Offset dish antenna

Lore & Background

The offset dish antenna was developed to address a fundamental limitation of front-fed parabolic dishes: the feed structure and its supports block a portion of the incoming radio waves, casting a shadow that reduces aperture efficiency and gain. By positioning the feed outside the beam path—typically below the dish on a boom—the offset design eliminates this obstruction. The beam axis is skewed at an angle to the plane of the dish mouth.

Before the 1970s, offset designs were mostly limited to radar antennas, which required asymmetric reflectors to create shaped beams. The advent of computer design tools in the 1970s made it easier to calculate the radiation pattern of offset dishes, removing earlier design difficulties and enabling more widespread use. The dish itself is an asymmetric segment of a paraboloid with different curvatures in the two axes, making it more complex to design than a front-fed antenna.

The design is most widely used for small parabolic antennas, such as common Ku band home satellite television dishes, where the feed structure is large relative to the dish and would block a significant portion of the signal. It is also employed on satellites, particularly direct broadcast satellites that beam television signals to homes, where limited transmitter power demands maximum efficiency. Radar antennas also use the offset design to collect as much signal as possible for detecting faint return signals.

Reader's Guide

The offset dish antenna's significance lies in its ability to improve aperture efficiency by removing the feed and its supports from the path of the radio waves. This is particularly important for small dishes, where the feed structure would otherwise block a significant proportion of the signal, and for satellite and radar applications where every bit of signal is critical. The design's adoption was historically constrained by the difficulty of calculating the radiation pattern for an asymmetric reflector, but the development of computer design tools in the 1970s removed this limitation. Since then, efficient offset designs have been used more and more widely. The offset dish antenna remains a key technology for home satellite television reception, direct broadcast satellites, and radar systems, where its ability to maximize signal collection directly impacts performance.

Did You Know?

• The feed is located at the focal point of the reflector, but offset to the side so it does not block incoming radio waves.
• Before the 1970s, offset designs were mostly limited to radar antennas that required asymmetric reflectors for shaped beams.
• The beam axis of an offset dish antenna is skewed at an angle to the plane of the dish mouth.