Definition
The three-dimensional shape of the radio signal radiated by an antenna, describing how signal strength is distributed in space around it. For an ILS localizer, the field pattern consists of two overlapping lobes — one modulated at 90 Hz and one at 150 Hz — that together form the on-course signal along the extended runway centerline.
Plain English
The shape of the invisible signal coming out of an antenna — where it's strong, where it's weak, and which directions it points. For the localizer, this shape is what the aircraft receiver reads to tell the pilot whether they're left, right, or on course.
Context Anchor
Seen in ILS localizer discussions, where the handbook explains how the ground antenna sends guidance signals toward approaching aircraft.
Derivation
Field' here refers to an electromagnetic field — the region of radio energy around an antenna — not a field of grass or an airfield. 'Pattern' is the shape that field takes in space. Knowing this prevents confusion with traffic patterns or airport fields.
Why Pilots Care
The field pattern determines the accuracy and usable range of lateral guidance during an ILS approach.
Analogy
Think of a flashlight beam. The bulb sends out light, but the reflector shapes it into a useful beam; a localizer antenna sends out radio energy shaped into a useful guidance area.
Intuition Check
Do not read field pattern as a traffic pattern over an airport or a painted pattern on the ground. Here, it means the invisible shape of a radio signal in the air.
Example Sentence 1
The localizer antenna's field pattern produces two overlapping lobes that the aircraft receiver compares to determine course deviation.
Example Sentence 2
During the approach briefing the instructor explained how the localizer field pattern provides course guidance in low visibility.