RF and Microwave Engineering: Fundamentals of Wireless Communications

6.10 Directional Coupler

If the amount of power that is separated is rather small or input and output require galvanic isolation, directional couplers can be used. A directional coupler basically consists of two coupled parallel lines with a length of = λ/4 separated by a distance s (see Figure 6.41).

Figure 6.41 Directional coupler consisting of two parallel lines ( = λ/4).

This kind of coupler is a backward coupler, since the fraction of power k²P that is coupled in the adjacent line is delivered to port 4. Port 3 at the other end of the line is isolated. The main part (1 − k²)P of the incident power is transferred onto the direct path to port 2.

The coupling coefficient is commonly expressed in logarithmic scale as

6.59

A 20 dB directional coupler (k = 0.1) decouples 1% of the incident power to port 4. 99% of the incident power propagates through the main line to port 2. Directional couplers are used in measurement, for example network analysers use directional couplers to evaluate forward travelling and backward travelling waves on transmission lines (see Section 5.7).

According ...