Simplex
In the simplex mode, only one frequency is used, that is, both stations transmitting and stations receiving use the same frequency. This is the simplest mode of operation and is used for radio communication over short distances, such as a neighborhood within a city. Because all hams using simplex transmit on the same frequency, they must, in some way, coordinate their transmitting such that only one ham at a time transmits. Generally, simplex operation is not used to obtain new contacts. Instead, simplex is used to provide communications between hams who have previously agreed to use simplex and to contact each other at a particular day, time, and frequency. This includes networks or nets of several hams who have common goals for their activity. Nets will be discussed in a separate page.
Repeater
Repeaters use two frequencies, one for receiving and one for transmitting. A repeater receives a signal on its input-frequency and simultaneously retransmits or repeats the signal on its output-frequency.
For example, refer to the diagram given above. Suppose two hams on opposite sides of a mountain can't communicate with each other via simplex, because the mountain blocks their signals. If a repeater is installed on the top of the mountain, it can communicate with each ham and thus can act as an intermediary between the hams. One ham will transmit, and the repeater will retransmit the signal received from the transmitting ham. The receiving ham will be able to hear the repeater and thus hear the signal from the transmitting ham. As long as both hams can hear and transmit to the repeater, the repeater makes communications possible between the two hams. Thus, repeaters are used to overcome obstacles and longer distances and are especially helpful to low-power stations, such as mobile and portable stations.
Hams use repeaters for making new contacts and for making contact with friends on an informal basis, such as while commuting to and from work. By tradition, stations using repeaters do not use CQ as a way to obtain contacts. Instead, they use normal voice sentences, such as, "W7ABC listening". In addition, not many Q-signals or word abbreviations are used when hams talk to each other via repeaters. Instead, they usually use normal words and sentences.
Repeaters are built to use specific frequencies for receiving and transmitting. The frequency-distance between the two frequencies is known as the offset or split, and by convention the offset in the 2-meter band is either -600 khz or +600 khz. Repeaters that have an output frequency that is less than 147.000 mhz have an offset of -600 khz. Repeaters with output frequencies 147.000 mhz and above use an offset of +600 khz. When you set your receiver to a repeater frequency, you set it to the output frequency of the repeater, that is, the frequency on which you will hear the repeater.
In order to reduce the interference or QRM on VHF/UHF bands, repeaters may use a tone (referred to as a CTCSS or PL tone) to separate signals intended for that repeater from extraneous signals heard by the repeater. Repeaters will ignore signals not having the correct tone. This allows repeaters with different tones to use the same frequencies.
In summary, to use a repeater, hams must set up their FM equipment to use the frequencies, offset, and PL tone used by the repeater.
Because you're dealing with a separate ham transmitter/receiver system, you must give the repeater time to disconnect from the other station before it can connect to your station. Thus, wait for two or three seconds after the other station stops transmitting before you press the talk-switch on your microphone. This will also allow hams with emergency traffic to break into the conversation and conduct their emergency business.
Hams also use repeaters for communication between several hams who have organized themselves into a network or net. Nets will be discussed in a separate page.
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