[Amps] disc ceramic capitors

[Manfred Mornhinweg]

Paul,

> What parameters or characteristics should one seek out when selecting 
> disc ceramic caps for power rf applications such as plate coupling and 
> padder caps used on the pi network.

You need to look at least at these specs:

- Correct capacitance
- Sufficient working voltage
- Sufficient current handling

Depending on the application, you might also need to look at the 
tolerance rating, the thermal stability or thermal coefficient, and the 
equivalent series inductance, which is mostly given by the length of the 
capacitor plus the wires.

Power loss is an important factor, but if the capacitor has suitable 
voltage and current ratings, that should imply low enough loss. High RF 
voltage, such as in PI network or filter use, causes high dielectric 
loss, so you want a very good dielectric in those applications. Bypass 
or coupling caps work at much lower RF voltage, but high DC voltage, and 
so dielectric loss is much less important. All of them are likely 
working at rather high current, so their equivalent series resistance is 
important in determining loss. Since all this is a bit hard to work out, 
it's best to use capacitors having firm voltage and current specs.

> I am assuming just buying high voltage disc ceramics doesn't necessarily 
> mean they will work well at rf in the 160 to 6m range?

Correct assumption. Many high voltage capacitors cannot tolerate high 
current. And in an amplifier you have high current through almost every 
capacitor.

Sometimes the diameter of the wires hints at the current handling. A 
high voltage capacitor having thin wires is certainly a low current 
type. Instead one having wide straps or very heavy wires is a high 
current one. One that has screw connections without being big and heavy 
enough to require screw mounting for mechanical reasons, is probably 
also a high current type. But it's better not to trust these hints too 
much, specially because they don't tell HOW high a current is 
acceptable. You have to find the specs of the actual candidate 
capacitors. If the current handling isn't specified, then it's better 
not to use that capacitor in high power applications. Unlike you enjoy 
special effects of the luminous and noisy kind, of course.

Manfred XQ6FOD

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