Q Alan, KZ1Y, asks, “What is the difference between simplex and duplex?”
A Simplex is a mode of communication in which stations transmit and receive at separate times, either on the same frequency or separate frequencies. One station transmits while the other listens, then the roles are reversed. Most amateur communication is simplex in nature.
Duplex (or full duplex) occurs when stations transmit on one frequency and receive simultaneously on another. FM repeaters, for example, are duplex stations; they receive and transmit at the same time (although the repeater users are operating “half duplex”). Amateur satellite operators often use duplex, transmitting to the satellite on one frequency while listening simultaneously on another so that they can compensate for frequency drift caused by the Doppler effect.
From QST February 2000
