FCC Exam Question: 3-70K4

What type of antenna is used in an aircraft’s Instrument Landing System (ILS) glideslope installation?

A. A vertically polarized antenna that radiates an omnidirectional antenna pattern.
B. A balanced loop reception antenna.
C. A folded dipole reception antenna.
D. An electronically steerable phased-array antenna that radiates a directional antenna pattern.
Correct Answer: C

Explanation: A folded dipole reception antenna is commonly used for aircraft Instrument Landing System (ILS) glideslope reception. Glideslope signals operate in the UHF frequency range (329.15-335 MHz) and are horizontally polarized. A folded dipole is a resonant antenna type that is robust, provides good impedance matching, and has sufficient bandwidth to cover the required frequency range while being sensitive to horizontal polarization when mounted appropriately. Option A is incorrect because ILS glideslope transmissions are horizontally polarized, not vertically. An antenna designed for vertical polarization would have poor reception. Option B, a balanced loop antenna, is generally less efficient for UHF reception compared to dipole variants and is not the standard choice for this application. Option D, an electronically steerable phased-array antenna, is overly complex and expensive for a fixed-frequency reception system like glideslope. Aircraft glideslope receivers use simpler, fixed antennas, as the guidance information is derived from the received signal's modulation, not from antenna steering.

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Includes Elements 1, 3, 6, 7R, 8, and 9.