Copper Capacitor Plates

[KL7AJ]

If you've disassembled a lot of boat anchors, you may have noticed that some high end units used COPPER plates on the local oscillator variable capacitor. Why was this?

Copper has a much higher thermal expansion coefficient than aluminum. If properly designed, the use of copper plates could compensate for thermal drift. In general, fixed capacitors have a positive frequency/temperature curve. A disk ceramic capacitor, for instance gets thicker with temperature so the capacitance of the metal plates goes down. The disc capacitor also gets bigger in diameter, which you would think would increase the capacitance. However, the change of capacitance varies as the SQUARE of the distance between the plates, while the capacitance only changes linearly with the surface area, so overall, the SPACING wins out in which way the capacitor drifts.

Stout copper variable capacitor plates however, will experience a much greater change in overlapping surface area than spacing, so this tends to have a negative frequency/temperature curve.

As you can see, designing a stable oscillator was a tricky balancing act, and the engineers who did it well were a lot smarter than us.

I would have loved being an engineer in the 30s!

Eric

[K8ERV]

I would have loved being an engineer in the 30s!

Eric

I was, and would have liked to have met you.

Oh, you mean NINETEEN 30's. Too late.

TOM K8ERV Montrose Colo

[K7KBN]

"I would have loved being an engineer in the 30s!"


My grandfather was. Union Pacific.

[KY5U]

Someone copped his copper clapper.

[WA9SVD]

Someone copped his copper clapper.

It was quite a caper.

Credit Johnny Carson with the "'Copped Copper Clapper Caper' stolen by a kleptomaniac" routine on the Tonight Show.

[AF6LJ]

That would have been an exciting time for radio.
All the work being done to take tube designs further toward their theoretical optimum.
That would have been a very fun time.....


The first place I had seen copper plates used on variable caps was in an old FM tuner I had. That makes a bunch of sense AFC circuits were primitive and needed all the help they could get.
Eric...
You ever notice all the really good UHF TV tuners from the sixties and seventies had copper plates in the LO section of the tuning cap.

[KY5U]

It was quite a caper.

Credit Johnny Carson with the "'Copped Copper Clapper Caper' stolen by a kleptomaniac" routine on the Tonight Show.

Give the kid a cookie! You're right. With Carson and Jack Web.

[W0LPQ]

http://www.youtube.com/watch?v=mhLLU...eature=related

No ads or dumb music on this one .. forgot I had it saved.

[KO6WB]

Cleaning woman Clara Clifford discovered your clean copper clappers kept in a closet were copped by Claude Cooper the kleptomaniac from Cleveland who is going to get clobbered, took them to make copper capacitor compensators very crudely.
How's that!!!!!
73
Gary

[K7ELP]

If you've disassembled a lot of boat anchors, you may have noticed that some high end units used COPPER plates on the local oscillator variable capacitor. Why was this?

Copper has a much higher thermal expansion coefficient than aluminum. If properly designed, the use of copper plates could compensate for thermal drift. In general, fixed capacitors have a positive frequency/temperature curve. A disk ceramic capacitor, for instance gets thicker with temperature so the capacitance of the metal plates goes down. The disc capacitor also gets bigger in diameter, which you would think would increase the capacitance. However, the change of capacitance varies as the SQUARE of the distance between the plates, while the capacitance only changes linearly with the surface area, so overall, the SPACING wins out in which way the capacitor drifts.

Stout copper variable capacitor plates however, will experience a much greater change in overlapping surface area than spacing, so this tends to have a negative frequency/temperature curve.

As you can see, designing a stable oscillator was a tricky balancing act, and the engineers who did it well were a lot smarter than us.

I would have loved being an engineer in the 30s!

Eric

Eric, I am interested in what they do the the construction of capacitors to give them a negative temperature coefficient and or the NPO charastics.
Ned