Rat-race coupler

In its simplest form, it is a 3 dB coupler and is thus an alternative to a magic tee.

Compared to the magic tee, it has the advantage of being easy to realize in planar technologies such as microstrip and stripline, although waveguide rat races are also practical.

Unlike magic tees, a rat-race needs no matching structure to achieve correct operation.

The rat-race coupler has four ports, each placed one-quarter wavelength away from each other around the top half of the ring.

The full scattering matrix for an ideal 3 dB rat-race is Rat-race couplers are used to sum two in-phase combined signals with essentially no loss or to equally split an input signal with no resultant phase difference between its outputs.