Antenna Length Calculator
Physical length for a resonant antenna at a given frequency, with the end-effect correction applied. Covers dipoles, quarter-wave verticals, loops and 5/8-wave elements.
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The formula
λ = c ÷ f length = λ × fraction × end-effect factor
An antenna resonates when it is an appropriate fraction of a wavelength long, because that is the length at which the current and voltage standing waves fit. The end-effect factor — about 0.95 for wire — accounts for the capacitance at the ends of a real conductor, which makes it behave as though it were slightly longer than it is.
What to watch out for
Three things this figure does not cover:
- Thicker conductors resonate shorter. The 0.95 factor suits wire. Tubing with a large diameter-to-length ratio can need 0.93 or less.
- Height above ground shifts it. A dipole less than half a wavelength up interacts with the ground, moving both the resonant point and the feedpoint impedance.
- A quarter-wave vertical is only half an antenna. It needs a ground plane or radials to work against; without them the feedline becomes the other half, usually badly.
Always cut long and trim. Removing wire raises the resonant frequency, and adding it back is considerably harder.
Frequently Asked Questions
How long should a half-wave dipole be?
About 143 ÷ frequency in MHz, in metres — roughly 0.98 m total for 145 MHz, split into two 49 cm legs. That is the free-space half wavelength reduced by the 0.95 end-effect factor.
What is the end effect on an antenna?
Capacitance at the tips of a real conductor makes it behave electrically longer than it is, so it resonates a few per cent below the frequency its physical length suggests. Multiplying by about 0.95 corrects for it.
Does a quarter-wave antenna need a ground plane?
Yes. It is half of a dipole, and the ground plane or radial system supplies the missing half. Without one, the outside of the coax braid takes that role, which puts RF back down the feedline and into the shack.