Antenna Calculator
Starting lengths for common wire antennas, plus coax cut length. Real-world results vary with height and surroundings — trim to resonance with an analyzer.
Coax cut length (electrical ¼λ / ½λ)
RG-58/RG-8 solid PE ≈ 0.66 · foam ≈ 0.80 · RG-6 ≈ 0.82 · LMR-400 ≈ 0.85
Frequently asked questions
How long should a half-wave dipole antenna be?
A half-wave dipole is roughly 468 ÷ frequency-in-MHz feet, end to end, with each leg half of that. For the 20 m band (about 14.2 MHz) that’s near 33 feet total. Enter your frequency above for the exact starting length, then trim to resonance.
How long is a quarter-wave vertical antenna?
About 234 ÷ frequency-in-MHz feet. For 2 m (146 MHz) that’s roughly 19 inches; for 40 m it’s about 33 feet. The calculator works it out for any frequency you enter.
What is the velocity factor of coax?
Radio signals travel slower in coax than in free space; the velocity factor (around 0.66 for RG-58, 0.82 for foam types) shortens any electrical cut length — like phasing lines or matching sections. The calculator applies it so your cut comes out right.
Why isn’t my antenna resonant at the calculated length?
Calculated lengths are starting points. Height above ground, nearby objects, and wire insulation all shift resonance. Cut a little long, measure SWR with an analyzer, and trim a few inches at a time.