RESONATICA
Antennas

Calculate wavelength and antenna length

From the frequency, the wavelength and the practical length of dipole, ground plane and full-wave loop, plus cable sections for matching and traps. The animation shows current and voltage on a half-wave dipole.

Free, no account needed

Abbreviations and symbols on this page (9)
λ
Wavelength: λ = c ÷ f, c = speed of light 299 792 458 m/s. At 14.2 MHz about 21.1 m.
λ/4
Quarter of the wavelength λ = c ÷ f (c = speed of light). A secondary coil resonates much like a quarter-wave radiator.
f
Frequency: oscillations per second, unit hertz (Hz).
Hz, kHz, MHz
Hertz, kilohertz (1000 Hz), megahertz (1,000,000 Hz).
VF
Velocity factor of a cable: how fast the wave travels in the cable compared with air. Solid polyethylene about 0.66, foam PE about 0.77–0.82 (check the datasheet).
k (shortening)
Factor by which a wire antenna is shorter than the free-space half wave; about 0.95 for thin wires (rule of thumb 468 ÷ f in feet). Height and surroundings shift it.
Dipole
Antenna made of two wires of equal length, fed in the middle; as a half-wave dipole about λ/2 long.
Ground plane (GP)
Vertical λ/4 radiator above radials or a conductive surface.
Coaxial cable
Cable with inner conductor and shield; characteristic impedance usually 50 Ω (radio, test equipment) or 75 Ω (TV).
Sign in · With an account you can save calculations and favorites.
MHz
Solid polyethylene (RG-58, RG-213) (0.66)
λ21.11 m
λ/2 · λ/4 · 5/8 λ10.56 m · 5.278 m · 13.2 m
Half-wave dipole10.03 m (each half 5.014 m)
Ground plane λ/45.014 m
Full-wave loop (circumference)21.53 m
Cable λ/4 (VF 0.66)3.484 m · λ/2 6.967 m
Feed pointλ/4λ/4━ I Current (maximum in the middle) · ━ U Voltage (maximum at the ends)
Standing wave on the half-wave dipole: current and voltage are offset by a quarter period.

Formula

λ = c ÷ f with c = 299 792 458 m/s. Dipole ≈ k × λ/2 (thin wire k ≈ 0.95, corresponds to 468 ÷ f in feet), ground plane ≈ k × λ/4, full-wave loop ≈ 1.02 × λ. In cable: λ_cable = VF × λ.

Example

14.2 MHz: λ = 21.1 m, half-wave dipole with k = 0.95 about 10.03 m (2 × 5.02 m), ground plane 5.01 m. A λ/4 section of RG-58 (VF 0.66) is 3.48 m long.

Notes

  • The rule of thumb 468 ÷ f is a starting value: simulations show values between about 466 and 483 depending on height above ground. Cut the antenna slightly long and trim it for minimum SWR.
  • A cable's velocity factor is given in the datasheet; solid polyethylene about 0.66, foam dielectric about 0.77–0.82.
  • Thicker conductors and insulated wire shorten it further.
  • Transmit only with the appropriate licence (amateur radio, ISM frequencies); observe field exposure limits.