A 400MHz duplexer/combiner with the cover removed. It’s. Bunch of resonant elements within a cavity.
https://furry.engineer/system/media_attachments/files/113/018/904/867/743/547/original/4824d59e7ebdeecf.jpeg
@ftg @Nixie RF stuff is weirder because it isn’t even just a more complicated version of something people are familiar with (like valves), it’s just pure invisible voodoo.
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