Ham Radio Antenna Calculator
Enter your operating frequency and pick an antenna type to get the exact element lengths to cut. Millimetres, centimetres, and feet/inches all at once, with a live diagram. Download a PDF for the workbench.
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Learn more: Ham radio antenna design and wavelength
How antenna length is calculated from frequency and wavelength
Radio antennas must be specific fractions of the electromagnetic wavelength at your operating frequency. Wavelength (lambda, λ) = speed of light / frequency. For example, 7.1 MHz on 40 meters: λ = 300,000,000 m/s / 7,100,000 Hz = 42.3 meters. A half-wave dipole is λ/2 = 21.15 meters per element. In practice, a velocity factor (VF) correction of 0.95-0.97 for copper wire shortens this to ~20m. Different antenna types use different fractions: quarter-wave vertical = λ/4, five-eighths vertical = 5λ/8. Each fraction optimizes impedance and radiation pattern for specific use cases.
Velocity factor, feedline choice, and practical construction considerations
Velocity factor (0.66-1.0) accounts for how fast an EM wave travels through a conductor or dielectric. Bare copper wire has VF ~0.97, insulated wire ~0.95, coax cable ~0.66-0.80. Lowering VF physically shortens the antenna needed for resonance. Your feedline (coax type) does NOT affect antenna length - only the antenna's physical medium (bare, insulated, or in-tube) does. A dipole in free air needs about 3% shortening from free-space theory. The calculator applies the correct VF for your chosen antenna and medium, but actual on-air tuning with an antenna analyzer may require small trim adjustments due to nearby objects, ground effects, and construction variations.
FAQ
How do I calculate half-wave dipole antenna length?
The formula for a half-wave dipole in free space is: length (metres) = 142.5 / frequency (MHz). In practice a velocity factor of 0.97 is applied for bare wire, giving 138.2 / MHz. For 40m (7.1 MHz) this produces an overall length of 19.5m (each element 9.75m).
What is velocity factor and why does it affect antenna length?
Velocity factor (VF) is the ratio of signal speed through a medium compared to free space. Bare copper wire has a VF of about 0.97, insulated wire 0.95, coaxial cable 0.66-0.80. A lower velocity factor shortens the physical length needed for resonance. This calculator applies the correct VF for your chosen antenna type.
What is a Yagi antenna and when should I use one?
A Yagi-Uda antenna uses a driven element, one reflector, and one or more directors to focus the signal in one direction, providing gain (typically 6-12 dBd) over a dipole. Yagis are ideal for point-to-point links, DX contacts, and working weak signals where directional gain improves range.