A rat-race coupler is a circular device often used to split or combine radio frequency and microwave signals. The size of the conductor ring in the coupler is typically equal to one-and-a-half wavelengths, and there are four ports that are separated by 90°. Output signals sometimes undergo a phase-shift of 180°, meaning the waveform on a graph will shift a certain distance based on the size of the change.
Also called a hybrid coupler, the device typically generates no negative effects on the calibration and function of a current receiver. The ports on a rat-race coupler are generally placed one-quarter of a wavelength apart from one another. These ports are usually located on the top half, while the bottom part of the ring runs for three-quarters of a wavelength with no inputs or outputs. If a signal enters port one, then usually it will be split between the second and fourth port; the third port will typically be isolated from the signal path.
The rat-race coupler can also be used as a splitter that keeps the signals in-phase. The third port is often isolated, but by using port four as an input, sometimes the second one will be isolated. In another configuration, the first and third ports can be isolated from one another. An unequal-split rat-race coupler typically pairs up the four segments, with each functioning at particular impedances, or resistance levels to electrical or signal currents. Signal bandwidth and frequency are typically dependent on these types of patterns.
High-power signals can usually be passed through a rat-race coupler without a loss in strength. The device can also combine signals that are mixed, and split those coming from amplifiers, and which need to be picked up by certain kinds of receivers. It is often suited for use with signal distribution systems in buildings.
While it is possible to divide output signals that are out of phase, a rat-race coupler typically combines two in-phase signals or splits a signal with no phase difference. Signals do not have to be matched by any structural components either. These types of couplers are often part of the connection between an antenna and a transmitter, and the option is usually cost-saving in comparison to equipment that can perform similar tasks. The function of a rat-race coupler is often described mathematically, so understanding how signal performance is calculated can help determine the best configuration to use.