Phased array antennas are widely used in the military and commercial systems. A typical phased array antenna consists of an array of many elements, an RF part consisting of transmitters and receivers, and a signal processing part for the beam forming and control. A multibeam array has a set of narrow beams(e.g. 9) radiating simultaneously from a single aperture. A typical multibeam array has a set of multiple sum and difference patterns for the search, detection and tracking of multiple targets.
In this thesis, a design and implementation of an active phased array with multiple sum and difference beams is presented. Using a single aperture of 24x24 Vivaldi elements, a sector pattern for radar transmission and 17 simultaneous sum patterns and 5 simultaneous difference patterns for radar reception are realized. To reduce the cost and the signal processing time, 24x24 elements are divided into 4x4 subarrays. Each subarray consists of 6x6 elements and each element in the subarray has its own phase shifter and attenuator(this is called the analog beam former). The 17 sum beams and 5 difference beams are clustered in a 10.5°x10.5° angular sector, which is scanned like a single beam.
In the receiving path, a received signal propagates down the receiver channel as follows. The signals from 6x6 subarray elements are combined, down-converted to an IF signal, which is then converted into a digital signal and multiplied with a weighting factor(this part is called the digital beam forming). The 16 weighted signals from the 4x4 subarray are combined and passed to the radar signal processing unit for the target search, detection, and tracking. 17 such receiver channels are required to form simultaneous 17 beams.
Scanning of the 10.5°x10.5° clustered beam is done by adjusting the phases of the subarray elements. The forming of 17 simultaneous beams is done by the digital beam forming. The radar signal processing unit analyzes 17 simultaneous signals from 17 beams to search, detect and track multiple targets.
In the transmitting path, a modulated and amplified radar transmitting signal is equally divided and fed to the 16 subarry inputs. The transmitting pattern is formed by controling the phases and amplitudes of 24x24(576) elements. The array is developed in the following steps.
Firstly, a receiving sum pattern is synthesized using a total of 24x24 elements. The element weights are realized by a combination of the subarray weights and the digital beam forming weights. A receiving difference pattern is realized using two closely-spaced sum patterns having an opposite polarity. A set of clustered 17 beams are formed by making 17 sum beams point to closely-spaced predetermined angles to cover a 10.5°x10.5° sector angle. A set of clustered 5 monopulse beams are realized in a similar manner covering a 7°x7° sector angle.
Secondly, a transmitting sector pattern is formed by weighting a total of 24x24 elements with weights derived by a genetic algorithm. The weighting of the transmitting pattern is done only at the analog beam forming. The transmitting beam pattern has a low ripple(less than ±0.5dB) in the sector region and low sidelobes(less than -25dB) outside the section region.
Thirdly, the array element is designed. The element chosen for this application is a linearly-polarized Vivaldi antenna fed by a microstrip line. The reason for the element choice is twofold. First the Vivaldi element has a wide impedance bandwidth which is advantageous in the impedance matching the elements in an active mode(i.e., when all elements are radiating with the array beams scanned up to ±40° in the horizontal and vertical directions). Secondly, the Vivaldi element has a wide H-plane beam width, which is advantageous in a wide angle beam scanning in one plane.
Next, a 8-element linear array and a 24-element array of the Vivaldi elements are designed, and fabricated and tested. Measured radiation patterns of two arrays agree well with the design.
Next, a total array of 24x24 elements are designed, fabricated and tested. The following characteristics are measured and verified: the active reflection coefficient of a sample center element, the sector transmitting pattern, the radiation pattern of a single sum beam, the scanning of a single sum beam, the radiation patterns of 17 sum beams, the radiation patterns of 5 difference beams, the monopulse slopes of 5 difference-sum beam pairs, the product of the transmitting beam pattern and the receiving beam pattern for the verification of the Tx-Rx-pattern-combined sidelobe reduction characteristics.
The construction of the thesis is as follows. In Chapter I, a review of the state of the related technology is presented followed by methods and techniques used in the thesis. Chapter II describes the theory related to the array pattern synthesis and related subjects. Chapter III presents the design of an active phased array having multiple sum and difference beams. Chapter IV is concerned with the fabrication and measurement of the designed active array. Chapter V presents the conclusion of the thesis.