1---
2app_version: 0.1.0
3precision: 3
4form_title: Dipole & Vertical Antenna Length
5form_description: How long should I cut this wire? Enter your design frequency — get the half-wave dipole and quarter-wave vertical length, in metres and feet.
6form_inputs: Design frequency (MHz)|f_mhz, End-effect factor|k
7form_outputs: Half-wave dipole — total (m)|half_wave_m, ...each leg (m)|leg_m, Quarter-wave vertical (m)|quarter_wave_m, Half-wave dipole — total (ft)|half_wave_ft
8---
9# Dipole & Vertical Antenna Length
10// "How long do I cut this wire?"
11//
12// speed_of_light/f gives the free-space wavelength; a real wire is
13// electrically a little longer than it looks (end effect, insulation, nearby
14// objects), so it resonates slightly short of a full free-space wavelength.
15// k folds that in.
16
17speed_of_light = 299792458 // m/s
18f_mhz = 14.2 // design frequency, MHz — the middle of your target band
19k = 0.95 // end-effect factor for bare wire (0.95-0.98 typical)
20
21wavelength_m = speed_of_light / (f_mhz * 1e6)
22
23half_wave_m = wavelength_m / 2 * k
24leg_m = half_wave_m / 2
25quarter_wave_m = wavelength_m / 4 * k
26
27"Free-space wavelength (m)": wavelength_m
28"Half-wave dipole, total length (m)": half_wave_m
29"...each leg (m)": leg_m
30"Quarter-wave vertical, incl. end effect (m)": quarter_wave_m
31
32// Same numbers in feet, for builders working in imperial.
33half_wave_ft = convert(half_wave_m, "m", "ft")
34quarter_wave_ft = convert(quarter_wave_m, "m", "ft")
35
36"Half-wave dipole, total length (ft)": half_wave_ft
37"Quarter-wave vertical (ft)": quarter_wave_ft
38
39// A classic rule of thumb for comparison — 468/f(MHz) in feet, 143/f(MHz) in
40// metres, both baking in k ≈ 0.95. Should land close to the numbers above.
41"Rule-of-thumb dipole length (ft, 468/f)": 468 / f_mhz
42