Antenna length
Estimate how long to cut a wire antenna for a frequency, and how long a quarter wave of coax is for matching stubs.
Solid polyethylene coax is about 0.66; foam polyethylene coax is typically about 0.78 to 0.86 (Belden data). Check the cable datasheet.
- Half-wave dipole, total length
- 3 ft 2.3 in (0.9736 m)
- Each dipole leg, or a quarter-wave vertical
- 1 ft 7.2 in (0.4868 m)
- Free-space wavelength
- 2.046 m
- Electrical quarter wave of feed line
- 0.3376 m (1.108 ft)
Uses the traditional 468 / f and 234 / f rules (feet, MHz), which allow for about 5% end effect. Real antennas vary with wire size, height and nearby objects, so cut a little long and trim while measuring SWR.
How it works
- Free-space wavelength in meters is 299.79 divided by the frequency in MHz; the exams round this to 300 / f.
- A free-space half wave is 492 / f feet. The 468 / f rule for a half-wave dipole is about 5 percent shorter (468 / 492 = 0.95), and ARRL suggests adding about six inches before trimming. A quarter-wave vertical or radial is 234 / f feet.
- Radio waves travel slower in coax than in free space; the velocity factor is that fraction. Solid polyethylene coax is about 0.66, and foam polyethylene coax is typically about 0.78 to 0.86. Check the cable datasheet.
Learn more: Antennas and feed lines
Sources: CODATA value: speed of light in vacuum (NIST); Single Band Dipoles (ARRL); Radio Mathematics (ARRL Handbook supplement) (ARRL); KX3 Owner’s Manual, Rev C5 (234 / f radials) (Elecraft); 8267 (RG-213, solid polyethylene, 66% velocity) technical data (Belden); 8214 (RG-8, foam polyethylene, 78% velocity) technical data (Belden); 7810A (foam polyethylene, 86% velocity) technical data (Belden)
Practice the exam questions: T9 Antennas and Feed Lines (Technician), G9 Antennas and Feed Lines (General)