Advice for capacitors selection for RF filter

[T3sl4co1l]

Hm. I don't know that a general statement is appropriate; there are certainly applications, mass-market / industrial applications, that use them -- consider the hundreds of thousands of cell towers for example, equipped with quite tight diplexing filters, constructed as 20-odd element cavity filters; or similarly, commercial broadcast where bandwidth is shared on an antenna tower (the antenna is broad, individual transmitters narrow).  In certain subsets that one may be familiar with, HAM radio for example, they're probably less common (lowpass most common AFAIK, being sufficient for impedance matching plus harmonic filtering purposes).

Though arguably, an antenna tuner is likely to have significant bandpass character so might still be considered a case, when one has a particularly ill-cut antenna for a given frequency of interest.  Perhaps even more interesting as an example, as it becomes a conditional one, probably with poor stopband attenuation (i.e. response is a peak above a plateau, or with an upper asymptote (overall lowpass character), or having mode breakup (and thus additional, accidental passbands) at/beyond upper band edge due to strays in construction, but perhaps still useful as a tuner despite that.

It is most likely true that, while not every transmitter has a tight bandpass on its output, nearly every receiver does -- at least, where "tight" is relative to the image bands relevant at a given stage in the signal path, generally getting narrower as the signal is whittled down to final IF and detection.

Tim

[Sp4]

Generally, band pass filters are used at low levels of RF, maybe up to +10 dBm. Low pass filters used on the output of a transmitter can have potentially damaging currents and Voltages, it is interesting that the Elecraft KPA500 (500 Watt) HF/6m linear amplifier uses 1206 size SMD capacitors in the low pass filter.

Actually, you are completely right about coming back to the question what we are actually trying to achieve BPFs are low power devices

Unfortunately, there is no service manual for the KPA500 and only sketchy details can be found on the web. Details of other transmitters/amplifiers can be found from more customer focused companies, such as Icom. You can download the service manual for the Icom IC-7610 (100W HF/6m) transceiver from https://www.manualslib.com/download/1542112/Icom-Ic-7610.html
The part numbers for at least the low pass filter capacitors are 1206 size SMD by Murata, C844 is GRM31A5C2J560JW01D (56pF, 630V, NP0, around $0.25 from Digikey). Take a look at the manual, it is refreshing that manufacturer part numbers are listed.

SJ

This is a great idea to look into the service manuals. I have one for Yaesu FT991A with all schematics and the part numbers. Looking how "big boys" solved a certain task is a great time saver.

As for the choice GRM over GQM - I have a solid theory that it was "good enough" for a given task to keep the cost lower. IC-7610 is a fancy rig, but ICom still needs to make some $$$ otherwise we will never see next model.

I do see some GQMs in 991A though which means that they do care about me .

Cheers,
Sergey

[VK4KYT]

I'm currently going through the process of building a mutli band Low Pass Filter, so I've kinda already walked the path you're going down. 

The filter design you are after is fairly generic. 
If you have a look at a book "Experimental Methods in RF Design" you'll get a heap of good information.
Additionally, to achieve the maths easily, have a look at "Elsie" (Kinda sounds like L C - the letters should be meaningful to you).  You can easily select the type of filter you want to design, the order of filtering, and heaps more ... that's using the free "student" version ...

Using the above tools, you'll be taking the guesswork out of determining both the capacitances and inductances required.

I hope this helps.

Luke  VK4KYT

[radiolistener]

For high-power filters and matching circuits, it is better to use ultra-low ESR, high-Q porcelain RF capacitors, such as those from ATC. However, they are tend to be quite expensive. Silver mica capacitors are also a viable option but may be difficult to source.

The C0G capacitors overheating excessively even with transmitter power below 100W, leading to capacitance drift and detuning of the circuit.