on of Where simultaneous reception and transmission is required, this spurious signals, thus further reducing the size and power consump- may be achieved in split frequency duplex mode by suitable filter tion of transceivers. circuits comprised within or external to the radio units, as shown in 46/4 PMR and Trunked Radio Systems Figure 46.2. This increases the physical size of the equipment somewhat due to the high-Q resonant filters required, the size of these normally being in proportion to the isolation required and in an inverse proportion to the frequency range used (DTI, 1987). Frequency A Frequency B Freq.A Freq.B Freq.A Freq.B Freq.B Freq.A SIMULTANEOUS UNIT 2 TRANSMIT/RECEIVE UNIT 1 CONTROL Figure 46.2 Full duplex operation employs simultaneous reception and transmission by the use of suitable r.f. filtering Figure 46.3 Talkthrough operation. Receive audio is It is common practice in personal radio communication usage for linked to the transmitter for re-transmission under manual the base station unit to operate in full duplex mode (DTI, 1981a), or automatic signalling control with mobile outstations operating in half duplex mode. This method allows the base to control communication, by disallowing uncoor- dinated portable to portable communication and allowing users with radio usage of each fleet occupying only a small percentage of emergency messages to interrupt the communication. By coupling available air time on the channel. the received audio from the base station receiver to the base station To prevent waiting users from other fleets being denied access for transmitter modulator, 'talkthrough' operation may be achieved. unreasonably long time periods, an electronically controlled time Here the base station transmitter with its normally advantageous site out timer is normally employed. Circuitry controlling this is fitted over the portable/mobile stations is used to significantly extend the both to the mobile transmitter circuitry to limit the maximum length range of communication to a defined cove