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RF Wavelength Calculator

Convert any RF frequency to free-space wavelength (λ = c/f) and apply a velocity factor for coax cables, waveguides, or PCB microstrip. Identifies the IEEE radio band (L/S/C/X/Ku/K/Ka/V/W).

Input

Frequency

Transmission medium (velocity factor)

Custom velocity factor (0 < VF ≤ 1)

Common RF frequencies

Result

Free-space wavelength λ₀

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Quick rule (300 / f_GHz, in mm)

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Wavelength in medium

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Half-wavelength (medium)

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Quarter-wavelength (medium)

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Velocity factor (selected)

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Period (1/f)

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Formulas

Free space: λ₀ = c / f (c = 299,792,458 m/s)

In medium: λ = λ₀ · VF (VF < 1 for any physical conductor)

Half-wave element: λ / 2 · Quarter-wave element: λ / 4

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Spectrum (1 kHz–1 THz, log scale) · IEEE band letters above baseline

About RF Wavelength & Velocity Factor

The RF wavelength is the most fundamental parameter in radio engineering — it sets antenna size, transmission-line cut lengths, PCB trace impedance, and even how a signal interacts with obstacles in its path. Free-space wavelength is simply λ₀ = c / f where c is the speed of light. But in real transmission lines and dielectric materials, EM waves propagate slower than c — and the wavelength shrinks accordingly.

Velocity factor (VF)

VF is the ratio of EM wave speed in a medium to the speed in free space. For typical coax cables, VF is 0.66–0.85. PCB microstrip can be as low as 0.45 due to the high dielectric constant of FR-4. The wavelength inside the medium is λ = λ₀ × VF — meaning your physical cable length to make a quarter-wave matching section is 0.66× (or whatever) the free-space length. Use the right VF for your medium or your matching network will be off-frequency.

Common coax VFs

RG-58, RG-8, RG-213 (solid PE dielectric): 0.66
RG-6, LMR-400 (foam PE): 0.82–0.85
LMR-600, hardline 7/8″ (foam, semi-rigid): 0.87–0.88
PTFE/Teflon coax: 0.695
Air-line coax (semi-rigid with air dielectric): 0.95

IEEE radio bands

Above 1 GHz, the IEEE convention uses letter band designators: L 1–2 GHz, S 2–4 GHz, C 4–8 GHz, X 8–12 GHz, Ku 12–18 GHz, K 18–27 GHz, Ka 27–40 GHz, V 40–75 GHz, W 75–110 GHz. Mnemonic for the lower bands: "Long Sandwich Crusts eXcept" (L/S/C/X). Below 1 GHz the bands are ELF/VLF/LF/MF/HF/VHF/UHF.

PCB microstrip wavelength

Microstrip lines on FR-4 PCB have an effective dielectric constant εeff ≈ 3 (between substrate's εr ≈ 4.4 and air's 1). VF in microstrip is 1/√εeff ≈ 0.58, but for typical 50-Ω microstrip dimensions on 0.063″ FR-4 it ends up closer to 0.45–0.50. For precise design at high frequencies, use a transmission-line calculator that accounts for trace width, substrate thickness, and copper thickness.

Frequently Asked Questions

Why is VF less than 1 in coax?

Because EM waves travel slower in a dielectric medium than in vacuum/air. The wave speed in a dielectric is c/√εr where εr is the relative permittivity. For solid polyethylene εr ≈ 2.25, so VF = 1/√2.25 ≈ 0.667 — matching the typical RG-58/RG-8 value. Foam dielectrics have εr ≈ 1.4–1.7 (more air mixed in), giving VF ≈ 0.77–0.85. The wave slows because the alternating E-field has to polarize the dielectric atoms each cycle, which takes time.

Does VF depend on frequency?

Slightly. For most coax types, VF varies less than 1% across DC to ~10 GHz. Above that, dispersion in the dielectric makes VF drop a few percent and varies with frequency — significant for wide-band applications. Manufacturer datasheets often quote VF at a specific test frequency (typically 100 MHz). For sub-GHz work, a single VF value is fine; for mmWave, use the manufacturer's frequency-dependent curves.

What's the wavelength of 2.4 GHz WiFi?

Free-space: λ₀ = 299,792,458 / 2.4e9 ≈ 124.9 mm (about 12.5 cm). That's why 2.4 GHz antennas are small — a quarter-wave antenna is just ~31 mm. In RG-58 coax with VF=0.66, the wavelength shrinks to 82.4 mm, so a quarter-wave matching stub is 20.6 mm of coax. In FR-4 PCB microstrip (VF≈0.45), it's only 56 mm free-space wavelength, and quarter-wave matching is 14 mm of trace.

How does WiFi go through walls then if 5.5 GHz wavelength is only 5.4 cm?

Walls are mostly transparent at WiFi frequencies because typical drywall is ~12 mm — much thinner than a wavelength, so it's "subwavelength" and the wave diffracts around small obstacles. Walls attenuate the signal (each ~3-7 dB at 2.4 GHz, more at 5 GHz, way more at 6 GHz). Concrete and metal block much more. Below 1 GHz (e.g., HF, MW), walls are nearly transparent — that's why AM/FM works inside buildings. Wavelength relative to object size is the key intuition.

Can I use this tool for fiber optics?

Yes — fiber has a "core refractive index" n ≈ 1.45 for silica, giving VF ≈ 0.69 (use Custom mode). At 1550 nm telecom wavelength, free-space wavelength is 1.55 µm and in-fiber wavelength is ~1.07 µm. That's the spatial period of the light wave inside the glass. The math is identical to RF — only the magnitudes are different.

What's a "waveguide" and is it the same as coax?

Different geometry, same idea. A waveguide is a hollow metal tube that guides EM waves above a certain "cutoff frequency". Inside the waveguide, the guided wavelength is different from both free-space and coax — it depends on the mode and tube dimensions: λ_guide = λ₀ / √(1 − (λ₀/2a)²) for the dominant TE10 mode in a rectangular waveguide of width a. This tool doesn't compute waveguide modes directly; for that you'd want a microwave-waveguide calculator. The "VF" displayed here for coax/PCB doesn't directly apply.

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