Why are two antennas usually required for an ILS system?

Why are two antennas usually required for an ILS system?

• A. The localizer requires more power than the glideslope.
• B. The localizer and glideslope signals are transmitted on different frequencies.
• C. They operate on the same frequency but different polarizations.
• D. One antenna monitors signals for redundancy.

Answer: (B)

The main idea is that the two parts of an ILS provide different kinds of guidance and must be received separately. The localizer gives horizontal guidance and operates in a VHF band, while the glideslope provides vertical guidance and uses a different frequency band (UHF-like). Because these signals live in separate frequency ranges, they can be transmitted and received without interfering with each other, allowing the aircraft to tune and process both streams simultaneously with dedicated receivers. This separation also lets each antenna be optimized for its specific beam pattern and coverage—one located at the far end of the runway for the localizer, and another near the approach end for the glideslope. The other ideas—more power, the same frequency with different polarizations, or relying on redundancy—don’t explain why two antennas are used for the dual guidance signals.

The main idea is that the two parts of an ILS provide different kinds of guidance and must be received separately. The localizer gives horizontal guidance and operates in a VHF band, while the glideslope provides vertical guidance and uses a different frequency band (UHF-like). Because these signals live in separate frequency ranges, they can be transmitted and received without interfering with each other, allowing the aircraft to tune and process both streams simultaneously with dedicated receivers. This separation also lets each antenna be optimized for its specific beam pattern and coverage—one located at the far end of the runway for the localizer, and another near the approach end for the glideslope. The other ideas—more power, the same frequency with different polarizations, or relying on redundancy—don’t explain why two antennas are used for the dual guidance signals.