A radio repeater is a transmitter operating on one radio frequency and a receiver operating on another radio frequency so connected that the transmitter transmits, repeats, what the receiver hears simultaneously. They are usually placed in a high location or the antenna on a tower so they can extend the range of mobile radios, handheld radios, and low level base or portable radios.
The operation as described above is called duplex operation. When the transmitter and the receiver are operating on the same frequency so one unit transmits while the other unit or units receive the transmitted RF energy directly from the transmitter it is called simplex mode or simplex operation.
A digipeater is a repeater designed to extend the range of digital communications. It receives a packet of information then the receiver shuts off and the transmitter sends the information back to the transmitting station for confirmation and to the receiving stations on the same frequency as it received the information. The transmitter and receiver are not operating simultaneously.
Most duplex ham radio repeaters use one antenna with a duplexer separating the transmitter’s output from the receiver’s input so the transmit frequency does not get into the receiver. A duplexer is a combination of a very high Q band pass and frequency reject filters usually in the form of tuned cavities.
A controller is used between the transmitter and the receiver so as to turn on the transmitter when the receiver receives an on frequency signal. The controller also discriminates when tones are used so the transmitter is only activated when the proper tone is received. The audio is passed through the controller and any tones used to activate the transmitter are stripped off and the transmitter generates it own tone if a tone is transmitted which normally not done with amateur radio repeaters.
Most amateur repeaters operate on the two meter ham band but there are certainly many other repeaters on the other amateur radio bands in the VHF and UHF spectrum. There are a few ham repeaters on the ten meter amateur band which is still considered HF.
Frequency Modulation (FM) is the most common mode for voice communications on ham radio repeaters. This is because FM has several advantages over Amplitude Modulation (AM) and Single Sideband (SSB) modes when it comes to repeater operation. Probably the most advantageous quality difference between FM and AM or SSB when it comes to repeater operation is that once a FM signal is strong enough to fully capture, some times called full quieting, the receiver the volume will not become any louder as the signal increases. Thus a strong signal does not over modulate or otherwise distort on a strong signal while a weak signal is easily heard. A second advantage FM has is AM and SSB signals can interfere with each other when the level differences are as great as 100 to 1 but an FM signal will over ride another signal at just 2 to 1. That is why FM is the choice mode for both ham radio repeaters and commercial radio repeaters in most instances.