Antenna Wavelength & Dipole Calculator
Enter a frequency to get wavelength, dipole and monopole lengths, loop size and quarter-wave coax stub length.
Step-by-step
- Enter valid values to begin.
Free-space wavelength uses c = 299,792,458 m/s. Dipole and monopole lengths are starting cuts: λ/2 × K for a straight wire in free space, with K covering end effect (about 0.95 classically, higher for thin wire, lower for thick or insulated elements). Height above ground, nearby objects, element diameter and a balun all pull the resonance, so cut slightly long and trim for lowest SWR with an analyzer. VF values are typical; check the cable datasheet. The loop uses the empirical 1005/f ft quad rule.
Dipole Length: Wavelength, End Effect and the 468 Rule
Core Engineering Principles
A half-wave dipole is resonant when the standing-wave current on the wire has a quarter-wave on each side of the feedpoint. In free space that is λ/2 = c/(2f), and with c = 299,792,458 m/s the wavelength is 2.053 m at 146 MHz. A real wire is shorter than that. Capacitance at the tips and the wire’s own velocity make it resonate at about 95% of λ/2, and that factor K is where the classical rule comes from: 492 × 0.951 rounds to 468/f in feet with f in MHz. Thicker or insulated elements push K lower, thin bare wire lets it climb toward 0.98.
Length is only half the job. A thin free-space dipole shows about 73 Ω at its feedpoint, a quarter-wave vertical over a good ground about 36 Ω, and both swing with height above ground, nearby metal and element thickness. Coax carries the signal slower than light, so a quarter-wave stub or matching section uses the velocity factor: 0.66 for solid polyethylene, about 0.80 for foam. The practical habit is to cut about 3–5% long, hoist it where it will live, and trim in small steps for the lowest SWR with an analyzer.
Loop ≈ 1005 / fMHz ft • Stub = (λ/4) × VF
NEC & Standard References
IEEE Std 145 defines the antenna terms used here, including radiation resistance, and IEEE Std 149 describes how antenna gain, pattern and impedance are measured. The 468/f rule and the 1005/f loop rule come from amateur practice and appear in the ARRL Antenna Book. IEC 61169 covers the RF connectors, and IEC 61196 the coaxial cables whose velocity factors you will use. Transmit limits come from the regulator, such as FCC Part 97 for amateur service or ETSI EN 300 328 for 2.4 GHz equipment.1. λ = 299,792,458 / 146,000,000 = 2.053 m.
2. Dipole = 1.0267 m × 0.95 = 0.9754 m = 3.20 ft = 38.4 in, so each leg is 19.2 in.
3. Check: 468 / 146 = 3.205 ft. The two methods agree.
4. A quarter-wave monopole is 0.4877 m (19.2 in), and the quarter-wave coax stub is 0.5133 × 0.66 = 0.339 m (13.3 in).
5. Cut each leg about 3% long (near 39.5 in total) and trim for SWR. Same rule at 7.1 MHz gives 65.9 ft, and at 2.4 GHz about 59 mm.
- Cut long, then trim. The calculator gives a start. Height, wire diameter and nearby objects shift resonance by several percent.
- Use a balun on coax. A dipole is balanced and coax is not, so without a choke the shield radiates and tunes the antenna.
- Insulation shortens the wire. Covered conductors resonate lower than bare ones, so a K of 0.92–0.97 only gets you in the neighbourhood.