Half-Wave Dipole Calculator
The classic center-fed half-wave dipole — simple, efficient and the foundation of many other designs.
What the physical dipole changes
The half-wave dipole is the reference antenna behind many of the wire designs in this site. The accompanying diagram shows the essential geometry: two radiator halves meet at a central feedpoint, with each half contributing approximately a quarter wavelength. That symmetry is why the dipole is such a useful starting point for understanding antenna length, current distribution, feedpoint impedance and the effect of installation height.
The calculator uses frequency to estimate the total wire length and the length of each side. The familiar free-space wavelength is shortened in the practical starting formula because a real wire has end effects and a finite diameter. The correction is only an approximation. Insulated wire, nearby conductors, the angle of the elements, and the height above ground can move the resonant frequency enough that a finished antenna normally needs trimming.
For a horizontal installation, keep the two sides as symmetrical as practical and provide a mechanically sound centre support. If the available space forces the ends downward, the antenna becomes an inverted-V and its feedpoint impedance changes. The feedline should leave the feedpoint cleanly; a common-mode choke can be useful where unwanted current on the outside of the coax is affecting measurements or operation. Build the wires slightly longer than the calculated value, raise them to operating height, measure the resonant frequency, and trim in small equal increments. The antenna that finally works is the one measured in place, not the one that only matches a formula on paper. Write the final cut length and the height above ground in your notebook — next season you will thank yourself.
Calculate Length
About the Dipole
Formula
Derived from free-space half-wavelength (492 / f ft) reduced by ~5 % for end-effect and length/end-effect correction in real wire.
Key Points
- Impedance ≈ 70–75 Ω in free space; closer to 50–60 Ω near ground.
- Use a 1:1 current balun at the feed point to keep RF off the coax shield.
- Height of λ/2 or more favors low-angle DX; lower heights raise the takeoff angle.
- Always start long and prune — it is easier to shorten than to lengthen.
Related
Assumptions & limitations
- Formula assumes free-space wavelength reduced by a simple end-effect factor (default ≈ 5 %). Real resonance also depends on wire diameter, insulation, height above ground and nearby conductors.
- Ground effect and feedline common-mode current are ignored in the length calculation.
- Results are starting dimensions only — measure and trim in the final installation.
Real-world gotchas
- Common-mode current on the coax shield can shift apparent resonance and radiate in the shack. A 1:1 current balun or choke at the feedpoint is cheap insurance.
- Height changes both resonant frequency and feed impedance. An antenna that looks perfect on the lawn often needs another trim once it is up.
- Insulated vs bare wire: insulation lowers velocity factor slightly; start a little longer with insulated wire and expect to prune more.
- Nearby metal (gutters, masts, guy wires) can pull resonance several percent — always measure in place.
Next step in the workflow
Use the calculation as a starting point, then move to the tool that answers the next practical question.