Simplex communication refers to information that can only be transmitted in one direction at the same time: you say I listen, I say you listen. This type of transmitter and receiver can only work alternately, and a radio intercom that cannot work simultaneously is called a simplex. The simplex machine works by pressing the key to control the conversion between receiving and transmitting. When the transmitting Control key is pressed, the transmitting is in the working state and the receiving is not in the working state; On the contrary, when the transmit button is released, the transmit is in a non working state and the receive is in a working state. According to frequency usage, simplex machines can be divided into co frequency simplex machines (also known as single frequency machines) and cross frequency simplex machines (also known as dual frequency machines, quasi duplex machines, and half duplex machines).

The same frequency simplex refers to a transmitter and receiver operating on the same frequency, with the advantage of using only one frequency to efficiently utilize frequency resources. Due to the intermittent operation of the transceiver, the circuit design is relatively simple and the price is also relatively cheap; The disadvantage is that both parties need to take turns speaking, that is, after the other party finishes speaking, we can only speak, which is not as convenient to use as making a phone call. But in terms of frequency resource utilization, simplex machines are the mainstream models. Heterofrequency simplex machines are only used in wireless communication systems with central turntables. Many simplex machines can work both on the same frequency and on different frequencies.