A power divider, also called splitter, divides an incoming RF-signal to a number of outputs while maintaining the input impedance. Power dividers are used in communication systems, laboratories and radar applications.

A combiner operates in the opposite direction. A number of incoming signals are combined to one output. Combiners are used for increasing power in RF amplifiers.

Resistive power dividers are typically small and can be used on wide frequency bands starting at DC. Resistive power dividers cannot handle high power, loss is high and isolation is not the best. The design is fairly simple but with the right resistors they can have good frequency response. 3-way splitters are most common, but there are also star- and delta formed splitters.

Reactive power dividers have one input and two or more outputs. They can handle higher power. The design is either Wilkinson- or toroidal design. Reactive power dividers covers a wide frequency spectra, they offer low loss and high isolation between the ports.

The Wilkinson splitter, designed by Ernest Wilkson in the 60, can divide the input signal in the same phase and amplitude to the outputs. Main advantages are very good isolation between the ports, low loss and an attractive price for the performance. They can however be limited on band width and sometimes size is the problem.

Directional couplers only connects signal in one directions. Power on the input is transferred to a terminated port and not to the connecter port. A coupler that divides the power equally on two ports is called a hybrid coupler.

Directional couplers with coaxial connectors or wave guides are used between a sender or an amplifier and the load in order to measure or monitor microwave power levels. A coupler provide the possibility to tap a signal on a port to do measurements without moving parts or adjustments.

Directional couplers of coaxial type are available from frequencies from a few MHz to about 50 GHz.
 
Typically they are designed for low VSWR (Voltage Standing Wave Ratio) and low loss. Directional couplers can be of single type or double type. The single version has one main signal path with a terminated path in parallel. The double typ is basically two single couplers that are connected so that both the forwarded signal and that the reflected signal can be measured.

Couplers and splitters are also available with wave guide connector or for mounting on printed circuit boards.

A common type of coupler is the 3 dB, 90° hybrid coupler with four ports, either for dividing the signal equal in amplitude with a 90° phase shift between the ports or to combine two signals with good isolation between the ports.

The rat race coupler is 3 dB, 180° hybrid coupler with four ports that either divides the input signal equal with 180° phase shift or it combines two signals.