Calculating antenna lengths

How long should I cut my antenna?

Antenna lengths come from a short formula. Learn the formula, then practise it until the numbers feel familiar.

The formulas

For a half-wave dipole:

$L\ (\text{m}) = \frac{150}{f\ (\text{MHz})} \times 0.95$

For a quarter-wave vertical:

$L\ (\text{m}) = \frac{75}{f\ (\text{MHz})} \times 0.95$

The factor $0.95$ shortens the antenna slightly so it resonates in practice. The frequency must be in MHz.

The worked answers

You now know

• Half-wave dipole: $L = 150 / f \times 0.95$. Quarter-wave vertical: $L = 75 / f \times 0.95$. Frequency in MHz, length in metres.
• A quarter-wave vertical is half the length of a half-wave dipole for the same frequency.
• Convert kHz to MHz before using the formula.