The formulas
A half-wave dipole's total length in feet is about 468 ÷ frequency in MHz (142.5 ÷ MHz in meters); each leg is half of that. A quarter-wave vertical is about 234 ÷ MHz in feet (71.3 ÷ MHz in meters), with radials about the same length.
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Dipole and vertical length calculator
Enter a frequency and your license class. You'll get cut lengths in feet and meters, and a check that the frequency is inside your privileges.
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- Runs in your browser
- Shows the working
Formulas are starting points: height, nearby objects and wire insulation all shift resonance, so cut a little long and trim while checking SWR.
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Good to know
Trimming for SWR
Cut 3 to 5% long, then trim in small steps. If SWR is lowest above your frequency, the antenna is too short; if lowest below, too long. Resonance moves in proportion to length, so shortening by 0.4% raises it by about 0.4%.
Stay inside your privileges
Your license class sets where you can transmit. A General can work 20 m phone from 14.225 to 14.350 MHz, but 7.150 MHz on 40 m is Extra and Advanced only. Remember that a sideband signal extends about 3 kHz from the dial.