Why are two antennas usually required for an ILS system?

Why are two antennas usually required for an ILS system?

• A. The redundancy of antennas allows maintenance without navigation loss.
• B. The localizer and glideslope signals are transmitted on different frequencies.
• C. They cover different azimuth and elevation sectors
• D. One antenna reduces interference from other navigation aids

Answer: (B)

Two separate guidance channels are needed for ILS: azimuth from the localizer and elevation from the glideslope, and they operate on different frequency bands. The localizer uses VHF frequencies to create the horizontal (left-right) guidance, while the glideslope uses a different, UHF range to provide vertical guidance. A single antenna cannot efficiently receive both frequency bands, so dedicated antennas are used for each system to ensure accurate reception, proper pattern formation, and minimal interference. The other reasons people mention, like redundancy or covering different sectors, aren’t the primary reason—the frequency separation is what drives the need for two antennas.

Two separate guidance channels are needed for ILS: azimuth from the localizer and elevation from the glideslope, and they operate on different frequency bands. The localizer uses VHF frequencies to create the horizontal (left-right) guidance, while the glideslope uses a different, UHF range to provide vertical guidance. A single antenna cannot efficiently receive both frequency bands, so dedicated antennas are used for each system to ensure accurate reception, proper pattern formation, and minimal interference. The other reasons people mention, like redundancy or covering different sectors, aren’t the primary reason—the frequency separation is what drives the need for two antennas.