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.

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, λ_{freespace} = c / f = | m (to 3 decimal places) | |

Free-space half wavelength, λ_{freespace} / 2 = | m (to 3 decimal places) | |

Free-space quarter wavelength, λ_{freespace} / 4 = | m (to 3 decimal places) | |

Practical wavelength, λ_{practical} = λ_{freespace} x b = | m (to 3 decimal places) ^{Note 1} | |

Practical half wavelength, λ_{practical} / 2 = y = | m (to 3 decimal places) ^{Note 1} | |

Practical quarter wavelength, λ_{practical} / 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.