How to use the Antenna Wavelength Calculator
- 1. Enter the frequencyLike 100M for FM radio or 2.4G for Wi-Fi.
- 2. Set the velocity factor1 for free space, lower for wire or coax.
- 3. Read the lengthsWavelength, λ/2, λ/4, 5/8 λ and the band name.
Wavelength formula
Radio waves travel at the speed of light in free space, c = 299,792,458 m/s, so λ = c / f. A half-wave dipole is λ/2 long (each arm λ/4) and a quarter-wave monopole over a ground plane is λ/4. In a cable or along a thick wire, waves are slower, so the physical length is multiplied by the velocity factor (VF).
Worked example: at 100 MHz, λ = 3×10⁸ / 10⁸ ≈ 3.0 m. The half-wave dipole is 1.5 m and the quarter-wave whip 0.75 m. With the end-effect factor of about 0.95 for a wire dipole, the cut length is about 1.42 m. At 2.4 GHz, λ = 12.5 cm and λ/4 = 3.1 cm.
Bands and exam questions
The ITU bands are VLF (3–30 kHz), LF, MF (300 kHz–3 MHz), HF (3–30 MHz), VHF (30–300 MHz), UHF (300 MHz–3 GHz), SHF and EHF, each ten times higher.
- Find the length of a half-wave dipole for 30 MHz (5 m).
- Find the wavelength of a 10 GHz microwave signal (3 cm).
- Find the length of a λ/4 coax stub at 145 MHz with VF 0.66.
- Which band does 433 MHz fall in? (UHF)
Frequently asked questions
Why is a real dipole shorter than λ/2?
Fringing at the wire ends makes the antenna look electrically longer, so it is cut about 5% short. The exact factor depends on wire thickness.
What is velocity factor?
It is the wave speed in a cable or structure divided by the speed of light. Solid polyethylene coax is about 0.66; foam coax is about 0.8 or more.
Should I use 3×10⁸ or the exact speed of light?
Exams usually use 3×10⁸ m/s. The tool uses the exact value, which differs by only 0.07%.
Is this tool free?
Yes. You enter your name and WhatsApp number once to see your result, and all other Uncodemy tools then open without the form.