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ILS Localizer Array

Aviation & Air-Traffic-Control Antennas

A wide row of antennas past the runway end that defines the approach centerline.

Band
VHF (108-112 MHz)
Gain
Directional along the runway axis
Polarization
Horizontal

Photos

Real-world photo of a ILS Localizer Array in use
Real-world example. Source: Wikimedia Commons (CC BY-SA 3.0; User Herr-K on de.wikipedia).

Radiation / wave patterns

Idealized radiation pattern of the ILS Localizer Array
Idealized azimuth radiation pattern (illustrative, generated). Radial scale in dB.

How & why it works

The localizer gives an aircraft left/right guidance onto the runway centerline. A long horizontal row of paired directional antennas—mounted beyond the far end of the runway—radiates a VHF carrier modulated with two tones, 90 Hz favouring one side of the centerline and 150 Hz the other. Exactly on centerline the two tones arrive equally; off to one side one tone dominates, and the aircraft receiver turns that imbalance into a left/right needle deflection. The array's width sets how sharply the two patterns overlap, which controls the precision of the course.

Real-world uses

Lateral guidance of the ILS precision approach used in low visibility at airports like Calgary.