[R-390] Paper Capacitor Replacement II

[David C. Hallam]

Just Radios  http://www.justradios.com/  carries of full line of 
capacitors, film, mica, and electrolytic.  Their prices are good.  I 
have purchased from the many times.

David
KW4DH

On 1/6/2012 6:36 PM, Dave Maples wrote:
> 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:
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>
>
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