In ILS, why are two antennas typically used?

In ILS, why are two antennas typically used?

• A. Because the localizer and glideslope signals are transmitted on different frequencies.
• B. Because it doubles the transmitted power.
• C. Because one antenna is used as a backup for maintenance.
• D. Because it reduces interference from other navigation aids.

Answer: (A)

Two antennas are used because ILS has two independent guidance signals: localizer for horizontal (azimuth) guidance and glideslope for vertical guidance. Each signal is broadcast in a different frequency band—localizer in the VHF range and glideslope in the UHF range—so they require separate transmitters and antennas. Using dedicated antennas allows each system to have its proper radiation pattern and prevents mutual interference, while the aircraft’s receiver can independently tune and interpret both signals to provide precise approach guidance. The other ideas don’t fit because the reason isn’t simply more power, redundancy, or reduced interference in general; it’s the need to carry two distinct signals in separate frequency bands that demands two antennas.

Two antennas are used because ILS has two independent guidance signals: localizer for horizontal (azimuth) guidance and glideslope for vertical guidance. Each signal is broadcast in a different frequency band—localizer in the VHF range and glideslope in the UHF range—so they require separate transmitters and antennas. Using dedicated antennas allows each system to have its proper radiation pattern and prevents mutual interference, while the aircraft’s receiver can independently tune and interpret both signals to provide precise approach guidance. The other ideas don’t fit because the reason isn’t simply more power, redundancy, or reduced interference in general; it’s the need to carry two distinct signals in separate frequency bands that demands two antennas.