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Antennas, or AerialsThe pages that follow will deal largely with transmitting antennas, and little need be said about receiving antennas. This is because of the application, by Carson, of the reciprocity theorem (page 151) to antennas. This theorem, often called the Rayleigh-Carson reciprocity theorem, because it was due to Rayleigh in its original form, may be stated: If a radio-frequency voltage E, when inserted at a given point x in antenna 1, causes a current I to flow at a given point y in distant antenna 2, then the same voltage E when inserted at point y in antenna 2 will cause the same current I to flow at point x in antenna 1. This theorem holds only if the transmission path remains constant. If a given antenna has certain directional properties in transmitting, according to this theorem it has the same directional properties in receiving, and vice versa. There is another important reason for concentrating on transmitting antennas. They are usually designed and constructed to radiate large amounts of power efficiently and to operate at a single narrow frequency band. On the other hand receiving antennas often must operate over a rather wide band and should not be efficient at one part of the band at the expense of other parts. An antenna, or aerial, is defined34 as "a means for radiating or receiving radio waves." Many types, shapes, and lengths of transmitting and receiving antennas are used, and they have various positions with regard to the surface of the earth and objects such as buildings. Attention will be focused on basic types, first as they behave in free space, and then as their characteristics are modified by the presence of the earth. In studying antennas, it is convenient to use the classification periodic antennas and aperiodic antennas, the latter being discussed on page 479.
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Last Update: 2011-05-30