Operators of the GB3ZZ amateur television repeater station
Radio and TV signals, particularly on the bands used for ATV will only travel in straight lines and are blocked by dense solid objects between the sending and receiving stations. Most repeaters are located on high ground or on top of buildings so they can see and be seen above obstructions and from a greater distance.
Picture two stations with directional antennas pointing at each other, they can look at each other and communicate successfully. Now imagine a third station wants to join the conversation but it is some way out of line of the other two. For all three to see each other they would each need two antennas, one pointing to each of the other stations. This starts to look impractical, especially if the antennas each have to be movable so they can point in any direction. The problem worsens if a fourth or fifth station try to join in. This is where a repeater comes in handy, being in an elevated position and being able to receive and reradiate signals, it provides a central spot that all the stations can point towards. Of course, only one station at a time can transmit but at least all the others can see it at the same time and with just one antenna.
To ensure that all stations have a chance to use the repeater, whichever direction they are from the repeater location, the repeaters antennas are omni-directional. This means they work equally in all directions. Signals from the repeaters transmitter must be sent in all directions so that everybody has a chance of seeing them. On some repeaters, there are several receiving antennas, one for all around coverage and one or more directional ones. In general the more directional an antenna is, the better its range and signal clarity becomes. Using a directional antenna improves the signal from a remote station but at the expense of making it harder for anyone out of line of the antenna to be received. For this reason, a link must first be established on the omni-directional antenna before changing to a directional one and as soon as reception is lost, the omni-directional antenna is selected again.
In order to change from one antenna to another there has to be a way for a user to tell the repeater to do it. Most repeaters use some sort of remote control system and how it is implemented depends on the person who designed the repeater. Unfortunately, there isn't a standard method but the most popular and easiest to use system uses telephone dialling tones. These aren't sent over a phone line though, the tones are sent along with any other sounds and speech over the normal sound channel.