Peter Chadwick wrote:
>So we are told that some manufacturers derate some mica caps above
>5MHz. Others have caps that in slightly different applications have
>ratings that are high. Some mica caps can handle high currents in the
>VHF range. Dissipation factor wouldn't, prima facie. seem to be the
>critical parameter between all these products. I can't say that I'm any
>the wiser - possibly a little better informed.
The things that limit RF power handling are:
* voltage breakdown
* temperature rise within the dielectric material.
Temperature rise is important because it may lead to other indirect
effects, eg melting, cracking, voltage breakdown or change in
capacitance.
Dissipation factor is only important insofar as it affects temperature
rise; but there are also other factors that can help reduce temperature
rise. For example, a VHF/UHF cavity amp may use large plate capacitors
for RF bypassing, which have two good effects for the dielectric. First
the large plate area reduces the current density through each square
inch of dielectric, which greatly reduces the I-squared*R losses.
Second, the metal plates act as a very good heatsink for the dielectric
that is sandwiched between them. Those two factors allow you to use
dielectrics with higher losses than would be possible if all the
capacitance were lumped together into a single small component.
But let's not be so quick to blame the mica. In the case of dipped or
block mica capacitors, the derating above 5MHz may not be due to the
mica at all. It could well be due to heating in the very thin
metallization, which would increase with frequency due to the skin
effect.
--
73 from Ian GM3SEK
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