464 BELL SYSTEM TECHNICAL JOURNAL 



may be considered as representing two waves transmitted in opposite direc- 

 tions, one (the incident wave) from the generator toward the load, the other 

 (the reflected wave) from the load toward the generator. The summation 

 is a standing wave pattern. The standing wave ratio (SWR) is defined as 

 the ratio of the wave amplitude at a maximum point (anti-node) to that at a 

 minimum point (node). If the standing wave ratio is stated as a numeric, 

 it is necessary to specify whether it applies to voltage (VSWR) or power 

 (PSWR). Possibility of ambiguity is avoided by stating the ratio in db. 



The ratio of the reflected current to the incident current is the reflection 

 coefficient, here designated as p. The value of the reflection coefficient is 

 given both in magnitude and phase by 



where Zq is the characteristic impedance of the line and Z is the load imped- 

 ance. The reflection coefficient is related to the standing wave ratio as 

 follows : 



VSWR = a = J-±^ , or, p = ^ (ll) 



1 - p 0- + 1 



Plots of the relationships are shown in Fig. 16. 



The reduction of radiated power due to reflection losses in a radar trans- 

 mission line, while important, is usually less serious than other effects of 

 impedance irregularities. Since the load impedance reacts on the oscillator 

 circuit, the frequency and output of most transmitter tubes are quite sensi- 

 tive to load impedance. If the line is electrically long, so that its impedance 

 varies rapidly with frequency, marked instability of oscillator frequency 

 may occur, a condition referred to as "long line effect." 



Since radar transmission lines contain many potential sources of imped- 

 ance discontinuity, including not only the antenna but a variety of couplings, 

 bends, wobble joints, rotating joints, switches, etc., measurements of stand- 

 ing wave ratio are frequently required. The need for such measurements 

 depends in part on whether the line is "preplumbed" or is provided with 

 field adjustments. 



Devices 



Standing waves may be detected and measured by several different types 

 of devices, including (1) a slotted line, (2) a squeeze section, (3) a directional 

 coupler and (4) a hybrid T. All of these furnish information on the mag- 

 nitude of the standing wave ratio. In some cases phase information may 

 be obtained also, which permits determination of impedance,' but this 

 knowledge, while useful in the laborator}', is seldom required in field work. 



