Coax Cable Length Calculator

Calculator Form

Cable type preset

Frequency

Frequency unit

Velocity factor

Calculation mode

Phase angle in degrees

Wavelength fraction

Extra full wavelengths

Target delay in ns

Known physical length

Known length unit

Extra trim allowance percent

Cable loss dB per 100 ft

Loss reference frequency

Reference frequency unit

Frequency loss exponent

Connector count

Loss per connector in dB

Input power in watts

Preferred output unit

Example Data Table

Use case	Frequency	Cable	Velocity factor	Target	Approximate length
VHF quarter wave jumper	145 MHz	RG-58	0.66	90 degrees	0.341 m, 13.43 in
TV half wave section	100 MHz	RG-6 Foam	0.83	0.50 wavelength	1.244 m, 4.08 ft
UHF full wave line	450 MHz	LMR-400	0.85	1 wavelength	0.566 m, 22.30 in

Formula Used

Free space wavelength: wavelength = speed of light / frequency.

Coax wavelength: coax wavelength = free space wavelength × velocity factor.

Phase length: length = coax wavelength × ((phase degrees / 360) + full wavelengths).

Fraction length: length = coax wavelength × (wavelength fraction + full wavelengths).

Delay length: length = delay seconds × speed of light × velocity factor.

Delay from known length: delay = length / (speed of light × velocity factor).

Estimated loss: total loss = cable loss + connector loss.

Power after loss: output watts = input watts × 10^{−loss dB / 10}.

How to Use This Calculator

Select a coax type, or choose custom values.
Enter the operating frequency and its unit.
Set the velocity factor from the cable data sheet.
Choose phase, wavelength fraction, delay, or known length mode.
Enter loss data when you want a power estimate.
Press Calculate to view results above the form.
Use CSV or PDF export for records.

Why Coax Length Matters

Coax cable length affects timing, phase, loss, and tuning. A short run may pass strong signals well. A long run can add delay and reduce power. In radio, test benches, video links, and timing systems, those changes matter. This calculator joins the common length checks in one place. It helps you plan before cutting expensive cable.

Electrical Length Basics

Signals do not travel through coax at light speed. They move at a lower speed set by the cable velocity factor. Solid polyethylene cable often uses a factor near 0.66. Foam dielectric cable can be higher. The calculator uses your chosen velocity factor to find the wavelength inside the cable. Then it turns phase angle, wavelength fraction, or delay into physical length.

Loss Planning

Cable loss rises with frequency and length. It also changes by cable model. A short jumper may lose very little power. A long feed line at high frequency may lose enough to affect performance. The calculator estimates total cable loss from attenuation per one hundred feet. It can also add connector loss. This gives a practical budget, not a lab certificate.

Useful Applications

Use the tool for antenna phasing lines, delay cables, matching sections, repeater jumpers, lab setups, CCTV paths, and satellite feeds. It can estimate a quarter wave, half wave, full wave, or custom phase length. It can also show how much delay a known cable creates. Those results are useful when matching two paths.

Good Building Practice

Always check the cable data sheet first. Velocity factor and attenuation vary by manufacturer. Measure twice before cutting. Leave a service loop when the installation allows it. For precise RF work, cut slightly long, test, and trim. Connectors, bends, adapters, and nearby objects can shift results. Clean connectors also reduce avoidable loss.

Final Check

The calculator gives planning values. Real cable may differ slightly. Temperature, manufacturing tolerance, and connector quality can change results. Use the exported CSV or PDF for job notes. Keep records for future repairs and repeat builds.

Result Review

Review the returned inches, feet, and meters together. Small rounding changes can matter at high frequency. Use the nearest cut, then verify with your analyzer, scope, or meter before final assembly.

FAQs

What is coax cable electrical length?

Electrical length is the length a signal experiences inside the cable. It depends on frequency, physical length, and velocity factor. A cable can be short physically but still represent a large phase shift at high frequency.

What is velocity factor?

Velocity factor is the signal speed inside coax compared with light speed in free space. A value of 0.66 means the signal travels at 66 percent of light speed.

Why is coax length important?

Coax length affects delay, phase, impedance matching, and signal loss. It is important for antenna phasing, matching sections, timing paths, and repeatable test setups.

Can this calculator find quarter wave coax length?

Yes. Choose wavelength fraction mode. Enter 0.25 as the fraction. Add full wavelengths only when you need a longer equivalent electrical length.

Does cable type change the result?

Yes. Cable type changes velocity factor and attenuation. Always use the actual cable data sheet when accuracy matters. Presets are useful starting points only.

Why does cable loss increase at higher frequency?

Higher frequency signals usually face more conductor and dielectric loss. The calculator uses a frequency exponent to estimate this increase from a known reference loss value.

Should I cut the exact calculated length?

For precise RF work, cut a little long. Then measure and trim gradually. Connectors and installation conditions can shift the final electrical length.

Can I export the result?

Yes. Use the CSV button for spreadsheet records. Use the PDF button after calculation to save a printable summary of the displayed result.