RF combiners and dividers, also known as combiners and splitters, are multi-port devices and modules that combine or split RF signals between a single port and the other ports. These include active splitters, flux coupled baluns, Wilkinson splitter/combiners and Doherty combiners.

Active RF splitters ICs are available for applications like RF distribution in cable splitter modules, multi-tuner set-top boxes and CATV systems. These are specified with fixed gain, port isolation, frequency band and amplifier noise figure (NF) to assess degradation in signal quality. Devices are available with 2, 3 or 4 output ports.  A passive flux coupled balun splitter is a transformer with the secondary winding center tapped to ground. A signal across the primary winding induces 180^o out-of-phase signals to be output on the ends of the secondary coil windings relative to each other.

RF combiners constructed from passive elements are reciprocal networks able to be used in reverse to split signals. These utilize quarter wave sections with implementations that include lumped passive, microstrip, transmission line, waveguide and coax.  Quarter wave sections cause addition or cancellation of signals at ports. A Wilkinson power splitter/combiner is a passive three port device. Quarter wave sections from two ports sum signals constructively at a combining port but isolation due to a half wave length phase difference between splitter ports. A resistor placed between the splitter ports dissipates any reflected power caused by mismatch to provide port isolation.

Doherty combiners are intended for use as part of a Doherty amplifier – an RF amplifier designed for efficient amplification that consists of two differently biased amplifiers in parallel (carrier and peaking). They consist of two offset lines and a 90-degree transmission line impedance inverter. The combined port presents a 25 W impedance due to how the amplifiers are summed. This is matched to 50 W by using a 35 W transmission line acting as a quarter wave impedance transformer. This transforms the impedance presented by the load to being its dual (reciprocal) impedance:

Z_in = Z_o²/Z_L

 

Where Z_o = 35 W characteristic impedance of the quarter wave section

Z_L = 50 W termination

Important combiner specifications include the isolation between the nodes being split or combined from, phase balance between ports, the insertion loss (attenuation) between the split ports and the combined port and the port match expressed as a return loss.