This isn't exactly HAM related, but it's RF related nonetheless.
I've always wondered what exactly the 'knob' does to the RF on some better TV rabbit ear sets. I remember the last set of these I owned had some sort of multi-position switch in there, and nothing else. Turning the knob would produce dramatic effects to the quality of reception on the TV, depending on the channel. These were passive devices, as they had no batteries or external power.
They're kind of hard to find nowadays, or else I'd just buy one and take it apart.
Hmm. May be time to make a stop at the thrift store