Skip to main content

Dipole and Inverted-V calculator

How it works

A dipole resonates when its overall length is a little under half a wavelength. How much "a little" is depends on the thickness of the conductor: thin wire comes up 5 % short, thick VHF tubing 7 %. Height barely changes the length, but it does change the feedpoint impedance — and that is what decides your SWR.

feederL/2L/2L = λ/2 · kh 12 m
An apex angle of 180° is a straight dipole; anything less is an Inverted-V
Frequency (MHz)
Conductor diameter (mm)
Height above ground (m)
Apex angle
Result
Overall length
Length of one leg
Shortening factor k
Wavelength λ
Height in wavelengths
Expected feedpoint impedance
Expected SWR against 50 Ω

📐 The formulas​

Overall length of the antenna:

L = k × 149.896 / f    [m]

Shortening factor — from the ratio of length to conductor diameter:

k = 0.930 + 0.022 × log10( (L/d) / 100 ) / 2      clamped to 0.930…0.952

Inverted-V: at an apex angle below 180° the resonant length drops by up to a further 4 %, and the feedpoint impedance falls by roughly 30 %.

Symbols:​

  • f — operating frequency, MHz
  • d — conductor diameter
  • L/d — ratio of overall length to diameter
  • k — the practical shortening factor; it already accounts for the end effect
Where this calculator stops:
  • The feedpoint impedance comes from an averaged curve for a horizontal wire over real ground — your particular soil will move it by tens of percent
  • Metal, trees and roofs nearby shift resonance more than any correction term can
  • For the Inverted-V the angle correction is a linear approximation; only modelling does better
  • Wire insulation is not accounted for: PVC shortens the antenna by another 2–3 %

🎯 In practice​

  • A first HF antenna. 40 m: 20.1 m of wire, fed in the middle, 8 m up or more.
  • Inverted-V on a single mast. A 120° apex gives about 50 Ω — the coax connects with no matching network at all.
  • Checking someone else's numbers. The classic 468/f rule (in feet) is k = 0.951, that is, thin wire on HF.
Practical advice:
  1. Cut the wire 2–3 % long and trim to the analyser — adding wire back is harder than cutting it off.
  2. Trim both legs equally, or the symmetry goes and current starts flowing on the braid.
  3. A dipole needs a choke balun at the centre; without one the coax becomes part of the antenna.
  4. A height of 0.25–0.5 λ is the best compromise between impedance and radiation angle.

© 2026 UR3PKI · CyberDev.Space · Content licensed under CC BY-NC-SA 4.0.

How to cite this calculator
UR3PKI. «Dipole and Inverted-V calculator». CyberDev.Space. https://cyberdev.space/en/radio/calculators/antennas/dipole (licence CC BY-NC-SA 4.0).

The licence lets you use this material freely, including in teaching materials, but only with attribution to the author and a link to the source.

Last updated on