MICROWAVE RADAR TESTING 



465 



A block diagram of an arrangement employing a slotted line for measuring 

 standing waves is shown in Fig. 17. The oscillator source is commonly 

 followed by a pad or attenuator to prevent frequency pulling. The slotted 

 section may be either a coaxial or a wave guide line employing a mode which 

 is not disturbed by the presence of the slot (e.g. normal coa.xial mode; TEi,o 

 in rectangular wave guide ; TMo,\ in round wave guide). A traveling pick-up 

 probe or loop projects through the slot and couples energy from the line 

 into a detector which delivers d-c. or audio-frequency to the indicator. The 



20 



8 

 SWR-db 



Fig. 16 — Relations in mismatched transmission lines. 



probe is moved longitudinally to find points of maximum and minimum field 

 strength. To avoid distortion of the field within the line, the probe should 

 be small and should project only a short distance inside the slot. For 

 accurate results extreme care must be exercised in design and construction 

 to avoid variation in depth of immersion as the probe is moved. Several 

 slotted lines employed for standing wave measurements are shown in Fig. 18. 

 \ squeeze section consists of a section of rectangular guide with slots 

 milled in the center of both broad faces so that the width of the guide can 

 be varied by external deforming means. This changes the wave length in 



