The sharing of antennas and feeders between a number of co-sited services is not only a precise method of controlling frequency isolation, it is an economic solution to the problems of antenna and tower management. An antenna may be shared between a number of receivers or transmitters, or shared simultaneously by both receivers and transmitters. Receiver sharing requires a splitter, i.e. a filter, to separate the frequencies for each receiver and an amplifier to make up the filter losses. Transmitters can share an antenna through circulator/isolators and filters. To obtain the required selectivity, stability
Txsystem Rxsystem− 50 dB Spectrum dividing
filter0 dB −1 dB Rx band
− 50 dB
Receiver splitter amplifier
Loss 0.7 dB 0 dB
−1 dB
−20 dB
−30 dB
and power handling capability the filters are often solidly constructed cavity resonators with an insertion loss of 1 dB and a bandwidth of 0.1% to the −20 dB points. Figure 19.4 shows the layout of a transmitter/receiver sharing system.