Antenna Length Calculator

How the calculator works

The calculator applies the classic wire-antenna formulas that hams have used for decades. You supply the frequency; it divides a fixed constant by that frequency to produce a length in feet.

• Half-wave dipole: total length L (ft) = 468 / f_MHz. Each leg is half of that total.
• Quarter-wave vertical: radiator length L (ft) = 234 / f_MHz.
• Ground-plane radials: each radial = 234 / f_MHz x 1.12, which makes the radials about 12% longer than the driven element.
• Full wavelength (for loops and reference): L (ft) = 984 / f_MHz.

To get inches, multiply the fractional feet by 12. For example, 6.34 ft is 6 ft plus 0.34 x 12 = about 4.1 inches.

Why 468 and not 492 (end effect)

In a vacuum, a half wavelength in feet is 984 / f_MHz, so a half-wave would be 492 / f_MHz. Real wire antennas come out shorter than that because of end effect: the electromagnetic field fringes at the open ends of the wire, and the insulators, wire thickness, and surrounding environment make the antenna behave electrically longer than its physical length.

To compensate, the physical wire is cut shorter. The traditional velocity-factor correction of roughly 0.95 turns 492 into about 468, which is why 468 / f_MHz is the accepted starting formula for a half-wave dipole. The same reasoning gives 234 / f_MHz for a quarter-wave element.

Reading the result

The output is a length in feet for the whole antenna and for each element. Here is a quick reference for popular Technician-band frequencies so you can sanity-check the calculator:

Always calculate for the center of the range you plan to use most. If you operate one repeater output at 146.94 MHz, cut for that frequency rather than the band edge.

Cut long, then trim: the workflow

The single most useful habit in antenna building is to cut a little long and trim down. You can always remove wire; you cannot easily add it back.

1. Calculate the length for your target frequency.
2. Add a few percent of extra length to each element (roughly 2-5%) before your first cut, and leave room to fold wire back at the connection point.
3. Measure the SWR across the band with an SWR meter or antenna analyzer. Note where SWR is lowest.
4. If the lowest SWR point is below your target frequency, the antenna is too long. Trim small, equal amounts from each end and re-measure.
5. Repeat in small steps. Resonance moves up in frequency as you shorten the antenna.

Trim in small increments, especially at VHF and UHF where a fraction of an inch shifts resonance noticeably.

Common mistakes

• Cutting each leg to the full dipole length. The 468 / f_MHz result is the total; each of the two legs is half.
• Forgetting to divide the total between legs. A 2 m dipole is about 38.5 in total, so each leg is about 19.2 in, not 38.5 in.
• Using the vacuum figure (492 or 984). Those are electrical half- and full-wavelength references, not the physical cut length for wire.
• Cutting for a band edge instead of your actual frequency. Center the antenna on where you operate.
• Skipping the SWR check. The formula is a starting length; height, wire type, and surroundings all shift the real resonant point.
• Trimming only one side. Keep the two legs of a dipole symmetric so the pattern and match stay clean.

Tools that finish the job

The calculator gets your wire close; a measurement tool tells you where you actually landed. A basic in-line SWR meter is enough to trim a single-band antenna, while an antenna analyzer sweeps the whole band and shows resonance without transmitting.

See our related pages on tuning an antenna for low SWR and picking an SWR meter for VHF/UHF.