tool · antennas
Antenna calculator
Choose the antenna type and band, or type the frequency. The measurements appear in the drawing. The shortening factor exists because the wave travels a little slower in the wire than in a vacuum and the tip of the wire "increases" the antenna; 0.95 works for thin wire in HF, thick tubing requires less.
Adjustment
Type
Band
Dimensioned drawing
out of scale
This is a starting point, not a final recipe. Height, nearby objects, soil, insulators, and the cable itself change the resonance. Cut the wire about 3% longer, measure with an SWR meter (or an antenna analyzer), and trim the ends equally on both sides until the minimum SWR falls in the middle of the band you use. If the minimum SWR is below the desired frequency, the antenna is too long.
Sources
The calculations run in your browser, nothing leaves here. Each formula was checked against the references below.
- ARRL Antenna Book: half-wave dipole (468/f feet), 1/4-wave vertical (234/f feet) and elevated radials
- ARRL Antenna Book, Yagi antenna chapter: reflector larger and director smaller than the driven element
- QST, October 1947, "Element Spacing in 3-Element Beams": reflector at 0.2 λ and director at 0.15 λ
- ARRL Handbook, VHF antenna chapter: J-pole as end-fed half-wave with 1/4-wave stub
- Speed of light: 299,792,458 m/s (BIPM, definition of the metre)