Antenna Calculator
Enter a frequency and get cutting lengths for dipoles, inverted-Vs, verticals and a 3-element Yagi — in metres and feet/inches. 输入频率，立即得到常用天线的导线长度。
Results计算结果
| Antenna天线 | Meters (m)米 | Feet / Inches英尺 / 英寸 |
|---|
公式按自由空间裸线计算。实际导线存在速度因子（约 0.95–0.98），建议先留长 3–5%，再边测驻波边修剪。离地高度和周围物体会影响谐振，请务必用驻波表或天线分析仪实测确认。
Every wire antenna starts with the same question: how long? This calculator answers it for the antennas hams build most — half-wave dipole, inverted-V, quarter-wave and 5/8-wave verticals, ground radials, and the elements of a three-element Yagi — from a single frequency input, in metric and imperial side by side.
The numbers use the standard practical formulas rather than raw physics. A free-space half wavelength is 150/f metres (f in MHz), but real wire resonates about 5% shorter because of end effect, so the dipole formula here is 142.5/f. At 14.200 MHz that gives a 10.04 m dipole — 5.02 m per leg. The quarter-wave vertical uses 71.5/f, the 5/8-wave uses 178.6/f, and the Yagi elements use the classic 150/f reflector, 143/f driven element and 137.8/f director spacings and lengths that have built generations of working beams.
Treat every result as a starting cut, not a finished antenna. The formulas assume bare wire in free space; insulated wire, height above real ground, nearby gutters and trees, and the diameter of the wire all move the resonant point. The professional habit is to cut 3–5% long, raise the antenna, measure with an analyser or SWR meter (the SWR Calculator turns those readings into losses), then trim both ends equally in small steps until resonance lands where you want it.
The wavelength readout (λ) is there for the spacings and matching tricks that are expressed in wavelengths — phasing lines, quarter-wave transformers and stacked antennas. Toggle the primary unit to highlight whichever column you cut by; both are always shown, so a metric tape measure and an imperial one are equally at home.
For verticals, remember that the radiator is only half the antenna: the radial rows matter just as much. Four quarter-wave radials are a practical minimum on the ground, and more — or an elevated radial system — lowers ground losses and lifts efficiency far more than squeezing the last centimetre out of the vertical element itself. If your SWR curve looks right but signals are weak, the ground system, not the length, is usually the missing piece.
Frequently asked questions
Are these lengths exact, or do I still need to trim?
Yes. The calculator uses 142.5/f (MHz) for a half-wave dipole — the free-space 150/f shortened by the ~5% end effect of real wire. That gets you close; height above ground, wire insulation and nearby objects still shift resonance, so cut long and trim.
How long is a 20 metre band dipole?
A half-wave dipole needs two quarter-wave legs (each 71.25/f metres). For 14.200 MHz that is a 10.04 m dipole with 5.02 m per side. The table shows total and per-leg values for every antenna type so you can cut directly.
Why is an inverted-V shorter than a dipole?
Its wide bandwidth and tolerance of low heights made the inverted-V popular: it is about 5% shorter than a flat dipole for the same frequency and needs only one tall support. The calculator's inverted-V rows already include that shortening.
Can I build a 3-element Yagi from these numbers?
Element lengths come from classic Yagi design formulas: reflector 150/f, driven element 143/f, director 137.8/f, with 0.15λ and 0.10λ spacing. Real Yagis are optimised with modelling software for gain and front-to-back ratio, so treat these as proven starting dimensions, not a final design.