 transmitter does not interfere with the receiver. This might result from spurious signals arising from the transmitter, or the receiver may become desensitized by the strong signal being received from the transmitter. The filters already mentioned are used to reduce these effects. Signals transmitted to satellites usually consist of a large number of signals multiplexed onto a main transmission. In this way one transmis- sion from the ground can carry a large number of telephone circuits or even a number of television signals. This approach is operationally far more effective than having a large number of individual transmitters. Obviously one satellite will be unable to carry all the traffic across the Atlantic. Further capacity can be achieved using several satellites on different bands, or by physically separating them from one another. In this way the beamwidth of the antenna can be used to distinguish between different satellites. Normally antennas with very high gains are used, and these have very narrow beamwidths, allowing satellites to be separated by just a degree or SO. Another variant of communications satellites is those used for direct broadcasting. This form of broadcasting has become very popular as it provides very high levels of bandwidth because of the high frequencies 234 Newnes Guide to Radio and Communications Technology Narrow beam width Satellites means only one satellite receives the signal Earth station Earth Figure 8.8 Separating satellites by position used. This means that large numbers of channels can be carried. It also enables large areas of the globe to be covered by one delivery system. For terrestrial broadcasting a large number of high power transmitters are required that are located around the country. Even then coverage may not be good in outlying areas. These DBS satellites are very similar to ordinary communications satellites in concept. Naturally they require high levels of transmitted power because domestic users do not want very large antennas on their houses to be able