What is a duplexer, and how does it work? How does a duplexer differ from a diplexer?
• Answer: Simply put, a duplexer is a device that allows a single antenna to be used for a repeater. Since a repeater must simultaneously transmit and receive (on two separate frequencies), the duplexer prevents the transmitted signal from feeding back into the receiver. There are several designs, but basically there are band-pass and band-reject filters on both the transmitter output and on the receiver input. On the transmitter output, the band-pass filter is tuned to the transmitter output frequency (e.g., 146.970 MHz in the case of a PPFMA repeater), and also on the transmitter output is a band-reject filter tuned to 146.370 MHz, the receiver’s frequency. On the receiver, there would be a band-pass filter tuned to 146.370 MHz, and a band-reject filter tuned to 146.970 MHz. There usually are multiple filters, or cavities, in series to improve the pass and reject qualities. In reality, it’s impossible to completely eliminate the transmitted signal from getting back into the receiver.
