What Use Is An F-call?

What use is an F-call? Recently I was spending some time with a group of amateurs and we got the urge to fix a local repeater. In the past I'd seen things called cavities, but how they really worked or how you might tune them seemed like a mystery. I can report that it's less of a mystery today than it was last week. I'm yet to understand the full implications, but I learnt something. A repeater is a transmitter and a receiver working on two frequencies. The repeater receiver listens for input on one frequency and transmits it on a different frequency. The frequencies are generally on the same band, a 600 KHz for 2m or 5 MHz for 70cm apart. When the transmitter is sending, some of the signal leaks back into the receiver, making it less able to hear your signal. A cavity acts like a filter. You have a gadget, that looks like a big Pringles can, that filters out the transmit frequency and you connect that to the receiver, in effect making the receiver less able to receive the transmit frequency. The cavit