In general, a waveguide directional coupler is a four-port junction equipment made from two waveguide sections joined together in such a way that a known fraction of a microwave signal propagating in one waveguide will couple a microwave signal into the other waveguide. 

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The direction of propagation of the coupled signal is determined by the direction of the signal in the first waveguide. Directional couplers are commonly used to measure signal parameters such as the power of a transmitter or reflected signal and reflection coefficients without disturbing the transmission. They are also used along with the appropriate circuitry, to regulate the output of microwave signal generators. 

There are different techniques for realizing waveguide directional couplers. One of the two main types is formed from two crossed waveguides sharing a common wall. A cross guide directional coupler is formed by the superposition of two waveguide sections at right angles to each other. Two cruciform openings are made in the common wall. Cruciform openings are preferred to circular openings because the discontinuity caused by a cruciform is smaller than that caused by a circle for the same degree of coupling. 

The main parameters which are used to compare directional couplers are: insertion loss which is the loss of signal power resulting from the insertion of a device in a transmission line, isolation is the ability to prevent a signal from appearing at the port in the circuit where it is unwanted and tap loss which is measure of how much lower the signal level at the tap output is, compared to the signal level at the input.