Cavities

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 cavities come in a set that are inter-tuned. So, you make the filter as deep as you can - making it deafer if you like - and each individual cavity has an effect on the next. With the right gear you can actually see what is happening and how changing a bolt on the top of cavity affects the filter. Suffice to say that we spent several hours playing and learning. That is what amateur radio is like. You start with a small problem, hours later, morning tea forgotten, you're still playing with connectors and adjusting things. I'm Onno VK6FLAB