Directional Couplers are Four Port devices. The design uses the coupling between two transmission lines. The distance between the main line where the energy is propagating through, and the coupling arm is a function of the energy coupled. All ports of the couplers are matched to 50 Ohms impedance.
Theoretically, the Power transmitted into port J1 is partially coupled into port J3. At port J2 the remaining power will be received. In reality however, the components behave differently:
Injected Power into port J1, minus Coupled Power at Port J3, minus Insertion Loss of the Circuit, minus power reflected into port J4, equals the Output Power at port J2. An ideal coupler, would not receive power at port J4.
The RF-Circuit between the input and output ports is called the Main Line.
The transmission line into which the main signal is coupled, is called the Coupled Line. Normally one port of the coupled line is terminated with a 50 Ohms load.
The Coupling Characteristics are limited over finite frequency bands. Using creative ideas, Spectrum Elektrotechnik GmbH has designed Directional Couplers for higher power applications, operating over wider frequency ranges of more than one octave.
High-power Directional Coupler, used in an L-Band Radar Aplication
The coupled power can be 40 dB below the power introduced into the main arm, depending on the specification. It varies with frequency at a limited amount. But the variation, or ripples can usually be optimized very well.
Every transmission line will cause loss, as energy will be dissipating into the circuit. A good design will offer good trade off between size of the component and its loss.
The power ratio between isolated port and coupled port is called Directivity. Ideal would be no power at Port 4, resulting in infinite Directivity, but in actual the isolated port never will become completely power free.
The power ratio between port 4 and port 1 is called Isolation. Isolation is also another term to measure reflections into port 4.
Every microwave component shows reflections and discontinuities within the circuit, as no design can be perfect, and manufacturing tolerances do not allow perfect designs anyway. Coupled port reduces the directivity by an amount, equal to the return loss of the mismatches.