (so consider this answer a placeholder until someone more knowledgeable comes along), but here are some speculations:

• The current density in the capacitor plates is much lower than in the connections to the capacitor, because (in a typical air-variable) there are many plates, and within each plate the current spreads out from the shaft (seeing a larger cross-sectional area) and decreases (because part of the capacitor area is behind it now). Therefore, there is less current flowing per volume of metal and thus less resistance loss.

• The plates must be stiff compared to their mass, so that mechanical shock will not change the spacing of the plates or short them out. The materials you propose might be inferior to aluminum for this purpose (I haven't looked up the info needed to verify this).

• My understanding is that variable capacitors for amateur use are typically bought used or otherwise scrounged — and their original use was likely not in a transmitting loop, and doesn't care about resistance as much. Building capacitors to spec for this purpose would be quite expensive.

Answer 3

Aluminum is cheap, lightweight, stiff, and easy to shape -- properties that pretty much trump any question of its electrical characteristics in this application.

It is also very inert, forming a non-conductive oxide layer that greatly improves the long-term stability of the capacitor.