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ADF sense antenna purpose

Aviation Mechanic Airframe (AMA) practice question 25 of 26 on Communications, Light Signals, and Runway Lighting Systems. ACS element AM.II.I.K8 · difficulty: standard. Want to answer it in a timed quiz? Use the Aviation Mechanic Airframe (AMA) practice mode.

Question bank updated 2026-09-22

QUESTIONA modern automatic direction finder (ADF) installation includes a sense antenna in addition to the loop antenna. What does the sense antenna accomplish?

  1. AIt removes the ambiguity of heading to or from the station.
  2. BIt increases the receiver range to about 1800 kilohertz.
  3. CIt transmits an identifying signal back to the ground beacon.
Show the answer and explanation

Correct answer: A. It removes the ambiguity of heading to or from the station.

  • A — correct In modern ADF systems an additional antenna, the sense antenna, is used to remove the ambiguity concerning whether the aircraft is heading to or from the transmitter.
  • B — incorrect Expanding the broadcast frequency range to include MF up to about 1800 kHz was an improvement of ADF over earlier radio direction finders, not a function of the sense antenna.
  • C — incorrect The ADF only receives; the identifying Morse code pulses are modulated onto the signal transmitted by the nondirectional beacon.

BACKGROUND

An automatic direction finder operates off of a ground signal transmitted from a nondirectional beacon, originally in the 190 kHz to 535 kHz range. A radio wave striking a loop antenna broadside induces a null signal, while a wave striking it in the plane of the loop induces a much stronger signal, and that property is what gives direction information. The loop alone, however, cannot tell the pilot whether the aircraft is flying toward or away from the station. The sense antenna has an omnidirectional reception field; when it is combined with the fields of the loop antenna, the result is a field with a single significant null reception area on one side. That single null is used for tuning and produces an unambiguous indication.

STUDY TIP

Associate "omnidirectional sense antenna" with "single null," and "single null" with resolving the to/from ambiguity.

KEY POINTS

  • The loop antenna gives direction but leaves a to/from ambiguity.
  • The sense antenna is omnidirectional and combines with the loop to leave one significant null.
  • That single null resolves whether the aircraft is heading to or from the transmitter.

SOURCES

  • ama-airframe/ch11.txt "Automatic Direction Finder (ADF)"
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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.