Antenna Length Calculator (Quarter-Wave / Half-Wave)
Frequency → physical antenna length for quarter-wave and half-wave designs, with velocity-factor correction for thin-wire construction.
Result
- Frequency146 MHz
- Velocity factor0.95
- Free-space wavelength2.053 m
- Adjusted wavelength λ1.951 m
- Quarter-wave (λ/4)0.488 m / 19.2 in
- Half-wave (λ/2)0.975 m / 38.4 in
- Full-wave (λ)1.951 m / 6.40 ft
Step-by-step
- λ_free = c / f = 299,792,458 / 146,000,000 = 2.0534 m.
- λ_adj = λ_free × velocity factor = 2.0534 × 0.95 = 1.9507 m.
- λ/4 = 0.4877 m; λ/2 = 0.9754 m.
How to use this calculator
- Enter the design frequency.
- Pick velocity factor (0.95 typical for thin wire).
- Read λ/4 (vertical / monopole) and λ/2 (dipole) lengths.
About this calculator
A resonant antenna is sized to a fraction of the operating wavelength λ = c/f. The quarter-wave vertical (λ/4) and half-wave dipole (λ/2) are the two most common practical lengths because they present near-resistive feed impedances at their respective resonance points (~36 Ω for λ/4 over a perfect ground; ~73 Ω for λ/2 dipole in free space). Thin-wire construction shortens the resonant length by ~5% (velocity factor 0.95) due to end effects.
What this calculator does
This calculator converts an operating frequency into the physical length of a quarter-wave, half-wave, or full-wave antenna. It applies a user-selectable velocity factor (default 0.95 for typical thin-wire construction) that accounts for the ~5% end-effect shortening between free-space wavelength and the actual resonant length of a real conductor. Outputs are reported in both meters and feet/inches so you can cut the wire directly from the result.
How it works — the formula
λ_free = c / f (c = 299,792,458 m/s)
λ_adj = λ_free × velocity_factor
Quarter-wave = λ_adj / 4
Half-wave = λ_adj / 2The speed of light c is fixed by SI definition. Free-space wavelength is c/f. Real conductors deviate from the free-space ideal by a few percent due to end-effect capacitance and surface currents; the velocity factor folds that into one constant. The traditional ham-radio shortcut 468/f(MHz) for half-wave-dipole length in feet bakes in velocity factor ≈ 0.95.
Worked examples
- Inputs:
- f = 146 MHz, vf = 0.95
- Output:
- λ/4 ≈ 0.487 m (19.2 in); λ/2 ≈ 0.975 m (38.4 in)
Standard 2 m ground-plane or J-pole reference.
- Inputs:
- f = 14.2 MHz, vf = 0.95
- Output:
- λ/4 ≈ 5.01 m (16.4 ft); λ/2 ≈ 10.02 m (32.9 ft)
20 m dipole — the classic full-size HF antenna.
- Inputs:
- f = 100 MHz, vf = 0.95
- Output:
- λ/4 ≈ 0.712 m (28.0 in); λ/2 ≈ 1.424 m (56.1 in)
Sets the length of a homemade FM whip.
When to use this vs other tools
Use this to size the conductor for a single-frequency resonant antenna. Multiband and broadband antennas (log-periodic, fan-dipole) need additional design tools.
- Wavelength ↔ Frequency
Use when you want the wavelength of a non-antenna application — sound, light, microwaves — at vf=1.0.
- SWR Calculator
Use after installation to verify the antenna is actually resonant at the design frequency.
- Coax Loss
Use to pick the right feedline — long coax runs at VHF/UHF can lose more than the antenna gains.
- Transmission Line Impedance
Use to match the antenna feedpoint impedance to the coax characteristic impedance.
Authority note
ARRL antenna design conventions (468/f shortcut, velocity factor ~0.95) are the de-facto standard in amateur and broadcast radio engineering, derived from the same Maxwell-equation field solutions used in IEEE professional antenna textbooks.
Limitations
- Assumes a simple straight-wire dipole or monopole; folded, looped, and helically-loaded geometries change the resonance equation.
- Velocity factor varies with wire diameter, insulation, and proximity to ground. 0.95 is typical; ±2% errors are normal.
- Real feedpoint impedance depends on antenna height above ground — ¼ λ above ground gives different Z than free-space ideal.
- For licensed transmit use, verify resonance with an SWR or antenna analyzer rather than trusting the cut-and-pray length.
Antenna construction near power lines or at significant heights carries physical risk. Licensed-amateur and commercial transmit operations have RF-exposure rules; consult the relevant regulator (FCC OET-65 in the US).