Method and apparatus for mounting a rotating reflector antenna to minimize swept arc

An apparatus and method for mounting a reflector antenna system on an outer surface of an aircraft which minimizes a swept arc of a main reflector. This allows the effective frontal area of the main reflector to be reduced such that a radome with a smaller frontal area can be employed to cover the antenna system. The preferred embodiments make use of a platform which rotates the main reflector about an azimuthal axis which is disposed forwardly of an axial center of the main reflector. In one embodiment, the azimuthal axis is located in a plane extending between the outermost lateral edges of the main reflector. In another embodiment the azimuthal axis is located forwardly of the outermost lateral edges of the main reflector.

FIELD OF THE INVENTION

The present invention relates to antenna systems, and more particularly to a method and apparatus for mounting a reflector antenna in such a manner as to minimize the swept arc of the antenna when the antenna is rotated about its azimuthal axis.

BACKGROUND OF THE INVENTION

The frontal surface area of an antenna mounted on an aircraft, under a radome, is of critical importance with respect to the aerodynamics of the aircraft. This is because of the drag created by the radome and the resulting effects on aircraft performance and fuel consumption. With reflector antennas that must be rotated about their azimuthal axes, the “swept arc” of the antenna is larger than the overall width of the main reflector of the antenna. This necessitates a commensurately wide radome, thus increasing the frontal surface area of the radome and consequently increasing the drag on the aircraft.

Referring toFIG. 1, the diameter of a swept arc “A” of a main reflector of a prior art antenna system can be seen when the azimuthal axis of rotation is located rearwardly, or behind, an axial center of the main reflector, as is conventional with present day reflector antenna systems. The outermost edges of the main reflector are also noted. This diameter is noted by dimension “B”. The diameter of the swept arc produced by the main reflector is considerably larger than the diameter of the main reflector itself when the azimuthal axis of rotation is located at, or rearwardly of, the center of the main reflector.

It is therefore extremely important that the height and width of a reflector antenna be held to the minimum dimensions consistent with the required electromagnetic performance of the antenna. More particularly, it is important for the main reflector of an antenna intended to be mounted on an outer surface of an aircraft, to be mounted in such a manner that the swept arc of the antenna is minimized when the antenna is rotated about its azimuthal axis. Minimizing the swept arc of the antenna would thus minimize the dimensions of the radome required to cover the antenna, and thereby minimize the corresponding drag created by the radome while an aircraft on which the radome is mounted is in flight.

SUMMARY OF THE INVENTION

The above drawbacks are addressed by an antenna system and a method for mounting the antenna system in accordance with a preferred embodiment of the present invention. The antenna system generally comprises a main reflector which is mounted on a mounting platform. The mounting platform is rotatable about an azimuthal axis to allow the azimuth angle of the antenna to be adjusted as needed. An azimuth motor is used for rotating the platform as needed to aim the main reflector in accordance with the desired azimuth angle.

A principal feature of the present invention is that the azimuthal axis about which the main reflector is rotated is disposed forwardly of the center of the main reflector, rather than at, or rearwardly of, the center of the main reflector. In one preferred form, the azimuthal axis is located at a point within a plane extending between the outermost ends of the main reflector. In another preferred embodiment, the azimuthal axis is located forwardly of the outer ends of the main reflector. With either arrangement, the swept arc of the main reflector is reduced from that which would otherwise be produced if the azimuthal axis was located coincident with the center of the main reflector, or rearwardly of the center of the main reflector. The maximum reduction in swept arc is provided by locating the azimuthal axis within the plane extending between the outermost ends of the main reflector.

By supporting the main reflector of the antenna at a position laterally offset (i.e., rearwardly) of the azimuthal axis about which the mounting platform is rotated, the swept arc of the antenna is reduced significantly, thereby decreasing the frontal surface area of a radome needed to house the antenna system when the system is mounted on an exterior surface of an aircraft. This mounting arrangement does not significantly complicate the assembly or construction of the antenna system itself or otherwise require significant modifications to the outer body surface of an aircraft on which the antenna system is to be mounted.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Referring toFIG. 2, a prior art antenna system10well suited to be mounted on an external surface of an aircraft is shown. The antenna system10includes a main reflector12having a center12aand outermost edge portions12b.A subreflector14is positioned forwardly of a feedhorn16located at the center12aof the main reflector12. A pair of low noise amplifiers (LNA)18and20are used, as are a pair of diplexers22and24, for performing signal conditioning operations on the received and transmitted signals. An elevation motor26is used to position the main reflector12at a desired elevation angle, while an azimuth motor28is used to rotate the main reflector12about an azimuthal axis to position the main reflector at a desired azimuth angle. An encoder30is used to track the azimuth angle of the main reflector12and to provide feedback to the azimuth motor28.

Referring now toFIG. 3, an antenna system100in accordance with a preferred embodiment of the present invention is illustrated. The antenna system100is similar to antenna system10by the use of a main reflector102having an axial center102aand outermost lateral edge portions102b. A feedhorn104is disposed at the center102aof the main reflector102. The main reflector102is supported on a platform106which places the azimuth axis of rotation108of the main reflector102in a plane which extends through the outermost edges102bof the main reflector. The platform106is rotated about the azimuthal axis of rotation108by an azimuth motor110to thus position the main reflector102at a desired azimuth angle. A two channel coaxial rotary joint112is preferably employed to enable the necessary electrical connections between the feedhorn104and a transmission line112awhich extends through an outer surface114of an aircraft. For simplicity, the radome which would ordinarily enclose the entire antenna system100has not been shown.

Referring toFIG. 4, a swept arc116is shown which is produced by rotational movement of the main reflector102, shown in highly simplified form, of the antenna system100. When the azimuthal axis of rotation108is located such that it extends through the outermost lateral edges102bof the main reflector102, as described in connection withFIG. 3, the radius of the swept arc116is approximately one-half that of the overall length118of the reflector102. Thus, locating the azimuthal axis of rotation108forwardly of the center102aof the main reflector102(i.e., to the right of center point102ainFIG. 3) dramatically reduces the swept arc produced by the main reflector. This reduction in the overall area, and volume, of the swept arc is also visible from a comparison ofFIGS. 1 and 4.

The antenna system100shown inFIG. 3, however, in some applications, may result in an unacceptable degree of blockage of the signal being transmitted and/or received by the antenna system100. Accordingly, it may be desirable to locate the azimuthal axis of rotation108shown inFIG. 3forwardly of the outermost edges102bof the main reflector102. Such a mounting arrangement is shown in FIG.5. Antenna system200shown inFIG. 5is identical with antenna system100shown inFIG. 3with the exception that mounting platform206has a longer overall length to allow the azimuthal axis or rotation108to be located forwardly (i.e., to the right inFIG. 5) of the outermost edges202bof the main reflector202. It will also be appreciated that components of the antenna system200in common with those of antenna system100have been designated by reference numerals increased by a factor of 100 over those used to denote the components of the antenna system100, such as motor210and transmission line212a. The swept arc produced by the antenna system200is shown in FIG.6. The swept arc is designated by dashed circle220. The maximum, effective frontal width of the main reflector202is thus represented by arrow222, which is only slightly larger than a diameter226of the main reflector. The radius of rotation of the reflector202is represented by line224. Comparing the swept arc220ofFIG. 6with the swept arc116illustrated inFIG. 4, it can be seen that the swept arc produced by the mounting arrangement of antenna system200is slightly greater than that produced by antenna system100. However, the location of the azimuthal axis forwardly of the outermost edges202bof the main reflector202helps to eliminate a degree of the blockage produced by the mounting platform206and the rotary joint212.

The preferred embodiments of the present invention thus provide a means for supporting a reflector antenna in a manner which minimizes the effective frontal area of the reflector antenna, and thus allows a radome having a smaller frontal area to be employed in covering the antenna when the antenna is located on an outer surface of an aircraft. The preferred embodiments do not significantly complicate the construction of the antenna system nor do they complicate the mounting of the antenna system on the outer surface of an aircraft. Furthermore, the preferred embodiments do not significantly add to the costs of construction of the antenna systems.