536 ANTENNAS AND RF COMPONENTS 



the proper transmission lines requires familiarity with the system specifi- 

 cations, environmental requirements, and the characteristics of the various 

 line types. 



There are many types of RF transmission lines, differing basically in 

 geometry and in mode of propagation. Fig. 10-11 shows cross sections of 

 several types of transmission lines and the electric field configurations of 

 representative modes. ^^ Each transmission line mode represents a different 

 and unique distribution of the electric and magnetic fields of the propa- 

 gating wave on or within the transmission line. There is no limit to the 

 number of modes which can propagate in a transmission line if no upper 

 frequency limit is specified. Transmission lines are generally operated in 

 the lowest-order mode, however, and an upper frequency limit is specified to 

 prevent propagation of energy in the higher-order modes. Higher-order 

 mode and multimode transmission lines are often used to advantage in 

 special applications. 



10-11 TYPES OF TRANSMISSION LINES AND MODES OF 

 PROPAGATION 



Transmission line modes are oi four general classes:^" (1) the TE or 

 transverse-electric modes (sometimes referred to as H waves) in which the 

 electric field is everywhere perpendicular to the direction of propagation; 

 (2) the TM or transverse-inagnetic modes (sometimes referred to as E waves) 

 in which the magnetic field is everywhere perpendicular to the direction of 

 propagation ; (3) the TEM or transverse-electromagnetic modes in which both 

 the electric and magnetic fields are perpendicular to the direction of 

 propagation of the wave; and (4) the HEM or hybrid electromagnetic modes 

 having components oi both electric and magnetic fields in the direction of 

 propagation. 



Electromagnetic energy always propagates at group velocity, which never 

 exceeds c, the velocity of light. The wavelength X^ in a transmission line 

 may be greater than, equal to, or less than the free-space wavelength, 

 X, depending upon the type of transmission line, the mode of propagation, 

 and whether the line is air or dielectric filled. T\ve. phase velocity Vp of a. wave 

 in a transmission line can be determined from a measurement of the 

 wavelength in the line and the relation 



Vp = ^c (10-20) 



where c is the velocity of light. 



'^For a very descriptive discussion of waveguide modes see George C. Southworth, Principles 

 and Applications oj Waveguide Transmission, Chapt. 5, D. Van Nostrand Co., Inc., Princeton, 

 N. J., 1950. 



20"IRE Standards on Antennas and Waveguides: Definition of Terms, 1953," (53IRE2.S1), 

 Proc. IRE,40, No. 12, (December 1953). 



