Dipole / Whip Length
Cut lengths for a half-wave dipole and quarter-wave whip.
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The engineering
A resonant dipole is a hair short of the free-space half wave — wire thickness and end capacitance slow the wave, hence the 0.95. The ham-radio 468/f feet rule is this card with the factor baked in.
Cut long, trim to lowest SWR; antennas are the last field-adjustable RF component.
Where this math comes from
Hertz's original 1887 radiator *was* a dipole — the antenna came before the radio. Amateur operators of the 1920s, exiled to the 'useless' shortwaves, turned dipole-cutting into folk science and the ARRL handbooks canonized 468/f, a number every ham still recites.
The end-effect physics was worked out as antenna engineering professionalized in the 1930s–40s (Schelkunoff and King's era), but the field practice hasn't changed: wire, insulators, and a trim.
- 1887Heinrich HertzThe dipole — the first antenna.
- 1926ARRL handbook era468/f enters amateur canon.
- 1943S. A. SchelkunoffRigorous antenna theory explains the shortening factor.
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