stub

A stub is a section of transmission line or waveguide deliberately terminated at one end, with the other end connected to the main signal path. Unlike a complete transmission line that carries signals from source to load, a stub acts as a reactive element: it reflects energy back into the circuit rather than absorbing it. The stub's physical length and the frequency of the signal determine whether it behaves as a capacitor, inductor, or resonator.

In coaxial cable systems operating below microwave frequencies, stubs are typically short lengths of 50-ohm or 75-ohm cable soldered or bolted to the main transmission line. In waveguide applications at gigahertz frequencies, stubs are precision-machined cavities or rod probes extending into the waveguide cross-section. The open end may be left bare, short-circuited with a metal plate, or terminated with a sliding short that allows the effective length to be varied.

Stubs serve three main purposes. A quarter-wave stub (stub length equal to one-quarter wavelength at operating frequency) presents the opposite impedance to what appears at its junction, making it useful for impedance matching. A half-wave stub has minimal effect on the circuit. At resonant lengths, a stub acts as a high-Q filter element, absorbing power at its resonant frequency while reflecting other frequencies. Stub-tuned circuits are common in antenna feeds, filter networks, and amplifier bias networks where precise phase and magnitude control matter.

The critical variable is electrical length, not physical length. A stub that is exactly one-quarter wavelength at 2.4 GHz behaves very differently at 1.2 GHz, where it becomes one-eighth wavelength. This frequency dependence makes stubs useful for narrowband tuning but problematic across wide frequency ranges. Sliding stubs, where a movable short circuit runs along the line, allow real-time adjustment of electrical length for impedance matching or frequency tuning without changing hardware.

Stubs fail when corrosion enters the junction, when the dielectric inside coaxial stubs absorbs moisture, or when mechanical movement of a sliding contact degrades electrical contact. In waveguide stubs, metal fatigue at the attachment point or accumulation of dust and moisture inside the waveguide reduces Q factor and shifts resonant frequency. The term comes from its appearance as a short branch off the main line, resembling a stub of a pencil or branch.