RF Couplers

RF couplers are devices used to couple energy from one structure transmission line, cable or circuit board trace onto an additional transmission line, cable or circuit board trace.
Often when operating at RF frequencies, it is not possible to simply connect two lines together to split the signal and have it available on two separate lines or wires. Anytime a signal is to be passed onto an additional line, creating a three-way split, a coupler of some sort is required. A coupler is needed because the introduction of a third line onto a high-speed transmission line or cable changes the overall impedance of that system at the junction of the split. It is desirable to have 50 ohm impedance at all locations of a transmission line. However, when two 50 ohm lines are introduced in parallel, or a split, the result is a lower impedance at the split than the desired 50 ohm impedance. Once the impedance discontinuity is observed by an electromagnetic wave or an RF signal, it will cause reflections on the line, and degrade the signal quality. 
An RF Coupler allows for a split or junction to be made, which will lower or change the impedance from the nominal 50 ohms. The coupler masks the effect of the split or junction and makes the impedance appear unchanged. In short, an RF coupler will couple energy onto the third line while maintaining the desired 50-ohm impedance. 
RF couplers can be implemented and built in several different ways. A common type of coupler is called a directional coupler, which only allows for energy to be coupled onto the third port in one direction. It is directionally dependent. Most RF lab equipment, such as vector network analyzers, use a form of a directional coupler in order to probe or sense energy being reflected or passed through the network under test.

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