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RF Electronics Chapter 7: RF Filters Page 271 2022, C. J. Kikkert, James Cook University, ISBN 978-0-6486803-9-0. For mobile radios, used by trucks or taxis to talk to their base, a coaxial diplexer is often used to provide isolation between the transmitter and the receiver in their mobile radio. Figure 7.99 shows a typical coaxial diplexer. Hills Industries [17] was sold and no longer make RF products. However figures 7.99 and 7.100, are a good illustration of the construction and operation of a diplexer. The receive frequency is 450.625 MHz and the transmit frequency is 460.125 MHz. These radios typically have a transmission power level of 25 W (43.98 dBm). The receiver typically has signals of -60 dBm applied to it, so that the difference between the transmitted and received signal is more than 100 dB. Connecting the transmitter directly to the receiver will result in severe damage to the input circuitry, but the diplexer prevents this. As shown in Figure 7.100, at the receiver frequency of 450.625 MHz, the diplexer of figure 7.99 causes a less than 1 dB loss between the antenna and the receiver ports and more than 60 dB isolation between the antenna and the transmitter ports, thus protecting the input from any spurious signals generated by the transmitter at the receiver frequency. At the transmitter frequency of 460.125 MHz, the diplexer provides less than 1 dB loss between the transmitter port and the antenna port, so that virtually all the 25W output signal is transmitted. At this frequency, there is more than 60 dB isolation between the antenna and receiver ports, thus protecting the input from the high power levels generated by the transmitter. Figure 7.100. Frequency response of the diplexer of figure 7.99. For low loss, high power microwave filters; it is often convenient to mill the whole filter out of a solid block of aluminium, as shown in figure 7.101. Martin [18] provides design tools for this. By including coupling between non-adjacent cavities, zeros of transmission can be included. In the filter of figure 7.101, non-adjacent cavity coupling has been applied to the first and last resonator. RF Electronics: Design and Simulation 271 www.cadence.com/go/awr