SIMPLEX and REPEATER channels are two types of communication channels used in radio communication. The key difference between SIMPLEX and REPEATER channels is in how they transmit and receive signals.
A SIMPLEX channel uses a single frequency to both transmit and receive signals. This means that when two radios communicate over a SIMPLEX channel, they take turns transmitting and receiving messages. For example, if Radio A wants to transmit a message to Radio B, it would use a single frequency to transmit the message, and Radio B would use the same frequency to receive the message.
On the other hand, a REPEATER channel uses two different frequencies: one for transmitting and another for receiving. The repeater station is typically located at a high elevation to provide a larger coverage area, and it re-transmits the signals it receives on a different frequency to extend the range of the communication. When two radios communicate over a REPEATER channel, they transmit on one frequency, and the repeater station receives the signal and re-transmits it on a different frequency that is picked up by the receiving radio. This allows for longer-range communication than would be possible using a SIMPLEX channel.
In summary, SIMPLEX channels use one frequency for both transmitting and receiving, while REPEATER channels use two frequencies to extend the range of the communication.