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Minimum bend for coaxial transmission line

Aviation Mechanic Airframe (AMA) practice question 10 of 26 on Communications, Light Signals, and Runway Lighting Systems. ACS element AM.II.I.R2 · difficulty: applied. Want to answer it in a timed quiz? Use the Aviation Mechanic Airframe (AMA) practice mode.

Question bank updated 2026-09-22

QUESTIONA coaxial antenna transmission line has an outside diameter of 0.25 inch. When the cable is routed around a bend, the bend must be at least how large?

  1. A10 inches
  2. B1.25 inches
  3. C2.5 inches
Show the answer and explanation

Correct answer: C. 2.5 inches

  • A — incorrect Ten is the multiplier in the rule, not the answer; it must be multiplied by the cable diameter.
  • B — incorrect That is five times the diameter, which is less than the stated minimum.
  • C — correct The bend must be at least 10 times the cable diameter, and 10 times 0.25 inch equals 2.5 inches.

BACKGROUND

Transmission lines connect a transmitter or receiver to its antenna and must transfer energy with minimal loss. When installing coaxial cable the handbook directs the technician to secure it firmly along its entire length at intervals of approximately 2 feet, and to avoid routing or tying it to other wire bundles so that performance is not degraded. When the cable is bent, the bend must be at least 10 times the size of the cable diameter. Transponders, DME, and other pulse type transceivers also require transmission lines that are precise in length, so the equipment manufacturer's instructions govern type and allowable length.

STUDY TIP

Work the bend rule as a simple multiplication of diameter by ten, and remember the companion routing rules: support about every 2 feet and keep coax out of other wire bundles.

KEY POINTS

  • Coax bends must be at least 10 times the cable diameter.
  • Secure coaxial cable at intervals of approximately 2 feet.
  • Do not route or tie coaxial cable to other wire bundles.

SOURCES

  • FAA-H-8083-31B ch11 "Installation of Aircraft Antenna Systems — Transmission Lines"
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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.