Coaxial Stub Length (λ/4)

Formula

Description

A quarter-wave coaxial stub is a length of transmission line used as a frequency-selective filter or impedance transformer. An open-circuited quarter-wave stub presents a short circuit at its resonant frequency and can be used as a notch filter to reject interference. A short-circuited quarter-wave stub presents an open circuit at resonance and passes the desired signal. Stubs are commonly used in amateur radio duplexers, harmonic filters, and phasing harness systems for antenna arrays.

Variables

• l — Physical stub length (m)
• f — Center frequency (Hz)

Practical Notes

The physical length must be multiplied by the velocity factor of the coaxial cable used. Common velocity factors: RG-58 (solid PE) = 0.66, RG-8X (foam PE) = 0.78, RG-213 (solid PE) = 0.66, LMR-400 (foam PE) = 0.85. Quarter-wave stubs have a bandwidth that depends on the cable impedance and the source/load impedances. Multiple stubs can be cascaded for sharper filtering.