re two basic ways in which a signal can be modulated. Either its amplitude or phase/frequency can be varied. However, there are a great many ways in which this can be achieved, each type has its advantages and disadvantages. Accordingly the choice of the correct type of modulation is critical when designing a new system. 4 Antenna systems Every receiver or transmitter requires an antenna if it is to receive or radiate radio signals. In some instances these antennas may be small, and incorporated into the equipment making them almost invisible as in the case of mobile phones. In other instances large antennas are used. Some good examples of very large antenna systems are used by short wave broadcast stations. Others might include a satellite communications centre such as Goonhilly in Cornwall, UK, the antennas used by NASA for communications with various space missions like that located at the Goldstone Deep Space Communications Complex or possibly a radio astronomy antenna. However, a vast number of antennas are more modest in size and are used in many applications. One very common example is a television antenna that can be seen on many domestic houses around the world. Despite the size of an antenna, its performance is of paramount importance. Whether small or large, every antenna needs to be optimized for its particular application to ensure that its performance is maximized. The function of the antenna for a receiver is to pick up the radio electromagnetic waves and convert them into electrical signals. Once they exist as electrical signals they are transferred from the antenna element itself into the receiver where they are amplified, filtered and demodu- lated to give the required audio output. Conversely in terms of a transmitter the antenna performs the function of converting the electrical energy into radiated electromagnetic signals. Antennas have a variety of properties. They can only operate efficiently over a given bandwidth. They also have an electrical impedance, and they are polarized, only pickin