MICROWAVE RADAR TESTING 469 



one end, a test circuit connected to the other end will be coupled to the 

 incident wave, while theoretically the two couplings to the reflected wave 

 will differ by X/2 and therefore cancel one another. 



With such a device measurements may be made of the characteristics 

 of the incident wave independently of reflections. If the coupling to the 

 main line is not too close there is no appreciable effect on the incident wave, 

 and continuous monitoring can be had. Conversely, test signals applied 

 through the directional coupler will travel in tlie main guide in the proper 

 direction for testing the radar receiver. 



If the locations of the termination and the test connection point in Fig. 

 19a are reversed, the couplings to the main transmission line are also re- 

 \'ersed. Such an arrangement tlierefore permits measurement of the re- 

 flected power which in turn makes it possible to adjust for minimum re- 

 flected power and hence for minimum SWR. Comparison of the reflected 

 power with the direct power determines the SWR. For convenience in 

 measurement, two directional couplers pointed in opposite directions are 

 frequently used, the combination being referred to as a bi-directional coupler 

 (Fig. 19b). One advantage of this arrangement is that the ability to measure 

 the reflected power from the antenna and that part of transmission line 

 beyond the coupler provides means for detecting trouble in that part of the 

 system. Directional couplers may be applied to any type of transmission 

 line. Figure 19c shows a simple form of directional coupler for a coaxial 

 line. 



One characteristic of importance in a directional coupler is the coupling 

 loss. A small value of coupUng loss affords increased sensitivity of measure- 

 ment, while a sizable value is desirable to minimize reaction on the main 

 transmission line as well as for other reasons. A loss of around 20 db has 

 usually been found a good compromise. It is now the practice to in- 

 corporate a directional coupler in every radar to obtain a test connection 

 point. 



Due to unavoidable imperfections, a directional coupler never gives com- 

 plete cancellation for the undesired direction of transmission. The de- 

 parture from ideality is indicated by the directivity (also referred to as 

 front-to-back ratio) which is defined as the scalar ratio of the two powers 

 measured at the test connection point when the same amount of power is ap- 

 plied to the main guide, first in one direction and then in the other. For 

 measurements of the direct wave and of receiver characteristics, a moderate 

 directivity, of the order of 15 db or better, is sufficient. In measuring re- 

 flected i)ower, however, the directivity determines the amount of direct 

 power which appears at the point of measurement and therefore controls 

 accuracy. The chart of Fig. 20 will facilitate determination of the maximum 



