548 ANTENNAS AND RF COMPONENTS 



Directional Couplers, A directional coupler is a device for coupling 

 signals directionally between two transmission lines, or between a main 

 guide and an auxiliary guide, with a fixed power division ratio. Referring to 

 Fig. 10-20, power entering the main guide at A divides between the main 

 guide and the auxiliary guide. The ratio between the power input and the 

 power coupled to the auxiliary arm C, expressed in db, is called the coupling 

 of the device. Thus a directional coupler in which 1 per cent of the input 

 power to the main guide reaches the auxiliary guide has a coupling of 20 db. 

 The balance of the input poWer continues down the main guide. The 

 directivity of the coupler is the ratio, expressed in decibels, of the auxiliary 

 guide power flowing to C and that reaching D, with input at A. To measure 

 directivity accurately, both B and C must be terminated with reflectionless 

 loads. Arm D of the directional coupler is normally terminated in a well- 

 matched load. 



The directional coupler^^ is often used as a power divider and for injecting 

 signals into the main guide through the padding or isolation provided by the 

 directional coupler. A more frequent use is for sampling or monitoring 

 power flow in the main guide. Thus if a transmitter is attached to A and 

 an antenna at B (Fig. 10-20), the amount of power flowing from A to B can 



Fig. 10-20 Waveguide Directional Coupler. 



be monitored by a detector on C. If the detector is instead attached to arm 

 D, it will sample the power reflected from the antenna on B to give a relative 

 indication of the condition of match of the antenna. Directional couplers 

 in which coupling ranges from 3 db to 30 db or more are most common, with 

 directivity of up to 40 db possible in high-quality couplers. 



A bidirectional coupler is a dual directional coupler which permits 

 simultaneous sampling of signal flow in both directions in the main guide. 

 A bidirectional coupler is often used in reflectometer setups. 



Hybrid Junctions. The term hybrid junction implies a multiport 

 transmission line component in which certain ports couple in common to 



sipor a detailed discussion of directional couplers see C. G. Montgomery, Technique of 

 Microwave Measurements, Chapt. 14, McGraw-Hill Book Co., Inc., New York, 1947. 



