in microwave engineering, a section of transmission line that is joined to a main line and through which microwave energy is transmitted from an oscillator to a load. The transmission line may be a hollow-pipe wave guide, a dielectric wave guide, a strip transmission line, a coaxial cable, or a two-wire line. A stub is regarded as a type of two-terminal network with a specified inductive or capacitive reactance. It may be connected to a load either serially or in parallel. A variable-impedance stub may be designed either as a short-circuited or open-circuited section of transmission line of variable length or as a section of transmission line of fixed length with a variable capacitive or inductive reactance.
In microwave engineering, stubs are frequently used to match lines and loads. A stub is connected to a load in parallel in a section of a main line where the conductance of the main line is equal to the line’s characteristic admittance 1/ρ, where ρ is the line’s characteristic impedance. The susceptance of the main line is balanced by the admittance of the stub, which is equal in magnitude but opposite in sign. Examples of two-wire matching stubs are the Tatarinov stub and the Nadenenko stub.
Quarter-wave stubs, which have a very high input impedance, are used in coaxial and strip transmission lines as quarter-wave isolators. Systems of open-circuited and short-circuited stubs are employed in the technology of strip-line and coaxial electric filters. Variable-impedance stubs, which can be moved along a line (especially in coaxial and two-wire lines), and systems consisting of several stationary variable-impedance stubs are used as general-purpose impedance transformers; a system of several stationary stubs usually consists of three stubs separated from one another by a distance of λ/8, where λ is the wavelength in the line.
Variable-impedance stubs with a short-circuiting plunger, which are usually coaxial or wave-guide stubs, are widely used to measure the characteristics of microwave four-terminal networks. Stubs with a variable reactive load are very promising in the technology of adjustable strip-line and coaxial phase shifters, switches, and attenuators. In such stubs, the reactive load may be a p-i-n diode, a varactor, or an electrical component containing a ferroelectric.
Systems of stubs in combination with certain types of ion devices are used in antenna switches to switch antennas from the receiving mode to the transmitting mode. The ion devices change the reactive load in the stubs as a function of the power level of the oscillations in the line.
R. I. PERETS