Stub Length Calculator
Calculate the physical length of an RF transmission-line stub from frequency, velocity factor, and electrical angle. Includes guided wavelength, propagation delay, and common wavelength fractions.
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Enter parameters and click Calculate to view results
Formula & Theory
lambdag = (c × VF) / f | L = (theta / 360°) × lambdag | t = L / (VF × c)This formula is used to calculate antenna parameters for stub length calculator.
Overview
The Stub Length Calculator converts a target electrical angle into the exact physical stub length needed for RF matching networks, resonant sections, and phase-shift applications. Enter your operating frequency in MHz, the transmission line's velocity factor, and the desired electrical angle in degrees (such as 90° for a quarter-wave stub or 180° for a half-wave section), and the calculator returns the guided wavelength, physical stub length in meters, centimeters, millimeters, and inches, the equivalent wavelength fraction (lambda/8, lambda/4, lambda/2, and so on), propagation velocity, propagation delay, and the typical RF application for that electrical angle. This angle-first approach makes it especially useful for designing quarter-wave matching stubs, half-wave resonant sections, phase-shifting elements, and any RF design where the required electrical behavior — not just the physical distance — is the starting point.
Input Guide
Enter Frequency exactly as shown on the calculator. Confirm every unit, selected option, and decimal position before calculating so the stub length result matches the intended transmission lines design case.
Output Guide
The output section reports Guided Wavelength (lambdag), Stub Length, Electrical Length, Propagation Velocity, Propagation Delay, Typical RF Application, Design Note for the values you entered. Use these values as design targets, then compare them with available space, component limits, feed system behavior, installation environment, and measured performance before finalizing the design.
How This Calculator Works
The Stub Length applies lambdag = (c × VF) / f | L = (theta / 360°) × lambdag | t = L / (VF × c) to the entered values. Calculate the physical length of an RF transmission-line stub from frequency, velocity factor, and electrical angle. Includes guided wavelength, propagation delay, and common wavelength fractions. Use the result as a first-pass transmission lines target, then validate it against losses, tolerances, mounting, nearby conductors, feed-line effects, and measurement conditions.
Design Notes
Unlike calculators that start from a physical length, this tool works backward from the electrical angle you need, using lambdag = (c × VF) / f to find the guided wavelength and then L = (theta / 360°) × lambdag to convert that angle directly into physical stub length — the same approach used in professional RF design workflows where the target electrical behavior (a 90° phase shift, a resonant half-wave section, and so on) is known before the physical dimensions are. A 90° (quarter-wave) stub is the most common building block for single-stub impedance matching, since it produces predictable, well-documented reactance behavior; a 180° (half-wave) section repeats the impedance seen at its input, making it useful as a resonant or isolating element; and angles beyond 360° represent stubs longer than one full wavelength, sometimes used intentionally in specific phasing or filter designs. Propagation delay is also calculated alongside physical length, since the electrical angle corresponds directly to a time delay at the given frequency — useful for phase-critical designs where timing, not just distance, must be controlled precisely.
Build and Tuning Notes
Use the calculated stub length as your fabrication target, but expect to trim slightly on final assembly, since the design note built into this calculator's results is accurate: PCB layout effects, connector transitions, fringing fields at open ends, and dielectric tolerance in the actual substrate or cable all introduce small deviations from the ideal calculated length. For PCB-based stubs, remember that the effective velocity factor is determined by the board's dielectric constant and trace geometry (microstrip vs. stripline), not the coaxial cable velocity factor typically quoted in datasheets, so use a velocity factor appropriate to your actual transmission medium. After fabrication, verify the stub's electrical behavior with a vector network analyzer at the target frequency — checking phase and impedance rather than just physical length — and trim in small increments if the measured electrical angle doesn't match the design target, since even a small physical length error translates directly into a phase error at the stub's working frequency.
Inputs used by this calculator
- Frequency — use MHz.
- Velocity Factor (VF).
- Electrical Angle — use °.
Frequently Asked Questions
How do I convert an electrical angle to a physical stub length?
First calculate the guided wavelength using lambdag = (c × VF) / f, where c is the speed of light, VF is the velocity factor, and f is frequency. Then multiply the guided wavelength by the fraction of the electrical angle out of 360° (L = (theta / 360°) × lambdag) to get the physical stub length. This calculator performs both steps automatically.
What is a quarter-wave (90°) stub used for?
A quarter-wave stub, corresponding to a 90° electrical angle, is the most common building block in single-stub impedance matching networks because it produces predictable, well-characterized reactance that can cancel unwanted reactive components in a load's impedance.
What is the difference between electrical angle and physical length?
Electrical angle expresses a stub's length in terms of signal phase at a specific frequency (0°–360° representing one full wavelength), while physical length is the actual measured distance. The relationship between them depends on the guided wavelength, which is determined by both frequency and the transmission medium's velocity factor.
What velocity factor should I use for a PCB microstrip stub?
PCB microstrip and stripline stubs have an effective velocity factor determined by the board's dielectric constant and trace geometry, not the velocity factor of coaxial cable. This is typically lower than common coax velocity factors and should be calculated or looked up for your specific PCB stackup rather than assumed.
Why does the calculator show propagation delay alongside stub length?
Because electrical angle corresponds directly to a time delay at a given frequency, propagation delay is useful for phase-critical RF designs where the timing relationship between signal paths matters as much as the physical stub dimensions, such as in phased feed networks or precision phase-shift applications.
Why might my fabricated stub not match the calculated electrical angle?
Fringing fields at open stub ends, connector transitions, PCB layout parasitics, and dielectric tolerance in the actual manufactured board or cable all introduce small deviations from the ideal calculated length, so stubs should be verified with a vector network analyzer and trimmed if needed after fabrication.
Alex Warren
B.Sc. in Electrical & Electronic Engineering (EEE)
Alex specialises in antenna design and wave propagation. His expertise helps ensure these calculators present practical RF concepts, useful design estimates, and clear engineering guidance for students, HAM operators, and wireless professionals.