Hank,
> I see a proliferation of blue encased caps appearing to possibly be
> disc ceramics being used in RF applications.
I have seen many blue disk ceramic caps used in high voltage
applications, but not where very high current is involved. For example
they are common in AC line filters, and many of these blue caps are
type-approved for direct connection to 250VAC and higher. This means
that they can safely handle not only the current produced by 250VAC
through them at 50 and 60Hz, but also the current that any higher
frequency harmonic on the power line can cause. And this with a large
safety margin. But still that is A LOT less current than what such a
capacitor would see if used in the low pass filter of a legal limit amp!
> For instance the MFJ1500
> watt autotuner , and Alpha is using them as padders .
There are many different types of ceramic disc caps coated in blue
resin. Some are indeed able to stand a respectable RF current, but you
really need to have a look at the data sheets for the exact capacitors
you can get.
> Does anyone know who is making these and if there is any published RF
> current data on them ?
TDK and Murata are two companies that make these blue high voltage
ceramic disc caps. But there are also other companies that make
equivalent capacitors that are not blue, such as Vishay. And surely
there must be several Chinese companies too, but they typically won't
publish as detailed data sheets as the mentioned ones.
Have a look at TDK's and Murata's websites to see the specs.
I investigated this matter severalo months ago, specifically searching
for capacitors suitable for HF low pass filters that can operate safely
at the legal limit, even with nonperfect SWR, and concluded that I
cannot buy any blue ceramic caps that would be up to the task. Instead
one needs to turn to either porcelain chip caps or to metal clad micas
(not the dipped silvered micas, those can't handle enough current). Both
are quite expensive.
In the end the best solution, and the most ham-like too, is to home brew
the required metal clad mica caps. That's easy to do and very
inexpensive, so it's the way to go, unless somebody can come up with a
good source for inexpensive capacitors that can actually handle 10
amperes and more of RF.
Manfred
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