Communications: radio wave antennas – Antennas – With housing or protective covering
Reexamination Certificate
1994-11-03
2001-04-17
Ho, Tan (Department: 2821)
Communications: radio wave antennas
Antennas
With housing or protective covering
C343S705000
Reexamination Certificate
active
06219005
ABSTRACT:
FIELD AND BACKGROUND OF THE INVENTION
The present invention relates to antennas and radomes, and particularly to a method and apparatus for reducing sidelobes of an antenna covered by a radome.
Radomes are structures designed to cover antennas and thereby to protect them from direct exposure to aerodynamic and environmental conditions, while being as transparent as possible to the antenna's electromagnetic (EM) radiation. However, many types of radomes include various forms of discontinuities or blockages. For example, radomes on high-speed, airborne platforms are usually equipped with a metallic tip to protect the radome against rain erosion, and/or with a pitot probe to measure the pressure of the fluid stream. Dual-sensor infrared/radio frequency (IR/rf) homing systems may include an IR seeker mounted at the radome's nose area. Small Streamline radomes commonly include dimensional and geometrical constraints which lead to an EM discontinuity in the radome's nose area. In all the above radome constructions, the radome may produce a blockage at the antenna's aperture.
When illuminated by the main beam of the antenna, the blockage acts as a secondary source that radiates in antiphase to the primary beam of the antenna. The exact pattern of this new source depends on the blockage dimensions. This radiation pattern interferes with the antenna's main beam and sidelobes, leading to degradation in the gain and sidelobes levels of the antenna's radiation pattern.
An object of the present invention is to provide a method, and also apparatus, for reducing the sidelobes of an antenna induced by such blockages in a radome.
SUMMARY OF THE INVENTION
According to the present invention, there is provided a method of reducing sidelobes of an antenna induced by a blockage in a radome covering the antenna, comprising providing at least one passive tuning element between the antenna and the blockage, and adjusting the position of the tuning element to reduce the blockage-induced sidelobes.
According to further features in the preferred embodiment of the invention described below, the passive tuning element is provided along the line of sight between the antenna and the blockage. Preferably, the passive tuning element is a circular metal member coaxial with the line of sight between the antenna and the blockage and is supported on a thin metal rod coaxial with the line of sight.
As will be described more particularly below, the provision of such a passive tuning element substantially reduces the sidelobes of an antenna produced by the above-described blockages in a radome. A single tuning element reduces sidelobes in a narrow angular range, but may cause energy increase of adjacent sidelobes. According to a further feature of the present invention, a broader angular effect may be achieved by using multiple tuning elements. Thus, the interelement distance shifts the angular position of the sidelobe null; the larger the separation between the discs, the closer the null to the mainlobe.
The radii of the passive tuning elements are typically about the same order as the blockage radius, preferably between 0.5 and 1.5 times of the blockage radius. In addition, the tuning elements are preferably spaced from the radome tip, and from each other, a distance of about 0.5 to 1.5 the operating (middle) wavelength of the antenna.
The invention also provides apparatus including an antenna and a radome, in which the sidelobes induced by the blockage in the radiation pattern of the antenna are reduced according to the above method.
Further features and advantages of the invention will be apparent from the description below.
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Friedman Mark M.
Ho Tan
Rafael-Armament Development Authority Ltd.
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