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[TowerTalk] Capacitor resistive values

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Subject: [TowerTalk] Capacitor resistive values
From: (Tom Rauch)
Date: Tue, 25 Sep 2001 19:56:20 -0400
> What resistive value would an air-variable or vacuum variable
> capacitor have?

It depends on the capacitor. The more "bulky" or compact the 
capacitor for a given insulation type the lower the loss will be.

The LAST thing we want is a long thin one made 
from coax. The best thing is a sandwich of plates in parallel, like 
chip capacitors.    

 If the mechanical connections to or within the
> capacitor are eliminated what then?

Mechanical connections can be a problem, but they are often a 
smaller part of the overall loss unless the capacitor is well-

 Dose the dielectric material or
> metals used to construct the capacitor determine this resistive value?

Yes, as well as the shape and size of the materials. The design is 
complex because current distribution in the component greatly 
affects loss.

> If there is a resistance being introduce into a circuit by the use of
> these types of capacitors what is its value?

ESR is Reactance/Q of the component. It varies widely. Vacuum 
capacitors with very very low dielectric losses (only in the housing) 
and concentric close-spaced rings of copper can have have Q's into 
the 50,000 range or higher. So ESR (equivalent series resistance) 
will be .02 ohms for a 1000 ohm reactance part.

Cube shaped air variables without wipers (butter-fly type with 
welded plates) can have  Q~15,000 for an ESR of .067 ohms in a 
1000 ohm reactance part.

Long conventional air variable capacitors typically have Q=2500 in 
1000 ohm reactance parts, so ESR might be .4 ohms.

Coaxial capacitors or single plate capacitors would be Q~50 to 500 
typically for an ESR of 2 to 20 ohms in the same reactance value.
73, Tom W8JI 

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