How it works
Dipole. A half-wave dipole is two wires, each a quarter wave long, fed in the middle. The ARRL's starting length is 468 ÷ f feet in total, with f in MHz; leave about six inches extra to trim.17 That is about 5% shorter than half a wavelength in free space (492 ÷ f feet, or 150 ÷ f metres): thick wire, height and nearby objects move the right length between 460 ÷ f and 490 ÷ f.2
Ground plane. A quarter-wave vertical is 234 ÷ f feet long. Its feed point is about 35 Ω with flat radials; sloping them down by about 45° brings it close to 50 Ω coax.2 The ARRL's 2 m version uses four radials at 45°, the 2020 handheld project two wires bent down about 30°, cut to the same length as the element.34
J-pole. The box scales two published 2 m designs by 146 ÷ f: Edison Fong's 300 Ω twin-lead J-pole (radiator 37¼″, stub 16½″, feed 1¼″ above the shorted end, ¼″ notch) and Michael Hood's copper-pipe J-pole (element 56¾″, stub 18¾″, feed clamps about 4″ up).56 The material sets the lengths, so each design stays with its own material. Mode range: 50 to 600 MHz.
Tuning. Every length is a starting point. Trim a little at a time with an SWR meter: if the SWR rises toward the top of the band, the antenna is long; if it rises toward the bottom, it is short.1 Feed line loss at your frequency: coax loss calculator.
