Resonant cavities are used by amateurs as a:

A resonant cavity is a highly conductive metal enclosure specifically sized to allow radio waves to reflect back and forth inside, creating a standing wave. Because of their physical geometry and lack of traditional component loss, cavities exhibit an extremely high "Q" (quality factor), acting as highly selective tuned circuits. Amateurs primarily use them as very narrow bandpass filters at VHF, UHF, and microwave frequencies. At these higher frequencies, standard LC (inductor-capacitor) circuits become lossy and impractical, whereas resonant cavities excel at passing a specific, narrow slice of the spectrum while fiercely rejecting nearby signals. They are entirely impractical for low-frequency applications (below 30 MHz) because the required physical dimensions of the cavity would have to be massively large to resonate at those long wavelengths. Furthermore, they are highly selective bandpass devices, not general-purpose high-pass or low-pass filters.