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Antenna Length Calculator

Calculate practical starting dimensions for a half-wave dipole, quarter-wave vertical or full-wave loop—or work backwards from an existing antenna length to an approximate resonant frequency. The calculator also estimates feed impedance and suggests a 50 Ω, 75 Ω or custom system match.

Calculate

Choose the antenna geometry whose common starting-length formula should be used.
Enter the intended centre or resonant frequency.
Choose the impedance your feed system should present. 50 Ω is the normal default for amateur transmitting systems.
Used when estimating the physical length of a quarter-wave impedance-transforming section. Typical solid polyethylene coax is often around 0.66.
Live antenna visualisation Not to scale
Enter a value to see the antenna drawn here.
Practical matching construction Goal: TX sees ~1:1 SWR

This panel uses the full available width and shows the practical feed arrangement from transmitter to antenna. Only common, easy-to-find coax families are considered for quarter-wave transformer sections.

How the calculation works

This calculator uses common amateur-radio starting formulas rather than pretending that a real antenna is simply an exact fraction of the free-space wavelength. Physical antenna resonance is affected by conductor size, insulation, geometry, installation height and nearby objects.

Length (feet) = 468 ÷ frequency (MHz)

For a simple half-wave wire dipole, 468/f MHz gives the approximate total starting length.

Worked example

Enter a value above to see the calculation worked through here.

Half-wave dipole

For a simple wire dipole, the traditional amateur-radio starting formula is:

Total length (feet) = 468 ÷ frequency (MHz)

The calculator also divides the result into two equal legs. Cut the wire slightly long and trim both sides symmetrically after measuring the installed antenna.

Quarter-wave vertical

The corresponding quarter-wave starting formula is:

Radiator length (feet) = 234 ÷ frequency (MHz)

This gives a useful starting radiator dimension. The final result depends strongly on the ground plane or radial system and the installation.

Full-wave loop

A commonly used amateur-radio starting estimate for a full-wave wire loop is:

Circumference (feet) = 1005 ÷ frequency (MHz)

The result is the total wire circumference. You can visualise the loop as a circle, a square or an equilateral triangle. The calculator shows the corresponding dimensions and a broad feed-impedance estimate for the chosen shape.

Feed impedance and 50 Ω systems

The feed-point impedance of an antenna is not automatically 50 Ω. The calculator therefore estimates the antenna feed impedance and compares it with a target system impedance such as 50 Ω or 75 Ω. Where the mismatch is significant, it also estimates the ideal impedance and physical length of a quarter-wave transforming section.

Why these dimensions differ from exact free-space fractions

The calculator separately displays the exact free-space wavelength and the ideal mathematical quarter-, half- or full-wave fraction. The practical starting formula can differ because a real wire antenna is not an infinitesimally thin conductor suspended in empty space.

Measure and trim

These values should be treated as construction starting points. Install the antenna in its intended position and use an antenna analyser, VNA or SWR measurement to determine where it actually resonates before making the final trim.

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