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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