In order to allow communication through an amateur radio or commerical repeater system, an input frequency and output frequency have to be chosen. A repeater will often use the same antenna to transmit and receive, so in order that the the high-power output signal from the transmit doesn't totally overpower the distant low-power signals from mobile stations, a duxplexer is used to keep each seperate.

Because of this, the frequencies chosen have to be a reasonable distance from one another to let the duplexer attenuate the output signal on the way to the receiver. Standard frequency distance between the input and output were chosen for most of the amateur radio bands to make it easier to access unfamiliar repeater systems. Those are:

For example, if the repeater output was on 146.940 MHz, then an operator could reliably assume that setting the input frequency 600 KHz lower to 146.340 MHz would allow him to communicate on this frequency.

The standard formats to describe this repeater would either be:

146.94/-

or

146.34/146.94

with the input frequency listed first.

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