[R-390] Paper Capacitor Replacement II

[Dave Maples]

All: OK, I am properly humbled.  It appears now that the only offerings for
the film-and-foil caps are radial-lead designs suitable for PC boards but
not for point-to-point wiring.

I am presuming that the issue with ceramic caps is that the electric field
actually interacts with the physical structure of the ceramic and causes the
small changes in capacitance, but I may well have that wrong as well.

Dave WB4FUR

-----Original Message-----
From: r-390-bounces at mailman.qth.net
[mailto:r-390-bounces at mailman.qth.net]On Behalf Of Randy and Sherry
Guttery
Sent: Friday, January 06, 2012 2:54 PM
To: n4buq at knology.net; R-390-List
Subject: Re: [R-390] Paper Capacitor Replacement II


On 1/6/2012 1:28 PM, Barry wrote:
> Can someone explain why ceramics are better in RF applications (such as
bypass) than film-and-foil

I can give you a short-form "reason"...  Ceramics are made
by plating both sides of a ceramic wafer with conductive
material - each of which form the plates of the capacitor.
As ceramic is an excellent dielectric - it can remain thin
and still have high withstand potential.  Being thin - a
given capacitance requires much less "surface area" than a
thicker dielectric - so the area of the plate is small -
having little resistance. The leads attach directly - then
the whole thing dipped in a coating to protect from
contamination. Simple - low inductance and low resistance

many poly caps are made by coating both sides of some
flexible insulating material (which becomes the dielectric)
with conductive material (which become the plates. The
flexible material stacked with another insulator layer -
then rolled into a "tube" to make it compact.  Now being in
a coil - (rolled up) it now exhibits considerably more
inductance than it would were it left flat. Also - having a
large area - the resistance is also greater than a similar
value capacitor that uses thinner dielectric (which would
have smaller surface area).

So ceramics have less inductive reactance losses at RF than
most polys - however - polys tend to have better stability
at audio.  Recall that ceramics use a thin wafer for a given
capacitance - that thin wafer may distort at audio
frequencies causing changes in capacitance. Ceramic's
tendancy to react electrically to physical distortion is the
principle behind ceramic phono cartridges.

This is - of course -  "over simplified" - but the basics
are pretty sound...

--
randy guttery

A Tender Tale - a page dedicated to those Ships and Crews
so vital to the United States Silent Service:
http://tendertale.com


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