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Directional coupler (20 MHz to 50 GHz)

As a RF technology component the directional coupler serves to decouple the electromagnetic waves that run in the waveguides in a predefined direction. This allows the separate measurement of the waves running to and fro within a cable. With the phase-stable decoupling of a small part of the high frequency signal of the wave that is running to or fro, for example, the power can be measured bi-directionally. The directional coupler's main area of application is, however, mostly in monitoring high frequency signals.

Checking the signal strengths in the area of an antenna system's supply line is an example of a possible measurement. In this case the voltage standing wave ratio (VSWR) is checked. The voltage standing wave ratio is the ratio between waves running to and fro. The optimum ratio between the two waves is "1". The better the result of the voltage standing wave ratio, the higher the output of the available transmitter power on the antenna. On the basis of the result, the supply line between antenna and transmitter output can be adjusted precisely.

Directional couplers and dual directional couplers in coaxial technology have a system impedance of 50 or 75 Ohm. For radio and radar technology, mostly directional couplers with an impedance of 50 Ohm are used. Antenna systems for satellite, cable or terrestrial television require an impedance of 75 Ohm. The directivity of directional couplers is at least 18 dB.

The coupling arms of conventional directional couplers are parallel to the main transmission line. A vertical positioning of the coupling arms is required, in particular, at high bandwidths. Cross-couplers meet these requirements. Depending on the integrated interface, cross-couplers are suitable for waveguides and for coaxial cables.

The loop coupler is another design type. When taking measurements the transmission signal is decoupled via a strip positioned in the conductor. Loop couplers are also used in ferrite antennae, where they serve to increase the voltage induction and improve fault elimination and directivity.









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