Recent advances in the use of coupled transmission lines as directional couplers

If two similar coaxial transmission lines are coupled by sharing a common outer conductor for part of their length, so as to form a 3-conductor line, they constitute a symmetrical directional coupler. In work on coaxial directional couplers at microwave frequencies up to 7 Gc/s, new designs of coupled transmission lines have been developed for use as power monitors and variable attenuators.