Dipole calculator and antenna lengths
A half-wave dipole is the simplest good antenna. The calculator determines its length for any frequency, accounts for the shortening factor and shows in three planes how height and setup affect the radiation pattern.
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What the page does
- Half-wave and quarter-wave length for any frequency
- Shortening factor depending on wire thickness
- Convert wavelength and frequency
- Dipole in three planes with radiation pattern
- Continue into the antenna simulation
Background
Free-space wavelength is λ = 300 / f with f in MHz and λ in metres. Because of the shortening factor of about 0.95 a half-wave dipole is slightly shorter than λ/2: as a rule of thumb L ≈ 142.5 / f m. For 14.2 MHz that is just over 10 m total length. Then trim to the desired frequency with an SWR meter.
In practice
How it works: enter the frequency, choose the wire thickness and cut the calculated length about five percent longer. Hang the dipole at the planned height, measure the resonance with an SWR meter or antenna analyser and shorten both halves equally. If the resonance is too low, the dipole is too long; if too high, too much was trimmed.
Frequently asked questions
Why is my dipole too long?
The rule of thumb is a starting value. Height, surroundings and insulation shift the resonance, so cut slightly long and trim.
Do I need a balun?
A current balun at the feed point prevents common-mode currents on the coax and is usually recommended.