Radio Propagation Fundamentals

The power radiated from a transmitting antenna is ordinarily spread over a relatively large area. As a result the power available at most receiving antennas is only a small fraction of the radiated power. This ratio of radiated power to received power is called the radio transmission loss and its magnitude in some cases may be as large as 1015 to 1020 (150 to 200 decibels). The transmission loss between the transmitting and receiving antennas determines whether the received signal will bo useful. Each radio * This paper lias been prepared for use in a proposed " A n t e n n a H a n d b o o k " to be published by McGraw-Hill. 593 ()02 T H E WELL SYSTEM T E C H N I C A L J O U R N A L , MAY 1 9 5 7 system has a maximum allowable transmission loss which, if exceeded, results in either poor quality or poor reliability. Reasonably accurate predictions of transmission loss can be made on paths that approximate the ideals of either free space or plane earth. On many paths of interest, however, the path geometry or atmospheric conditions differ so much from the basic assumptions that absolute accuracy cannot be expected; nevertheless, worthwhile results can be obtained by using two or more different methods of analysis to "box in" the answer. The basic concept in estimating radio transmission loss is the loss expected in free space; that is, in a region free of all objects that might absorb or reflect radio energy. This concept is essentially the inverse square law in optics applied to radio transmission.