Free Quarter-Wave Coaxial Stub Filter Calculator
Calculate precision quarter-wave (λ/4) and half-wave (λ/2) coaxial stub notch filters for ham radio, repeater isolation, and RF harmonic rejection.
📡 Operating Frequency & Coax Type
📊 Cut Lengths & Harmonic Behavior
Recommended Tools & Equipment
Tested hardware and components for high reliability
Frequently Asked Questions
How do I fine-tune a coax stub filter?
Always cut your coaxial stub about 1/2 to 1 inch longer than the theoretical calculation. Connect the stub via a T-connector to a Vector Network Analyzer (NanoVNA) or antenna analyzer, and snip 1/8 inch off the end at a time until the resonant dip aligns precisely with your target frequency.
How much attenuation does a quarter-wave coax notch stub provide?
A quality coax stub typically provides between 25 dB and 35 dB of notch attenuation at its resonant frequency. Lower-loss cables (like LMR-400 or RG-213) have higher Q factors, creating deeper notches with sharper attenuation skirts.
Can a coax stub handle high transmitter power (100W to 1,500W)?
Yes, but be careful with open-circuit stubs. Because an open quarter-wave stub inverts impedance, extremely high RF voltages develop at the open tip. For high power (500W+), cut back the braid 1/2 inch, seal the dielectric with silicone or heat-shrink, and use high-voltage cable.
What is the difference between a band-stop stub and a harmonic stub?
An open-circuit quarter-wave stub creates notches at the fundamental and all odd harmonics (1f, 3f, 5f). A short-circuited half-wave stub passes the fundamental frequency and notches out the second harmonic (2f).