468 BELL SYSTEM TECHNICAL JOURNAL 



the guide, so that maximum and minimum values may be determined by a 

 fixed probe and indicator. 



The use of a directional coupler for standing wave measurement is dis- 

 cussed in the following section. 



Another useful device for standing wave measurement is the "hybrid T" 

 or "magic T". This is a sort of microwave bridge, consisting of a main 

 wave guide to which an E plane branch and an H plane branch are joined in 

 the same physical plane. With matched terminations of the two ends of the 

 main guide, the two branches are conjugate. Terminating one end of the 

 main guide in the unknown impedance and the other in a matched termina- 

 tion, the degree of impedance mismatch of the unknown is indicated by the 

 magnitude of the reflected wave which appears in one branch when energ>' 

 is fed into the other. 



Directional Couplers 



Accurate measurement of transmitter power and receiver sensitivity 

 requires a couplmg path of known loss between the radar and the test set. 

 The first method employed for this was to place a portable test antenna 

 (see Fig. 2) in the field of the radar antenna. Depending on the frequency 

 range, this test antenna took the form of a dipole,^^ with or without a small 

 reflector, or an electromagnetic horn." With this method it was necessary 

 to calibrate the loss of the space coupling path between the two antennas. 

 Smce it proved difficult to locate the test antenna at exactly the same 

 point and to be sure that the main antenna pattern remained the same, a 

 separate calibration of the coupling loss was usually required whenever a 

 measurement was made. 



An alternative method was to place a single probe in the radar transmission 

 line. This introduced another sort of difficulty. Accuracy of measurement 

 was vitiated by the presence of standing waves which rendered the probe 

 pick-up a function of frequency and of location with respect to the irregu- 

 larities. A highly satisfactory answer to the entire problem was found in 

 a device which is called a directional coupler because it couples only to the 

 wave propagated in one direction. In its simplest form a directional coupler 

 consists of two couplings to the main transmission line, which add for one 

 direction of transmission and cancel for the other. Thus, for example, 

 Fig. 10a shows a form of directional coupler for wave guide which is placed 

 in the radar transmission line at the point indicated schematically in Fig. 1 . 

 An auxiliary wave guide is coupled to the main guide through two identical 

 orifices spaced Xff/4 between centers (or more generally n\g/4: where n is 

 an odd integer). Assuming the incident and reflected waves in the main 

 guide to be directed as shown, and the auxiliary guide to be terminated on 



