Use this tool to calculate of the correct lengths for cutting the wire elements of an HF dipole. Note that the lengths that are cut will be intentionally too long, so that the wire at the dipole ends can be wrapped around the end insulators and twisted back on itself (see the diagram below). This allows the effective length of the dipole to be adjusted.

Dipole Diagram.

Desired Dipole Resonant Frequency, f = MHz
Ground proximity and end capacitance factor, b = dimensionless Note 1
Adjustment and centre connection allowance, d = m Note 2

Velocity of light in free space, c = m/s
Free-space wavelength, λfreespace = c / f = m (to 3 decimal places)
Free-space half wavelength, λfreespace / 2 = m (to 3 decimal places)
Free-space quarter wavelength, λfreespace / 4 = m (to 3 decimal places)
Practical wavelength, λpractical = λfreespace x b = m (to 3 decimal places) Note 1
Practical half wavelength, λpractical / 2 = y = m (to 3 decimal places) Note 1
Practical quarter wavelength, λpractical / 4 = x = m (to 3 decimal places) Note 1
Cut length of each wire element = m (to 3 decimal places)

Note 1: As a real dipole will not be in "free space", it will be affected by the proximity of the ground and surrounding objects. The capacitance between the dipole elements and the surroundings will tend to lower the frequency at which the dipole is resonant, hence the dipole needs to be shorter than we might expect from "free space" calculations. Unfortunately, it is very difficult to calculate these affects. In practice, the free space wavelength is multiplied by a factor of 0.95 to give a "practical" wavelength that is shorter to account for the ground affects. As the 0.95 factor is just a guess, the dipole must be tuned by lengthening or shortening the dipole ends so that it resonates at the correct frequency for the particular installation.

Note 2: This is the amount of extra length that will be allowed on the wire for making up the connections at the centre insulator and for twisting back on itself at the dipole ends. This may only need to be 0.5m or so for the higher frequency HF bands (eg 10m), but for the lower bands (eg 80m) may need to be increased to 2m or more to give enough length for adjustments.

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