Ham Radio Tools

Coax Electrical Length Calculator

Calculate the physical length of coax needed for a target electrical length. Enter frequency, coax velocity factor, and phase length to find quarter-wave, half-wave, full-wave, or custom cable sections for ham radio feedlines, stubs, and phasing lines.

What this tool does It converts a desired electrical phase length into a real cable length using frequency and velocity factor.

Rule of thumb A quarter-wave coax section is 90 electrical degrees; a half-wave section is 180 electrical degrees.

Practical note Always check the cable datasheet. Solid polyethylene coax is often near 0.66 VF, while foam and air-spaced cables are usually higher.

Calculator

Ham band preset

Frequency

Coax type / velocity factor Velocity factor is the wave speed in the line as a fraction of the speed of light. Presets are typical values; use your exact cable datasheet when precision matters.

Custom velocity factor

Electrical length

Custom phase length

Results

Physical length

0.00ft

Metric length

0.00m

Free-space wavelength

0.00m

Electrical phase

0deg

Length status

Enter values to calculate a coax electrical length.

Band	Frequency	1/4 wave	1/2 wave	Full wave

How the Calculation Works

A radio wave has a free-space wavelength set by frequency. Inside coax, the wave travels slower, so the physical cable is shorter than the same electrical length in free space. The velocity factor scales the result.

free_space_wavelength_m = 299.792458 / frequency_MHz coax_wavelength_m = free_space_wavelength_m * velocity_factor physical_length_m = coax_wavelength_m * electrical_degrees / 360

Rules of Thumb

Use 90 degrees for a quarter-wave transformer or shorted/open stub starting point.
Use 180 degrees when you need a half-wave line that repeats impedance at the far end.
Solid dielectric RG-58 and RG-213 are often near 0.66 velocity factor.
Foam dielectric and LMR-style cables are commonly around 0.78 to 0.85, depending on the exact cable family.
Cut slightly long, measure, then trim when the length is critical.

Measuring Velocity Factor

Real cable velocity factor varies by manufacturer, construction, and even production run. For phasing harnesses, matching stubs, or VHF/UHF work, use the exact cable datasheet and verify with an antenna analyzer or VNA. Connector length and exposed braid can also shift the final electrical length.

To measure your own coax velocity factor, cut or use a known physical length of cable, leave the far end open or shorted, and find the frequency where it behaves as a quarter-wave stub with an antenna analyzer or VNA. Then calculate the free-space quarter-wave frequency for that same physical length. A simple working formula is VF = measured_stub_resonant_frequency / free_space_quarter_wave_frequency. Equivalently, VF = 4 * cable_length_m * measured_stub_frequency_MHz / 299.792458.

The preset list uses typical published velocity-factor values where available, including Times Microwave LMR data and Belden 9913 data. Treat generic RG, heliax, ladder-line, and open-wire entries as planning estimates.