📡 Antenna Length Calculator

The fundamental formula is: Wavelength (meters) = 299.79 / Frequency (MHz). For a half-wave dipole, divide the wavelength by 2. For a quarter-wave antenna, divide by 4. In feet, use 983.57 / f(MHz) for full wavelength. Multiply by the velocity factor (typically 0.95 for wire) to get the practical length. The well-known 468 / f(MHz) formula gives dipole length in feet with a built-in correction factor.

A quarter-wave (1/4 λ) antenna is one of the most common antenna types. It requires a ground plane (counterpoise) to operate effectively and is widely used in mobile installations, vehicle antennas, handheld radios, and ground-plane verticals. Its impedance is approximately 36–37 Ω over a perfect ground, making it easy to match with proper radials. It's compact compared to a half-wave dipole, which makes it ideal for space-constrained setups.

A half-wave dipole is a balanced antenna approximately λ/2 long, fed at the center, with an impedance of ~73 Ω. It does not require a ground plane and is the reference standard for antenna gain (0 dBd). A quarter-wave vertical is λ/4 long and requires a ground plane or radial system to form the "missing half" of the antenna. Verticals are omnidirectional and popular for DX due to their low takeoff angle, while dipoles are bidirectional and easier to deploy at height.

Velocity Factor (VF) is the ratio of the speed of an electromagnetic wave in a conductor compared to the speed of light in a vacuum. In coaxial cable, VF ranges from 0.66 to 0.85; for bare copper wire in air, VF is approximately 0.95–0.98; insulated wire is around 0.90–0.95. When building antennas, applying the correct VF ensures your antenna is cut to the right physical length. Ignoring VF can result in an antenna that is 5–15% too long, shifting its resonant frequency downward.

The 492 / f(MHz) formula calculates the free-space half-wavelength in feet. The 468 / f(MHz) formula is an empirically derived version that accounts for the "end effect" — the slight elongation of the electrical length due to capacitive effects at the wire ends. The ratio 468/492 ≈ 0.951, which effectively applies a built-in velocity factor of ~0.95. Most hams use the 468 formula as a starting point, then trim the antenna slightly during tuning for precise resonance.

CB radio operates at approximately 27 MHz (channel 19). A full-wave is about 36 feet (11 meters), hence the "11-meter band" nickname. A 1/4 wave CB antenna is approximately 8.5–9 feet (2.6–2.7 meters) — the classic "102-inch whip." A 1/2 wave dipole for CB is about 17–18 feet (5.3 meters) total length. Use our calculator with 27 MHz and VF 0.95 for practical CB antenna dimensions.

A 5/8 wavelength (5λ/8) antenna provides approximately 3–4 dB gain over a 1/4 wave vertical, concentrating radiation toward the horizon for improved DX performance. It's extremely popular for VHF/UHF base station antennas (2m and 70cm bands) and mobile installations. The 5/8 wave requires a matching network (typically a loading coil or base matching stub) because its feedpoint impedance is complex (not purely resistive). It offers an excellent balance of gain, size, and omnidirectional coverage.

Always cut your antenna slightly longer (2–5%) than the calculated length. It's much easier to trim material away than to add it back! After initial cutting, use an antenna analyzer or SWR meter to measure the resonant frequency. If the SWR dip is below your target frequency, the antenna is too long — trim small amounts (1/4 inch at a time for VHF, 1–2 inches for HF) and retest. This iterative approach ensures you hit your target frequency precisely.