Wave Propagation in Free Space

The concept of propagation refers to the various ways by which an electromagnetic (EM) wave travels from the transmitting antenna to the receiving antenna. Propagation of EM wave may also be regarded as a means of transferring energy or information from one point (a transmitter) to another (a receiver). The transmission of analog or digital information from one point to another is the largest application of microwave frequencies. Therefore, understanding the principles of wave propagation is of practical interest to microwave engineers. Engineers cannot completely apply formulas or models for microwave system design without an adequate knowledge of the propagation issue. Wave propagation at microwave frequencies has a number of advantages (Veley, 1987). First, microwaves can accommodate very wide bandwidth without causing interference problems because microwave frequencies are so high. Consequently, a huge amount of information can be handled by a single microwave carrier. Second, microwaves propagate along a straight line like light rays and are not bent by the ionosphere as are lower frequency signals. This straight-line propagation makes communication satellites possible. In essence, a communication satellite is a microwave relay station that is used in linking two or more grounded-based transmitters and receivers. Third, it is feasible to design highly directive antenna systems of a reasonable size at microwave frequencies. Fourth, compared with low-frequency electromagnetic waves, microwave energy is more easily controlled, concentrated, and directed. This makes it useful for cooking, drying, and physical diathermy. Moreover, the microwave spectrum provides more communication channels than the radio and TV bands. With the ever-increasing demand for channel allocation, microwave communication has become more common. EM wave propagation is achieved through guided structures such as transmission lines and waveguides or through space. In this chapter, our major focus is on EM wave propagation in free space and the power resident in the wave. EM wave propagation can be described by two complimentary models. The physicist attempts a theoretical model based on universal laws, which extends the field of application more widely than currently known. The engineer prefers an empirical model based on measurements, which can be used immediately. This chapter presents the complimentary standpoints by discussing theoretical factors affecting wave propagation and the semiempirical rules allowing handy engineering calculations. First, we consider wave propagation in idealistic simple media, with no obstacles. We later consider the more realistic case of wave propagation around the earth, as influenced by its curvature and by atmospheric conditions.