Antenna Length Calculator
Wire length from frequency — half-wave dipole and quarter-wave vertical, end effect and velocity factor included.
Antenna Length Calculator
Results recalculate instantly on every keystroke. Nothing you type is transmitted.
What this result does not account for
- Thin-wire resonance — no height or ground model
- Feedpoint impedance quoted in FAQ only
In short: A dipole for the 7.15 MHz band runs 468 over 7.15: about 65.5 feet tip to tip, two 32.7-foot legs off the centre feed. The same antenna in metres is just the feet times 0.3048 — 19.95 m. The 468 already carries the end-effect trim: a raw half wavelength would be 492 divided by frequency, but current crowds the wire's tips and the real resonant antenna is a few percent shorter. Cut long, trim for the dip in SWR, and let the field tune the last foot.
Formula
L(ft) = 468 ÷ f(MHz) — = 492 × vf ÷ f
A half wavelength in free space is 492 feet divided by frequency in megahertz; real wire ends make the antenna electrically longer than it is physically, so the classic formula trims to 468 — the 0.95 velocity factor of bare copper. This screen scales that trim with your wire: insulated or closely surrounded conductors run slower and want a shorter cut. Quarter-wave verticals are half a dipole, 234 feet over frequency at the same factor.
Worked Example
- Pick the band centre frequency.
- 468 over frequency gives the dipole in feet.
- Scale by velocity factor; cut long, trim to tune.
Defaults: 7.15 MHz, bare wire — 65.5 feet total, 32.7 per leg, 32.7 feet for a quarter-wave vertical. The citizens' band preset at 27.185 MHz cuts the same maths to 17.2 feet.
Strengths & Limits Of This Model
Where this engine is strong
- Feet and metres both, legs split for you
- Velocity factor as an input, not an afterthought
Where it stops
- Real installations always need field trimming
Practical Use Cases
HF wire antennas
the first cut of the dipole
Verticals
quarter-wave against a ground plane
Receiver experiments
any wire near half-wave hears
Methodology & Editorial Standards
Computation runs in IEEE-754 double precision at full internal precision; rounding to two decimal places occurs strictly at the display layer, so no cumulative drift enters the result. All monetary outputs use accounting presentation — grouped thousands, two decimals, negatives in parentheses — so figures can be transcribed directly into a model or working paper. Division-by-zero and out-of-domain inputs return an em-dash rather than a misleading number.
This engine was reconciled against an independent reference implementation and hand-verified for the worked example above before release. Our full five-stage review process is published on the About Us page.
Disclaimer. This calculator is provided for informational and modelling purposes only and does not constitute financial, tax, legal, medical, or engineering advice. Verify all figures with a qualified professional before acting on them.
Antenna Length Calculator — 8 Expert FAQs
8 analyst-written answers to the questions practitioners actually ask — optimised for voice and answer-engine retrieval.
Why 468 and not 492?
492 is the free-space half wavelength in feet; 468 is that times 0.95, the end-effect trim for bare copper. The wire's tips hold charge that makes it resonate long, so the physical antenna is cut a few percent short of the textbook wave.
What does velocity factor change?
It scales the whole answer: insulated wire runs the wave slower, so the resonant length shortens proportionally. Type the factor your wire's data quotes; 0.95 remains the honest default for bare radial wire.
Where do the legs split?
At the centre feed, each leg exactly half the total. The feedpoint sits near 73 ohms for a half-wave dipole in the clear, which is why coax and baluns get the conversation after the wire is cut.
Does wire diameter matter?
Slightly — thicker elements are broader-banded and a whisker shorter at resonance. The 468 lore assumes ordinary 12 to 14 AWG wire; fat tubing trims another percent or two.
Why cut long before trimming?
You can shorten wire but not lengthen it, and every installation bends, sags and neighbours differently. Cut to the computed length plus a few percent, then fold the ends back until the SWR dips at your frequency.
What about the quarter-wave vertical?
Half a dipole stood on end, 234 feet over frequency, working against a ground plane of radials. Its feedpoint sits near 36 ohms with a good ground — the radials are half the antenna, which is why skipping them hurts.
Can I use this for receive-only loops?
Loops are whole wavelengths — about 1005 feet over frequency — and a different feedpoint entirely. The dipole maths here brackets wire antennas that are half or quarter wave.
How exact is any of this?
It is the honest first cut: height, ground, nearby objects and insulation all nudge resonance. Amateur practice cuts from this number and tunes in the field — the analyser has the final word.