g up waves of a certain polarization. Many of these factors are very important and if the antenna is to give the optimum performance it is necessary to ensure it has the correct properties and that it is set up correctly. Resonance and bandwidth An antenna is a form of tuned circuit that acts like an inductor at some frequencies and a capacitor at others. Like a tuned circuit it has a resonant Antenna systems 77 Figure 4.1 The 70m dish antenna at the Goldstone Deep Space Communications Complex, located in the Mojave Desert in California, is one of three complexes which comprise NASA's Deep Space Network (DSN). The DSN provides radio communications for all of NASA's interplanetary spacecraft and is also utilized for radio astronomy and radar observations of the solar system and the universe (courtesy NASA/JPL/Caltech) frequency, and most antennas are operated at resonance. In view of this there is only a limited band over which the antenna operates efficiently. These characteristics are governed largely by the dimensions of the antenna. The larger the antenna or more strictly the antenna elements, the lower the resonant frequency. For example, antennas for UHF terrestrial television have relatively small elements, while those for VHF broadcast sound FM have larger elements indicating a lower frequency. Antennas for short wave applications are larger still. The bandwidth is particularly important where transmitters are concerned. If the transmitter is operated outside the bandwidth of the antenna, it is possible that damage may occur. In addition to this the signal radiated by the antenna may be less for a number of reasons. For receiving purposes the performance of the antenna is less critical in some respects. It can be operated outside its normal bandwidth without any fear of damage to the set. Even a random length of wire will pick up signals, and it may be possible to receive several distant stations. However, for the best reception it is necessary to ensure that the performance of the antenna is optimum. Ofte