{"success":true,"version":"v1","request":{"tool":"get_calculator_schema","calculator_id":"antenna-length"},"result":{"entity_type":"calculator","id":"antenna-length","calculator_id":"antenna-length","canonical_url":"https://tttkmbb.com/engineering/antenna-length","name":"Antenna Length Calculator","title":"Antenna Length Calculator – Half-Wave Dipole, Quarter-Wave Vertical, Full-Wave Loop and 5/8-Wave Lengths from Frequency","category":"engineering","category_name":"Engineering & Automotive","tool_name":"calculate_antenna_length","featured_mcp_tool":false,"description":"Computes the free-space wavelength for a frequency and the physical wire length of a half-wave dipole (with the length of each leg), a quarter-wave vertical, a full-wave loop or a 5/8-wave radiator, applying a velocity (end-effect) factor as in the 468/f ft rule.","use_when":"You are cutting wire or tubing for an amateur-radio, CB, FM, Wi-Fi, LoRa or ISM-band antenna and need the starting length for a given frequency.","do_not_use_when":"You need a Yagi, patch, helical or loaded (shortened) antenna design, impedance matching, or just the wavelength without an antenna (use wavelength-frequency).","inputs":[{"name":"frequency_mhz","label":"Frequency","type":"number","unit":"MHz","required":true,"max":1000000,"exclusive_min":0,"description":"Operating (centre) frequency in megahertz, e.g. 146 (2 m band), 27.185 (CB channel 19), 433.92 (ISM), 2437 (Wi-Fi channel 6).","example":146},{"name":"antenna_type","label":"Antenna type","type":"enum","required":false,"default":"half_wave_dipole","values":[{"value":"half_wave_dipole","label":"Half-wave dipole (two legs, centre-fed)","aliases":["dipole","half_wave"]},{"value":"quarter_wave_vertical","label":"Quarter-wave vertical / ground plane","aliases":["quarter_wave","vertical","monopole"]},{"value":"full_wave_loop","label":"Full-wave loop (total wire length)","aliases":["loop","full_wave"]},{"value":"five_eighths_wave","label":"5/8-wave vertical","aliases":["5/8","five_eighths"]}],"description":"Radiator geometry.","example":"half_wave_dipole"},{"name":"velocity_factor","label":"Velocity factor","type":"number","required":false,"default":0.95,"min":0.3,"max":1,"description":"Shortening factor for end effect and conductor thickness: 0.95 for thin wire (the classic 468/f ft rule), 1.0 for the theoretical free-space length, 0.66–0.85 for coaxial-cable elements (coax dielectric).","example":0.95}],"outputs":[{"name":"wavelength_m","label":"Free-space wavelength","type":"number","unit":"m","decimals":4,"description":"λ = 299.792458 / frequency_mhz."},{"name":"wavelength_ft","label":"Free-space wavelength","type":"number","unit":"ft","decimals":4,"description":"λ in feet (1 ft = 0.3048 m)."},{"name":"total_length_m","label":"Total length","type":"number","unit":"m","decimals":4,"description":"Overall radiator length (tip to tip for a dipole, total wire for a loop)."},{"name":"total_length_ft","label":"Total length","type":"number","unit":"ft","decimals":4,"description":"Overall length in feet."},{"name":"total_length_in","label":"Total length","type":"number","unit":"in","decimals":2,"description":"Overall length in inches."},{"name":"each_leg_m","label":"Each leg","type":"number","unit":"m","decimals":4,"description":"Length of each of the two dipole legs (half of the total; dipole only)."},{"name":"each_leg_ft","label":"Each leg","type":"number","unit":"ft","decimals":4,"description":"Each dipole leg in feet (dipole only)."},{"name":"each_leg_in","label":"Each leg","type":"number","unit":"in","decimals":2,"description":"Each dipole leg in inches (dipole only)."},{"name":"formula_used","label":"Formula used","type":"string","decimals":4,"description":"Length expression applied for the chosen antenna type."}],"input_schema":{"type":"object","properties":{"frequency_mhz":{"description":"Operating (centre) frequency in megahertz, e.g. 146 (2 m band), 27.185 (CB channel 19), 433.92 (ISM), 2437 (Wi-Fi channel 6). Unit: MHz.","type":"number","maximum":1000000,"exclusiveMinimum":0,"examples":[146],"x-unit":"MHz"},"antenna_type":{"description":"Radiator geometry.","type":"string","enum":["half_wave_dipole","quarter_wave_vertical","full_wave_loop","five_eighths_wave"],"default":"half_wave_dipole","examples":["half_wave_dipole"]},"velocity_factor":{"description":"Shortening factor for end effect and conductor thickness: 0.95 for thin wire (the classic 468/f ft rule), 1.0 for the theoretical free-space length, 0.66–0.85 for coaxial-cable elements (coax dielectric).","type":"number","minimum":0.3,"maximum":1,"default":0.95,"examples":[0.95]}},"additionalProperties":false,"required":["frequency_mhz"]},"output_schema":{"type":"object","properties":{"wavelength_m":{"description":"λ = 299.792458 / frequency_mhz. Unit: m.","type":"number","x-unit":"m"},"wavelength_ft":{"description":"λ in feet (1 ft = 0.3048 m). Unit: ft.","type":"number","x-unit":"ft"},"total_length_m":{"description":"Overall radiator length (tip to tip for a dipole, total wire for a loop). Unit: m.","type":"number","x-unit":"m"},"total_length_ft":{"description":"Overall length in feet. Unit: ft.","type":"number","x-unit":"ft"},"total_length_in":{"description":"Overall length in inches. Unit: in.","type":"number","x-unit":"in"},"each_leg_m":{"description":"Length of each of the two dipole legs (half of the total; dipole only). Unit: m.","type":"number","x-unit":"m"},"each_leg_ft":{"description":"Each dipole leg in feet (dipole only). Unit: ft.","type":"number","x-unit":"ft"},"each_leg_in":{"description":"Each dipole leg in inches (dipole only). Unit: in.","type":"number","x-unit":"in"},"formula_used":{"description":"Length expression applied for the chosen antenna type.","type":"string"}}},"formula":"wavelength_m = 299.792458 / frequency_mhz; half_wave_dipole: total_length_m = wavelength_m / 2 × velocity_factor (≈ 468 / frequency_mhz ft at 0.95), each_leg_m = total_length_m / 2; quarter_wave_vertical: wavelength_m / 4 × velocity_factor (≈ 234 / f ft); full_wave_loop: wavelength_m × 1.02 (≈ 1005 / f ft, velocity_factor not applied); five_eighths_wave: 0.625 × wavelength_m × velocity_factor","method":"Free-space lengths scaled by an empirical shortening factor (0.95 for thin wire, per the ARRL 468/f rule) that accounts for end effect; loops are electrically longer than free-space wavelength, hence the 1005/f ft (1.02 λ) rule. Results are starting lengths: cut slightly long, then trim for minimum SWR, because height above ground, wire diameter, insulation and nearby objects shift resonance by a few percent.","sources":[{"name":"Wikipedia – Dipole antenna","url":"https://en.wikipedia.org/wiki/Dipole_antenna","type":"reference","retrieved_at":"2026-09-24"},{"name":"ARRL Antenna Book for Radio Communications (American Radio Relay League) – dipole and loop length formulas","url":"https://www.worldcat.org/search?q=ARRL+Antenna+Book","type":"textbook","retrieved_at":"2026-09-24"},{"name":"Wikipedia – Velocity factor","url":"https://en.wikipedia.org/wiki/Velocity_factor","type":"reference","retrieved_at":"2026-09-24"}],"freshness":{"type":"static","max_age_seconds":null,"note":"Deterministic formula with fixed constants; results never go stale. Inputs supplied by the caller determine the output."},"examples":[{"name":"Half-wave dipole for 146 MHz (2 m band), vf 0.95","inputs":{"frequency_mhz":146,"antenna_type":"half_wave_dipole","velocity_factor":0.95},"expected":{"wavelength_m":2.0534,"wavelength_ft":6.7368,"total_length_m":0.9754,"total_length_ft":3.2,"total_length_in":38.4,"each_leg_m":0.4877,"each_leg_ft":1.6,"each_leg_in":19.2},"url":"https://tttkmbb.com/api/v1/calculate/antenna-length?frequency_mhz=146&antenna_type=half_wave_dipole&velocity_factor=0.95"},{"name":"Quarter-wave vertical for CB channel 19 (27.185 MHz)","inputs":{"frequency_mhz":27.185,"antenna_type":"quarter_wave_vertical"},"expected":{"wavelength_m":11.0279,"total_length_m":2.6191,"total_length_ft":8.5929,"total_length_in":103.11},"url":"https://tttkmbb.com/api/v1/calculate/antenna-length?frequency_mhz=27.185&antenna_type=quarter_wave_vertical"}],"faq":[{"q":"Why is the dipole shorter than half a wavelength?","a":"Capacitance at the wire ends (end effect) and the finite conductor diameter make a wire resonate at a length about 5 % shorter than λ/2 in free space, which the 0.95 velocity factor and the 468/f ft rule build in."},{"q":"Does the velocity factor apply to the loop?","a":"No. A full-wave loop resonates when its total wire length is slightly longer than one wavelength, so the calculator uses the empirical 1.02 λ (1005/f ft) rule and ignores velocity_factor for loops."},{"q":"Does a quarter-wave vertical need anything else?","a":"Yes, a ground plane: radials or a conductive surface of about the same length as the radiator form the other half of the antenna. The 5/8-wave vertical also needs a base loading coil to match 50 Ω."}],"tags":["antenna length","dipole length calculator","quarter wave antenna","468 divided by frequency","full wave loop","ham radio antenna"],"related":[{"calculator_id":"wavelength-frequency","reason":"Wavelength for any frequency and propagation speed."},{"calculator_id":"lc-resonance","reason":"Tuned circuits and traps used with antennas."},{"calculator_id":"length","reason":"Convert the cut length between metres, feet and inches."}],"links":{"html":"https://tttkmbb.com/engineering/antenna-length","markdown":"https://tttkmbb.com/engineering/antenna-length.md","json":"https://tttkmbb.com/engineering/antenna-length.json","api":"https://tttkmbb.com/api/v1/calculate/antenna-length","schema":"https://tttkmbb.com/api/v1/calculators/antenna-length","openapi":"https://tttkmbb.com/openapi.json","mcp":"https://tttkmbb.com/mcp"},"version":"v1","updated_at":"2026-09-24"},"timestamp":"2026-09-24T03:46:13Z","next_actions":[{"tool":"run_calculator","calculator_id":"antenna-length","reason":"Run Antenna Length Calculator with the inputs above."}],"links":{"markdown":"https://tttkmbb.com/engineering/antenna-length.md"}}