[ARC5] Coaxial variable capacitors

[Roy Morgan]

On Sep 29, 2009, at 4:00 PM, Michael Tauson wrote:

> YEs, I know, way too many messages but it has been a bad night and
> this is better than fighting the night demons ...
>
> Anyway, I was thinking about coaxial capacitors and realized I had no
> clue what to use to determine the plate size - the surface of the
> inner piece, that of the outer one or some average between them.

Michael,

I take it you don't mean capacitors made from coax cable.  These are  
VERY useful as (PART of) parasitic suppression in RF amplifiers.  The  
capacitance is figured from the cable table value for pF per foot.   
Pull the center conductor out a bit on one end for the plate end, and  
flare out the braid or solid aluminum sheath if using solid coax for  
the base.  A short piece of  heavy coax is good for many kilovolts.

If you mean the Transmitting Neutralization capacitors that have two  
concentric aluminum tubes with one adjustable into and out of the  
other one, I suggest this:
  Find the formula for flat plate capacitors. Use the circumference of  
the middle of the air gap as one dimension, and the length of overlap  
of the tubes for the other.  This will get you close enough.  The  
first book I'd turn to for information is the RDH-4.  One is handy  
here, so I'll have a look later tonight.

Roy

Roy Morgan
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Groton NY, 13073
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