Rat-race coupler
Encyclopedia
A rat-race coupler is a type of coupler
used in RF
and Microwave
systems. In its simplest form it is a 3dB 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 bottom half of the ring is three quarter wavelengths in length. The ring has a characteristic impedance of factor
compared to port impedance.
A signal input on port 1, will be split between ports 2 and 4, and port 3 will be isolated. The full scattering matrix
for an ideal 3dB 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 out and inputs. It is also possible to configure the coupler as a 180 degree phase-shifted output divider or to sum two 180 degree phase-shifted combined signals with almost no loss.
Power dividers and directional couplers
Power dividers and directional couplers are passive devices used in the field of radio technology. They couple a defined amount of the electromagnetic power in a transmission line to another port where it can be used in another circuit...
used in RF
Radio frequency
Radio frequency is a rate of oscillation in the range of about 3 kHz to 300 GHz, which corresponds to the frequency of radio waves, and the alternating currents which carry radio signals...
and Microwave
Microwave
Microwaves, a subset of radio waves, have wavelengths ranging from as long as one meter to as short as one millimeter, or equivalently, with frequencies between 300 MHz and 300 GHz. This broad definition includes both UHF and EHF , and various sources use different boundaries...
systems. In its simplest form it is a 3dB coupler and is thus an alternative to a magic tee
Magic Tee
thumb|right|A magic T consisting of four rectangular [[waveguide |waveguide]] meeting in a single three-dimensional junctionA magic tee is a hybrid or 3dB coupler used in microwave systems. It is an alternative to the rat-race coupler...
. Compared to the magic tee, it has the advantage of being easy to realize in planar technologies such as microstrip
Microstrip
Microstrip is a type of electrical transmission line which can be fabricated using printed circuit board technology, and is used to convey microwave-frequency signals. It consists of a conducting strip separated from a ground plane by a dielectric layer known as the substrate. Microwave components...
and stripline
Stripline
Stripline is a transverse electromagnetic transmission line medium, that was invented by Robert M. Barrett of the Air Force Cambridge Research Centre in the 1950s.- Description :...
, although waveguide
Waveguide
A waveguide is a structure which guides waves, such as electromagnetic waves or sound waves. There are different types of waveguides for each type of wave...
rat races are also practical. Unlike magic tees, a rat-race needs no matching
Impedance matching
In electronics, impedance matching is the practice of designing the input impedance of an electrical load to maximize the power transfer and/or minimize reflections from the load....
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 bottom half of the ring is three quarter wavelengths in length. The ring has a characteristic impedance of factor
compared to port impedance.A signal input on port 1, will be split between ports 2 and 4, and port 3 will be isolated. The full scattering matrix
Scattering parameters
Scattering parameters or S-parameters describe the electrical behavior of linear electrical networks when undergoing various steady state stimuli by electrical signals....
for an ideal 3dB 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 out and inputs. It is also possible to configure the coupler as a 180 degree phase-shifted output divider or to sum two 180 degree phase-shifted combined signals with almost no loss.