Expensive capacitor alternative!!

[XXL]

What can I use in place of the overprice ATC100b caps usually found on the filter of an RF amp?

Will 3kv 1206 work or will they just catch fire ?
There will be around 250w going though the filter.

[thewisepranker]

Those capacitors are expensive because they have an expensive combination of performance characteristics that cheaper ones don't, like an absolute value of capacitance that doesn't change with temperature and more importantly applied voltage. For example, X5R or X7R capacitors massively decrease in value as you increase DC bias voltage (not the amplifier bias but the DC bias of the capacitor) and increase in value as you increase AC bias, but to a lesser extent than the DC effect. In other words, you could tune your filter at 0 dBm using a network analyser or whatever and not see any problems, but then when you run 250 W through the filter it will be between far and very far out of tune (maybe not even in the band you wanted it in) because all of the capacitors are now some value other than what you think is in there, depending on whether any of the capacitors see DC or not.

It's not uncommon to see a ceramic capacitor down at 40% or even less of its supposed value at its supposed working voltage.

Datasheets might give you a graph of DC bias vs capacitance change but they won't give you data on capacitance vs AC bias vs frequency so you can't even interpolate it. If you're lucky they'll specify it at DC, 50 Hz and 60 Hz but it's often not even mentioned.
You could measure it if you've got the equipment to do so but it's not worth all that hassle.

Also, ATCs are very high Q, general ceramics probably won't be.

C0G dielectric material doesn't suffer from these problems but they're expensive, much like ATCs.

[sinus trouble]

WP is spot on!

250W is a serious amount of power in terms of RF! Mistakes will inevitably lead to injuries either to you or the filter!

The moral of the story is, Stop being a tight git and proceed with the circuit design as it was meant to be!

[XXL]

Yeh often on older amplifiers you will see c0g big ceramic orange capacitors on the output.