Coax Wavelength Calculator: Formula & Uses

This calculator works out the wavelength of a radio frequency signal inside a coaxial cable. You enter the operating frequency and the cable’s velocity factor (or choose a common cable type), and it returns the physical wavelength in the cable, plus the wavelength in free space for comparison. The result is the distance between two identical points on the wave as it travels through the coax, which is shorter than in air because the dielectric slows the signal.

Formula

The wavelength in a coaxial cable is calculated using the standard relationship between frequency, speed, and wavelength:

λ_cable = (c / f) × VF

• λ_cable = wavelength in the cable (meters)
• c = speed of light in a vacuum ≈ 300,000,000 m/s (or 300 × 10â�¶ m/s)
• f = frequency (Hz)
• VF = velocity factor of the cable (dimensionless, typically 0.66 to 0.95)

The velocity factor is the ratio of the speed of the signal in the cable to the speed of light in a vacuum. It is determined by the dielectric material between the center conductor and the shield. For example, solid polyethylene has a VF around 0.66, while foam polyethylene can be 0.78–0.88.

Worked Example

Using the calculator’s default values: frequency = 100 MHz, cable = RG-58 (VF = 0.66).

1. Calculate free-space wavelength: λ_air = 300 / 100 = 3.0 m
2. Multiply by velocity factor: 3.0 × 0.66 = 1.98 m
3. So the wavelength in the coax is 1.98 m (about 6.5 feet).

The calculator shows 1.98 m for the cable wavelength and 3.0 m for free space. Notice the cable wavelength is shorter because the signal travels slower in the dielectric.

Why Wavelength in Coax Matters

Knowing the wavelength inside a coaxial cable is essential for designing impedance-matching sections, stub filters, phasing lines, and measuring cable length. For example, a quarter-wave transformer at 100 MHz needs a physical length of λ/4 = 1.98/4 = 0.495 m (49.5 cm) of RG-58. If you used the free-space wavelength (3.0 m), you’d cut the cable too long by 25%, and the transformer would not work as intended.

Also, when measuring cable length with a time-domain reflectometer (TDR), the instrument uses the velocity factor to convert time delay into distance. A wrong VF gives wrong distance readings.

How to Choose the Right Velocity Factor

Selecting the correct VF is critical for accurate results. Here are the common sources:

• Manufacturer datasheet – the most reliable source. RG-58/U typically lists 0.66, but some variants are 0.65.
• Cable type – solid PTFE (Teflon) has VF ≈ 0.70, foam PE ≈ 0.80–0.88, air-spaced ≈ 0.95.
• Measured value – if you have a network analyzer or antenna analyzer, you can measure the electrical length and derive VF.
• Generic rule – if unknown, use 0.66 for solid PE, 0.80 for foam PE, 0.85 for LMR-400 (which is foam PE).

Note that VF can vary slightly with frequency and temperature, but for most amateur and professional work, the datasheet value is sufficient.

Common Mistakes and Misconceptions

One frequent error is forgetting to convert frequency to Hz before using the formula. If you plug in MHz directly, you must use c = 300 (in million meters per second) to get meters. The calculator does that automatically.

Another misconception is that the wavelength in coax determines the physical length of a half-wave dipole. It does not – antennas are usually cut for free-space wavelength, but the feed line length can affect impedance matching. For example, a 50-ohm dipole fed with a half-wave of 75-ohm coax will still see 50 ohms at the transmitter, but a quarter-wave section would transform the impedance.

Also, remember that the velocity factor applies to the entire cable, including connectors. If you have a long run with different cable types, you must calculate each segment separately.

Practical Applications

Beyond stubs and transformers, wavelength calculations are used for:

• Phasing harnesses – for antenna arrays, each feed line must be an exact multiple of a wavelength to keep signals in phase.
• Measuring cable length – if you need to cut a cable to a specific electrical length, you divide the desired electrical length (in degrees) by 360 and multiply by the wavelength.
• Designing delay lines – in radar and test equipment, precise delays are achieved with specific cable lengths.
• Determining resonant frequency of a stub – a shorted or open stub of a given length resonates at frequencies where the length is a quarter or half wavelength.

For example, a 1-meter length of RG-58 (VF=0.66) has an electrical length of 1 / 1.98 = 0.505 wavelengths at 100 MHz. That means it is just over half a wavelength, which could be used as a half-wave transformer.

Limitations of the Formula

The formula assumes a lossless cable and a constant velocity factor across the entire frequency range. In reality, the velocity factor changes slightly with frequency due to dispersion, especially in low-cost cables. Also, the formula gives the wavelength of a continuous wave; for pulsed signals, the group velocity may differ slightly from the phase velocity, but for most practical purposes, the difference is negligible.

Another limitation: the velocity factor is defined for a specific impedance and construction. If you use a cable with a braided shield versus a solid foil shield, the VF can differ by a few percent. Always use the manufacturer’s value when possible.

Frequently Asked Questions

What is the velocity factor of RG-58?

Most RG-58/U cables have a velocity factor of 0.66 (66% of the speed of light). Some low-loss versions may be 0.65 or 0.70, so check the datasheet. The calculator uses 0.66 as the default for RG-58.

How do I convert wavelength to feet?

Multiply the wavelength in meters by 3.28084. For example, 1.98 m × 3.28084 = 6.50 feet. The calculator gives meters, but you can easily convert.

Does the length of a coax cable affect SWR?

Yes, but not the SWR of the antenna itself. The SWR seen at the transmitter depends on the cable’s characteristic impedance, loss, and the load impedance. A half-wavelength line repeats the load impedance, while a quarter-wave line transforms it. Using the correct wavelength helps predict this.

Why is the wavelength in coax shorter than in free space?

Because the signal travels slower in the dielectric. The frequency stays the same, so the wavelength (distance between peaks) must be shorter. The velocity factor tells you the ratio: λ_cable = λ_air × VF.

Can I use this calculator for any frequency?

Yes, as long as you know the velocity factor. The formula works from DC to light, but in practice, coax has frequency limits due to attenuation and mode propagation. Above a few GHz, higher-order modes can appear, and the simple formula may not hold.

What is the difference between electrical length and physical length?

Electrical length is expressed in wavelengths or degrees, while physical length is in meters or feet. A cable that is physically 1 meter long might be electrically 0.5 wavelengths at a certain frequency. The calculator helps you convert between them.

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in f | Frequency | MHz | 100 | 0
sel cable | Cable type | RG-58=0.66, RG-8=0.66, LMR-400=0.85, RG-213=0.66, Custom=1
in vf | Velocity factor (if Custom) | - | 0.66 | 0.1
out wl_cable | Wavelength in cable | m | (300 / f) * vf | 3
out wl_air | Wavelength in free space | m | 300 / f | 3