TheRadioBoard

You are correct, except that the larger plate area should be at the hinge. Also by trial and error (cutting away more material) you would come to a linear configuration.

The only problem you run into, is that the minimum capacitance is raised and, (dependant on the final plate shape), the physical size may be doubled (more or less) for the same maximum capacitance. Some "authoritative" sources suggest that larger is not better WRT "Q", due to "environmental" losses.

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Another way to do it, may be to make the moving plate from light gauge spring brass such that the plate curves as it is pulled away from the fixed plate. This, however, would involve clamping, rather than hinging the join, (unless weighted in the centre) which would increase dielectric losses and the minimum capacitance. (This might work on your "tape loop" setup, though the brass would introduce some resistive loss compared to the copper overlap.)

Don't think a rotary cap. could be beaten (in most cases) for simplicity, longevity, adjustment control and overall size.

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Probably something approaching a diamond or teardrop shape would work with the hinged type. Fast "maximum C" reduction would be minimised and a small "minimum C" (with correspondingly tighter "minimum C" to "plate angle" ratio) could be maintained.

You still end up with a physically larger cap though.

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Right - it's all a question of which imperfect capacitor configuration has the least loss. I wish there were an easy way to test the overall in-circuit loss. For the loop antenna, maybe field strength measurements could do it?

"Teardrop" shape aptly describes the plate shape that I was thinking of for the book capacitor. I see what you mean about the thinner area needing to be farther away from the hinge (since the far end of the hinge separates more quickly, we need to reduce this otherwise large effect this would have on capacitance).

However to keep a very low minimum capacitance it seems the base of the teardrop should not be right at the hinge, but slightly offset (towards the book edge), so that greater maximum separation of near-hinge areas of the plates can be achieved (ensuring lower minumum capacitance).

Still not sure if this will lead to a usable design, but sure is fun to think about.

Mine separates 10mm at 90° for ~5pF, 2-3pF of which is hinge capacitance. Capacitance is ~6.5p at ~45° and ~9p at ~22.5° (outer edge separated by 25mm).

I used spring brass plates to keep tension on the conical sapphire pivots to maintain accuracy but if these were replaced with a non metallic, high Q equivalent (I don't know of one that would wear well and maintain it's tension) then the hinge capacitance could be reduced. An adjustable socket on a fixed bracket would work but would be more involved for a homebrew effort (and require periodic adjustment).

The unit however, as constructed, matched the "Q" (16000) of a 500pF, Johnson, silverplated, ceramic variable. There are half a dozen improvements I could incorporate that would probably make it extremely good, but size is always the killer (and the cost of silver . )

Extending the length of the adjustor-screw outrigger (and using a finer screw pitch) decreases the rate of "C change" but again, adds to the bulk. (See earlier concept drawing below.)



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By the way, what exactly are the "environmental losses" incurred by larger plates?

I read somewhere that stray inductance can start to become a factor, but can't find the reference right now.

Don't know personally if it's true. The theory is (I think), that there will losses to the floor, walls .... anything close by. The bigger the physical size of the capacitor = a greater interaction with the immediate "environment".

BTW, I am talking WRT crystal radios where these losses are of some interest but are probably negligable WRT amplified applications.
Others may know better.

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There's a thread going on right now at regenrx about homebrew variable capacitors. One guy posted some pictures of 1920s articles including one with a spiral-shaped variable capacitor. Pretty interesting stuff, even if not directly applicable to a low-loss magnetic loop antenna capacitor.

http://groups.yahoo.com/group/regenrx/message/14369

One of the simplest and most reliable homebrew caps, with a reasonable capacitance value, is the "trombone" tube type, with some sort of insulation (non air) between the tubes.
(FWIW).

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@golfguru,

re "bookcap"

I think those bookcaps only are used for some degrees of rotation, right?

In this case I recommend to replace the hinges by simple springs eg
made from razor blades or spring brass.
(Think of the springs used for the pendulum in wallclocks)
Such a spring is much simpler than a sapphire pivot and very
reprocucible in action.
If You need the utmost precision, crossed springs (90°) can
be applied.
Georg

The sapphire pivot was easy for me as I have a faceting machine.
http://hervey-bay.freeadsaustralia.com/ ... g_1590.jpg

The sapphire pivots (conical) and adjustor-screw plate, are the insulators between the two plates (and the pivots also do "double duty" as the hinges). Sapphire (aluminium oxide) was used, as it is one of the lowest loss-tangent materials. Contact area between the two plates is limited to the sharp point on the adjustor-screw, and a 2mm circle, of negligable width, on each pivot. Total contact area between the 2 plates, via the low loss material, would be less than 1.0 square mm.

It was designed to specifically reduce (to an absolute minimum) the insulator bulk and contact area (as a "Q" experiment).

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From http://forums.qrz.com/showthread.php?29 ... ost2259458

Any ideas why stray inductance is bad?

Current flowing through an inductance induces a magnetic field. Stray inductance would cause the capacitor to become part of the antenna's radiating structure and possibly unbalance the loop. Remember, with an antenna like this, any added loss is bad news.

73,

_________________

http://kr1s.kearman.com/
http://qrp.kearman.com/

Here's another variant of a sliding-plate capacitor that doesn't look overly difficult to build:

http://www.standpipe.com/w2bri/article1.htm


Personally I think the use of braid to connect to the capacitor plates introduces unnecessary loss; I would use copper straps instead.

Any opinions on if the wood backing will introduce significant loss? If I build something like this I would probably use a thick polyethylene cutting board for the backing.

Why do you think braid would would be lossier than solid straps? It's all copper.


I expect wood would be horrible for losses.