Ratrace microstrip coupler
Image of the Ratrace microstrip coupler.

The rat-race may be considered as the microstrip equivalent of the waveguide Magic-T already designed by Antenna Magus. It is a four-port network which produces a 180 degree phase shift between the two output ports, hence the name "180 degree hybrid junction".

With reference to the port numbers shown below - when fed at port 1 (also known as the Sum port), two equal and in-phase signals are received at ports 2 and 3; while when fed at port 4 (also known as the Difference port), two equal but out-of-phase signals are received at ports 2 and 3. Good isolation is achieved between the two input ports (1 and 4).

The rat-race ring hybrid with its relevant line lengths and impedances
Typical reflection coefficient behaviour versus frequency with an input applied at port 1
Typical phase behaviour versus frequency with an input applied at port 1
Typical reflection coefficient behaviour versus frequency with an input applied at port 4
Typical phase behaviour versus frequency with an input applied at port 4