What is a repeater? As the name implies, a repeater…repeats but what does it repeat? How does it work? What is the purpose of a repeater? When it comes to wireless technologies, a repeater is typically used to extend the coverage of a wireless signal in order to defeat terrain obstructions or allow the use of relatively low power radios to reach out to a larger area and is simply accomplished by re-transmitting the message.
There are two basic types of repeaters, half-duplex and full-duplex. Half-duplex meaning the message is received, stored in memory and then re-transmitted. The cool thing about this is that receive can take place on the same frequency as transmit (sometimes referred to as simplex or parrot repeating in the amateur radio world). Full-duplex meaning the repeater is simultaneously receiving and transmitting. In order to accomplish this, two frequencies are needed along with enough isolation (in the form of physical antenna separation) or filtering to keep the transmitter from affecting the receiver.
Repeaters come in different sizes and different shapes but consist of two major components, a receiver and a transmitter. In order to function, a repeater also needs some form of controller in order to tell it when it is receiving a message and when to transmit as well as a power source. Other accessories may be added, such as a power amplifier, secondary receiver, and extra filters. Here is one of my favorite repeaters, the Motorola Quantar which though initially released in the mid 1990’s is still in service all over the United States, as it’s modular (and labeled) chassis make it easy to show the main parts of a repeater.
As you can see in above picture, the parts are clearly labeled at the top of the chassis cards with the receiver being at the right, the Station Control Module (SCM) which controls repeater function being mounted next to the power supply, the exiter (low power output) and power amplifier making up the transmit section of the repeater on left.
The basic operation of a full-duplex repeater consists of a handful of steps. Frist, a signal will be transmitted to the repeater upon which the receiver will detect and unsquelch a valid signal. This will cause the receiver to set the Carrier Operated Relay (COR) to active which will notify the controller the a signal is being received. The receiver will also demodulate the signal to obtain audio (usually at line level) and pass to the controller. The controller may or may not (depending on configuration) further process the audio and in-turn, activate the Push to Talk (PTT) of the transmitter and pass the audio through to be mixed and retransmitted. From carrier detect to transmit typically takes 250 ms or less but the audio path is typically much lower latency ~1 ms. The primary difference in operation of a half duplex repeater is that the controller stores the demodulated audio in memory until the COR signal from the receiver is no longer active, then then the controller keys the transmitter and transmits the stored audio (this allows the use of a simple transceiver).
As next to no signal processing is performed (especially with analog FM repeaters), repeaters typically are strategically located in areas where the receiver will have optimum coverage (direct Line of Sight) to the transmitting radio which is why repeaters are typically found on top of tall buildings, towers and on top of hills (or all of the above). They can come in many different sizes from a simple handheld radio with a half-duplex module, to two mobile radios tied together, to massive machines that weigh enough to give one a hernia.
If one is interested in learning more about repeaters such as more detail on how they work, how to build them and other unique information, I highly recommend checking out the Repeater Builder site located at http://www.repeater-builder.com as they have a plethora of information regarding the topic.
