Communications: directive radio wave systems and devices (e.g. – Directive – Including a steerable array
Reexamination Certificate
2002-07-16
2004-07-20
Blum, Theodore M. (Department: 3662)
Communications: directive radio wave systems and devices (e.g.,
Directive
Including a steerable array
Reexamination Certificate
active
06765530
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to an antenna apparatus and, in particular, a small antenna array that generates an endfire pattern in a desired direction while simultaneously forming a null in the opposite direction. Employing the particular antenna apparatus in a larger antenna array of many elements provides a wide field of view by increasing the scanning ability of the array element to near grazing angles.
BACKGROUND OF THE INVENTION
Antenna array systems for transmitting and/or receiving data or other information have been devised in a variety of configurations. Phased array antenna systems require many costly components that contribute to a design complexity that may not be acceptable or appropriate for certain situations. The most general implementation of a phased array produces an array design capable of focusing the energy from all antenna elements to any desired point in space. Phased array antennas have their elements arranged in rectangular or triangular grid lattices and are capable of focusing the antenna array pattern from broadside to the array to angles nearing 50 degrees off of broadside without difficulty. Scanning the array to angles exceeding 50 degrees becomes increasingly more difficult. In some applications, however, it may be desirable to operate an array in an endfire mode, which directs the radiation along the axis of the array at a scan angle of 0°, corresponding to 90° from broadside.
Endfire operation is the most difficult mode in which to use a phased array. Upon attempting to use a phased array to scan in the endfire direction, several problems arise which severely limit the array's ability to scan to angles approaching endfire. Traditional designs used for antenna arrays which are required to scan in the endfire direction call for very specialized antenna elements with limited fields-of-view (FOV). If an application requires an antenna array which is able to scan beyond the maximum scan angle of the antenna element, multiple arrays must be used. For example, if an application required 0°-90° of scan angle, three arrays might be needed, one for scan angles of 0°-15°, another for scan angles of 15°-30°, and a third for scan angles beyond 30°. While the use of multiple arrays can increase the scan angle of the antenna system, it can increase the cost and complexity of the antenna system.
In addition to the scan angle limitations, traditional endfire antennas have other physical problems. For example, grating lobes will be generated if the inter-element spacing exceeds &lgr;
0
/2 and the array is used to scan to angles exceeding a nominal value. This includes scanning the array to angles approaching endfire. A grating lobe is a lobe other than the main lobe produced by an antenna array when the inter-element spacing is sufficiently large to permit the in-phase addition of radiated fields in more than one direction. Grating lobes are undesirable because the antenna is less efficient due to the energy that is being directed in the direction of the grating lobe instead of in the desired direction of the main beam of the antenna pattern. Additionally, grating lobes result in possible target ambiguities and false targets which arc difficult for a radar to resolve. In order to reduce grating lobes produced by the application of an antenna for endfire applications, elements in an array are typically arranged such that the distance between the elements is less than one-half wavelength of the center operating frequency of the array (i.e. &lgr;
0
.) However, this element spacing constraint can increase the difficulty and cost to manufacture the array and increase the mutual coupling between elements causing increased mismatch with scan angle. Phased array antennas typically have certain components which are required for each element within the array. This hardware includes transmit and receive modules (T/R modules), phase shifters, low noise amplifiers, high power amplifiers, and limiters. If the elements are spaced relatively closely, as discussed above, this results in an antenna which is difficult and expensive to build, expensive to maintain, and may have reliability issues.
In addition to the cost and complexity of phased array antennas designed for use in endfire applications, the antenna systems ability to transmit and receive can also be degraded. As described above, scan limitations are common for phased array antennas scanning in an endfire mode. Scan limitations result from mutual coupling between elements in an array. Mutual coupling is the mechanism by which fields present at one element due to a forced excitation produce significant fields in other elements. Due to mutual coupling, a fraction of the energy incident on each element in the array will be scattered off the elements in all directions, allowing the elements themselves to behave as secondary radiators. Mutual coupling results in an active impedance which is a function of scan angle. If the active impedance of the elements in an array is not controlled by some means, large reflection coefficients will result and the individual elements will reflect power that is incident on them from the transmitter. In other words, the antenna will not transmit the power input into the phased array antenna. The reflection coefficient is the ratio of reflected to forward voltage at a specified reference plane. In traditional array antennas as the scan angle approaches endfire, the active impedance causes the reflection coefficient to increase towards a value of one. As the reflection coefficient approaches a value of one, only a very small percentage of the power input into the array is transmitted, while the remaining power is reflected back to the transmitter. Additionally, the reflected power creates heating within the antenna, which must be dissipated.
When scanning an array antenna, it is desired to have the magnitude of the reflection coefficient as small as possible. In many applications, an acceptable magnitude of the reflection coefficient is approximately 0.33, which results in a voltage standing wave ratio (VSWR) of 2:1. For an application with a FOV extending from 0° to 90° (where 90° corresponds to the plane of the array), the VSWR must be below 2:1 for satisfactory performance. Another way to consider the detrimental effects of excessive reflection coefficient is to consider the effective transmission loss due to reflection coefficient. An effective transmission loss can be computed for any value of reflection coefficient. Transmission loss is a measure of output power compared to input power and can be measured in dB by taking 10 log
10
(x) where x is the ratio of output power over input power.
The net result of the uncontrolled active impedance of N elements in an array antenna is to produce an excessive VSWR or excessive transmission loss at specific angles over the FOV the antenna will be used to scan through, as well as a trend of severely degraded performance over angles nearing endfire. The occurrence of high VSWR at specific angles is referred to as scan blindness. Traditionally the effects of scan blindness have been dealt with by mitigating the effects of high VSWR by designing the array with a lattice structure favoring performance over some regions while compromising performance over other regions. Although design measures can be taken to mitigate scan blindness effects and the effects of severely degraded performance as an array is scanned near endfire, technology has not been available to altogether eliminate or reduce these effects to a satisfactory level.
In addition to active impedance performance of an array another very important characteristic of the array is the reduced radar cross section (RCS). Reduced RCS is directly attributed to the reduction in impedance mismatch or reflection coefficient as an array antenna is scanned throughout its FOV.
SUMMARY OF THE INVENTION
In accordance with the present invention, an apparatus is disclosed for providing an array antenna capable of scanning zero through ninety degrees. The apparatus includes on
Boone Brian
Lalezari Farzin
Ball Aerospace & Technologies Corp.
Blum Theodore M.
Sheridan & Ross P.C.
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