RF Wavelength Calculator
This RF wavelength calculator converts any frequency to free-space wavelength (λ = c/f) and applies a velocity factor for coax cables, waveguides, or PCB microstrip. It also identifies the IEEE radio band (L/S/C/X/Ku/K/Ka/V/W).
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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 (299,792,458 m/s). But in real transmission lines and dielectric materials, electromagnetic waves propagate slower than c — and the wavelength shrinks accordingly. The same relationship applies at audio frequencies too; if you need wavelength for sound or other wave types, the frequency-to-wavelength converter covers 19 different propagation media.
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). The microstrip velocity factor 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. Rogers RO4003C and similar high-frequency laminates have lower loss tangent and tighter εr tolerances, which matters for stable impedance matching above 10 GHz. For precise design at high frequencies, use a transmission-line calculator that accounts for trace width, substrate thickness, and copper thickness — the values here give a useful first approximation for quarter-wave matching stubs and filter element sizing.
Frequently Asked Questions
Why is VF less than 1 in coax?
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?
What's the wavelength of 2.4 GHz WiFi?
How does WiFi go through walls then if 5.5 GHz wavelength is only 5.4 cm?
Can I use this tool for fiber optics?
What's a "waveguide" and is it the same as coax?
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.