MICROWAVE RADAR TESTING 



471 



error that nia\- occur in measuring different values of SWR with various as- 

 sumed directivities. 



With a simple two-hole coupler, the directivity deteriorates rapidly as 

 the frequency departs from that corresponding to quarter-wave spacing. 



2.2 2.6 



MEASURED VS WR 



Fig. 20 — Error in standing wave measurements caused by directivity of directional 

 coupler. 



By providing additional couplings suitably spaced, the residuals from 

 different sets of couplings can also be cancelled against one another and the 

 directivity versus frequency characteristic can be materially broadened. 

 With multiple hole couplings a minimum directivity of 26 to 30 db over a 

 frequency band of 10 to 20% is readily practicable in quantity production, 



