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Half-wave antenna length calculation

Aviation Mechanic Airframe (AMA) practice question 13 of 26 on Communications, Light Signals, and Runway Lighting Systems. ACS element AM.II.I.K3 · difficulty: applied. Want to answer it in a timed quiz? Use the Aviation Mechanic Airframe (AMA) practice mode.

Question bank updated 2026-09-22

QUESTIONUsing the handbook formula for the ideal length of a half wavelength antenna, what is the length of an antenna resonant at 117 MHz?

  1. A2 feet
  2. B4 feet
  3. C4 meters
Show the answer and explanation

Correct answer: B. 4 feet

  • A — incorrect Two feet is one-quarter of the wavelength; the formula given produces the half wavelength.
  • B — correct The formula is length in feet equals 468 divided by the frequency in megahertz, and 468 divided by 117 equals 4.
  • C — incorrect The formula returns a result in feet, so the number is right but the unit is wrong.

BACKGROUND

An antenna that is half the wavelength of the applied frequency is resonant and radiates the strongest signal. The handbook gives the ideal half wavelength antenna length in feet as 468 divided by the frequency in megahertz, a formula derived from the propagation speed of radio waves and adjusted for the dielectric effect of air at the antenna ends. Applying the formula to the VHF communication band of 118 to 136.975 MHz gives half wavelengths of about 3.96 to 3.44 feet, which is why VHF antennas are relatively long. Quarter-wave antennas are often substituted on metal aircraft because the skin forms a ground plane that supplies the missing quarter wavelength.

STUDY TIP

Practice the 468 divided by frequency calculation until it is automatic, and always check whether the question asks for the half wavelength or the quarter wavelength.

KEY POINTS

  • Half-wave antenna length in feet equals 468 divided by frequency in MHz.
  • At 117 MHz the result is 4 feet.
  • A quarter-wave antenna is half that length and relies on the aircraft skin as a ground plane.

SOURCES

  • FAA-H-8083-30B ch11 "Antennas — Length"
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SkyDrill is not affiliated with the FAA. This is an original practice item based on FAA-H-8083-31B Aviation Maintenance Technician Handbook – Airframe, and 14 CFR part 43. Regulations and handbooks are revised; confirm details against the current edition.