Communications: radio wave antennas – Antennas – Wave guide type
Reexamination Certificate
2000-02-25
2001-09-25
Wong, Don (Department: 2821)
Communications: radio wave antennas
Antennas
Wave guide type
C343S7810CA, C343S7810CA, C343S835000, C343S837000
Reexamination Certificate
active
06295034
ABSTRACT:
TECHNICAL FIELD
The present invention relates generally to an antenna, and more particularly to a common-aperture antenna with a high-efficiency feed and a method for designing the same.
BACKGROUND OF THE INVENTION
Common aperture antennas are generally known. For example, U.S. Pat. No. 5,214,438 describes a millimeter wave and infrared sensor in a common receiving aperture. In the past, false target acquisitions have degraded the cost effectiveness of single sensor seekers. Weather conditions and the time of day can adversely affect the ability of the sensor to acquire the target. Millimeter wave (MMW) energy is useful under adverse weather conditions. However, the resolution is not as precise as exhibited by optical systems operating in the infrared (IR) region. In an optical system, resolution is adversely affected by rain, fog or humidity. These conditions can reduce the effectiveness of such sensors in the optical spectral region. Target acquisition can be substantially improved by combining millimeter wave and infrared optical signals, substantially reducing the influence of climatic conditions. IR and MMW are also susceptible to known countermeasures of various kinds and therefore a combined aperture system is less susceptible to a single type of countermeasure.
Despite the aforementioned advantages associated with such common aperture antennas, applicants have found that various problems exist with conventional designs. For example, a prime-focus reflector antenna design may have an abnormally large amount of central blockage (much larger than the feed would normally induce) created by another part of the overall system. In such a situation, it is left to the antenna designer to maximize the reflector antenna performance in the presence of this blockage.
As a more specific example, an IR sensor within the common aperture antenna may share the same main reflector surface as an RF (microwave or millimeter wave) reflector antenna. In such a situation, the reflector configuration is often dictated by the more stringent IR system requirements. This typically has an adverse affect on the performance of the RF system. That is to say what is advantageous for the IR system is typically not what is advantageous for the RF system.
In view of the aforementioned shortcomings associated with conventional designs, there is a strong need in the art for a common aperture antenna which can provide efficient operation with respect to each of the systems. For example, there is a strong need for a common aperture reflector antenna which may be optimized for an IR system and also efficiently configured for an RF system. Moreover, there is a strong need in the art for a method of designing such an antenna.
SUMMARY OF THE INVENTION
A common aperture reflector antenna and feed are presented for use in common aperture sensor systems. In an exemplary embodiment of the invention, the feed includes an array of individual elements. The array elements are configured to increase the overall efficiency of a reflector antenna by flattening the aperture illumination, and also by nullifying the illumination within the centrally-blocked-portion of the reflector antenna surface. More specifically, the array elements are carefully configured with respect to spacing and excitation, for example, such that the array illuminates only the non-blocked portion of the main reflector. In addition, the array pattern is optimized such that the non-blocked portion of the reflector antenna is quasi-uniformly illuminated.
According to one aspect of the invention, a common aperture reflector antenna is provided. The antenna includes a main reflector having a generally parabolic reflective surface and a boresight axis extending from a vertex of the main reflector through a focal point of the main reflector. In addition, the antenna includes a feed located generally at the focal point for illuminating the main reflector with and/or receiving from the main reflector radio frequency (RF) energy of a predefined RF wavelength to transmit/receive RF energy; and at least one of a sub-reflector and a sensor located generally at the focal point for reflecting or receiving energy of a predefined wavelength different from the predefined RF wavelength. A blockage of the main reflector due to the sub-reflector or the sensor along the boresight axis is equal or greater than a blockage of the main reflector due to the feed. In order to counteract such blockage, the feed is configured to direct a majority of RF energy from the feed towards regions of the main reflector which are not blocked by the sub-reflector or the sensor.
According to another aspect of the invention, a method is provided for designing such an antenna. The method includes the steps of selecting an initial estimate for a feed array making up the feed on a basis of blockage of the main reflector due to the sub-reflector or the sensor and at least one of a number of array elements, spacing of the array elements, amplitude excitation of the array elements, diameter of the main reflector and focal length of the main reflector; evaluating a performance of the feed array based on the initial estimate; computing a figure of merit indicative of the RF efficiency of the antenna based on the evaluated performance; and optimizing the RF efficiency by altering the initial estimate and reevaluating the performance and figure of merit.
REFERENCES:
patent: 4482897 (1984-11-01), Dragone et al.
patent: 4636797 (1987-01-01), Saffold et al.
patent: 4689632 (1987-08-01), Graham
patent: 4866454 (1989-09-01), Droessler
patent: 5041840 (1991-08-01), Cipolla et al.
patent: 5214438 (1993-05-01), Brusgard et al.
patent: 5298909 (1994-03-01), Peters et al.
patent: 5327149 (1994-07-01), Kuffer
Brown Kenneth W.
Drake Thomas A.
Chen Shih-Chao
Raytheon Company
Renner , Otto, Boisselle & Sklar, LLP
Wong Don
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