MEASUREMENT OF IMPEDANCE MISMATCHES 453 



branch D so that the voltages due to the flanges of the hybrid junction are 

 cancelled. 



The vector diagram of the voltage at C might be represented as in Fig. 6. 

 Vector 0-1 represents the voltage at C when input is applied to A , due to 

 the impedance mismatch at the coupling BE. Vector 1-2 represents that 

 due to the mismatch at coupling D. Vector 2-3 represents that due to 

 the mismatch at the variable attenuator, (which will usually change in 

 magnitude and probably in phase for different settings). Vector 3-0 repre- 

 sents the voltage at C due to the cancelling voltage from the branch D. 

 Its phase can be varied by changing the position of the movable shorting 

 piston. Its magnitude can be varied by changing the setting of the variable 

 attenuator. When the adjustment is accomplished effectively no power 

 reaches the detector. It is necessary that the reflection coefficients of 



Fig. 6 — Vector diagram of voltages at terminal C. 



couplings A, B, and C be small so that multiple reflections caused by them 

 will not affect the accuracy of measurement. 



The reflected power from coupling FG may be measured when wave- 

 guide GH is connected to waveguide EF as shown in Fig. 5 and termination 

 Z is located within waveguide GH. The detector attenuator setting might 

 he A5 . The incident power may be measured as before when termination 

 Z is withdrawn from the waveguide EF and the piece of waveguide GH is 

 replaced by a fixed shorting plate. 



Wi, (due to reflection coefficient of the coupling FG) = Ai — A^ (17) 



IV. Termination Z Imperfect, Four Couplings on Hybrid Junction 



In Fig. 5 if the movable termination Z is not perfect, there will be two 

 reflected voltages in branch B when the adjustment is being made. The 

 vector diagram of the voltage at C might be as in Fig. 7. This is the same 

 as Fig. 6 except that a new vector 0-5 represents the voltage due to the 

 mismatch of the movable termination Z. The adjustment is accomplished 

 the same as in the last section except that the criterion is to have no change 

 in detector output as the movable termination Z is moved axially over a 



