286 BELL SYSTEM TECHNICAL JOURNAL 



difficult to evaluate the economic and practical problems. This 

 paper will, therefore, confine itself to a discussion of some of the 

 fundamental properties of wave guides derived either from calculation 

 or experiment. These properties include characteristic impedance, 

 attenuation and velocity 'of propagation as well as frequency, selec- 

 tivity and radiation. 



Nature and Properties of Wave Guides 



Analysis has shown that there are many kinds of waves that may be 

 propagated through cylindrical guides. However, four of them are 

 of unusual interest and are such as merit special consideration at this 

 time. All four have been experimented with in our laboratory and 

 their more important characteristics have been determined. This 

 experimental work has been paralleled by a mathematical theory ^ to 

 which it conforms most satisfactt)rily. 



A good mental picture of the nature of the waves propagated 

 through guides can probably best be had by abandoning the ordinary 

 concept of current electricity flowing in a "go and return" circuit 

 in favor of that of lines of electric and magnetic force. This latter 

 concept has, of course, always been applicable even for low-frequency 

 transmission over parallel wires or coaxial conductors but due to its 

 complexity in pictorial representation it has usually been avoided. In 

 the form of transmission with which we are now concerned, the field 

 point of view is almost necessary. 



Figure 1 is a pictorial representation based on this point of view 

 of the four types of waves mentioned above as found in a guide sur- 

 rounded by a metallic conductor. In these models the lines of electric 

 force have been represented by solid lines and the lines of magnetic 

 force have been shown by dotted lines. In the longitudinal sections, 

 the small open circles represent lines of force directed toward the 

 observer. The solid circles represent lines directed away from the ob- 

 server. The designations £o, -Ei, Ih and Th are convenient reminders 

 of certain characteristics of these waves. 



The first two waves have been designated as electric because there 

 is a component of electric force in the direction of propagation. For 

 similar reasons the latter have been known as magnetic waves. Such 

 a designation is, of course, rather arbitrary and should not be con- 

 strued to mean that either component resides alone. It is true here 

 as in other forms of electromagnetic waves with which we are generally 

 familiar, that both the electric and magnetic components are essential 

 to the very existence of the wave and that they may conveniently be 

 considered as different aspects of the same thing. 



