Fan-beam antenna
A directional antenna with a narrow beam in one dimension and wide in the other.
A fan-beam antenna is a type of directional antenna whose main beam is narrow in one direction and wide in the other. This shape comes from using either a truncated or a circular paraboloid reflector. Because the reflector itself is narrow vertically and wide horizontally, the beam it produces is wide vertically and narrow horizontally—since a larger antenna dimension creates a narrower beam.
In a standard parabolic antenna, the feed horn sits at the focal point and illuminates the reflector, which sends back a tightly focused, pencil-like parallel beam. If part of the paraboloid is removed, the rays from that missing section are lost. By keeping only the horizontal or vertical portion of the antenna, only the rays at right angles to that remaining section are focused. This gives a narrow beam in that direction and a wide beam in the other.
Fan-beam antennas are commonly used in radar systems. For example, primary airport surveillance radars often use a fan-beam with the antenna section oriented horizontally, producing a narrow beam in azimuth. However, because this setup cannot determine altitude, it must be paired with a height-finder radar. The height-finder’s reflector is rotated 90 degrees, creating a beam that is wide horizontally but narrow vertically.
- Type
- directional antenna
- Beam shape
- narrow in one dimension, wide in the other
- Reflector types
- truncated paraboloid or circular paraboloid
Lore & Background
In a parabolic antenna, the feed horn is placed at the focal point and irradiates the reflector, which sends back a highly focused parallel beam described as pencil-shaped. When a section of the paraboloid is removed, rays coming from that section are lost. By keeping the antenna only in the horizontal or vertical direction, only the rays at right angles to the remaining antenna section are focused, creating a narrow beam in that direction while the beam is wide in the other direction.
Reader's Guide
Fan-beam antennas are used in radar sets. Primary radar at airports often uses a fan-beam antenna with the section of the antenna oriented horizontally to give a narrow beam in azimuth. Because such a radar cannot locate altitude, it must be complemented with a height finder. The height finder has its reflector rotated 90 degrees, producing a beam that is wide in the horizontal plane but narrow in the vertical. This pairing allows the radar system to determine both azimuth and elevation of targets. The design principle—that the larger the antenna dimension, the narrower the beam—underlies the fan-beam's ability to provide precise angular resolution in one axis while covering a broader area in the other.
Did You Know?
- A fan-beam antenna produces a beam that is narrow in one dimension and wide in the other.
- The beam pattern is achieved by a truncated paraboloid or circular paraboloid reflector.
- Primary airport radar uses a fan-beam oriented horizontally to give a narrow beam in azimuth.
- Height finder radars rotate the reflector 90 degrees to produce a beam wide in the horizontal and narrow in the vertical.
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