9.1 Duplexers

A repeater contains a receiver and transmitter which operate simultaneously. Separate antennas can be used for the receiver and transmitter as long as the isolation between the two antennas is sufficient to prevent the transmitter from interfering with the receiver. Sometimes, sufficient vertical separation between the antennas can be obtained to achieve this objective. This largely depends upon the frequency separation of the receive and transmit frequencies the greater the frequency separation, the less isolation required.

Since tower space is usually at a premium, it is most common to use a single antenna for the receiver and transmitter. A duplexer allows the transmitter and receiver to use the same antenna at the same time. The duplexer isolates the transmitter and receiver, thus preventing interference. Isolation requirements depend upon frequency separation, transmitter sideband noise level at the receiver frequency, and the receiver desense at the transmitter frequency. Some transmitters produce more sideband noise at a given frequency offset and some receivers are more susceptible to desense at a given frequency offset. Thus, by using the highest quality receiver and transmitter for the repeater, less isolation will be required from the duplexer.

As an example, suppose that the transmit and receiver frequencies for a VHF high-band repeater are separated by 1MHz. The transmitter sideband noise at an offset of 1MHz is specified to be ?100dBc, 100 decibels...