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The design of a small wideband phased-array radar antenna follows a process that involves radiating-element design, the feed network, and performance analysis when installed on an aircraft. As an example of this process, a simulation-based design flow can be applied to a previously published antenna system[1], and the work can be extended to include effects of the feed circuits in addition to the antenna performance when installed behind a radome on an aircraft. It becomes apparent from this example that complex antenna system behavior can be predicted by leveraging commercial 3-D electromagnetic (EM) and advanced circuit simulations.

The goal of this work is to demonstrate a simulation-based design flow for a low-cost phased-array system mounted on an aircraft platform. Figure 1 shows an overview of the antenna system and platform installation. The system consists of a four-element Vivaldi antenna array mounted within the radome of a fixed-wing aircraft. The array is fed by active Tx/Rx circuits that use traditional microstrip circuit technologies and MMIC LNAs and Pas.

The microwave circuits are designed and simulated following procedures that are familiar to microwave engineers. Circuits such as filters or matching networks are constructed using distributed models for transmission line circuit components. By cascading these components together, engineers can design and simulate circuits on the computer long before building prototypes. Simulated results are accurate as long as the distributed models are used within their operating range in terms of frequency, dimensions, and substrate parameters. Advanced EM simulators complement the circuit simulators by providing more detailed physical extraction that captures component performance and interactions between components. The combination of circuit simulations with EM simulation allows engineers to cycle through extraction, simulation, and validation to optimize designs on the computer. This concept can be extended to antenna design. However, new, sophisticated techniques that couple electromagnetics with circuits are required, because antennas generally do not have circuit models for simulation. Dynamic Link, Pushed Excitations, and Data Link are all software technologies that provide such techniques to enable complex antenna-system simulations.

Dynamic Link is a technology that provides bidirectional connection between circuit and EM simulators. Microwave engineers are familiar with using EM simulation to generate S-parameter models for components and circuits not contained in circuit simulation libraries. Those S-parameters are traditionally added to a circuit as a static black-box element. Dynamic Link automates and extends this process. A fully parameterized EM model is linked into the circuit and behaves just like any other circuit model. The bidirectional link allows parameters, such as dimensions and material properties, to be passed to the EM simulator, and S-parameter results are then passed back. Advanced multidimensional interpolation between solved dimensions in the EM models provides the speed of circuit simulation with the accuracy of full-wave electromagnetics.

Pushed Excitations is a technology that closes the loop between circuits and electromagnetics. Circuit simulation produces the voltages and currents on all nodes and all branches of the circuit, respectively. Those voltages and currents can be used as the excitation to the EM model so that engineers can visualize fields and compute secondary radiation patterns.

Data Link couples multiple EM simulation projects by exploiting the equivalence principle of electromagnetics. In the 3-D field solver high-frequency structure simulator (HFSS), the finite element method is used to compute fields in a finite 3-D volume. The tangential fields on the outer surface of that volume can be used to compute radiated fields in the near or far zone. Data Link technology uses the tangential fields on the surface of a first HFSS project as the excitation for a second HFSS project. This linkage between projects allows engineers to efficiently simulate very large and complex geometries. For example, the first HFSS project may contain a highly detailed model of an antenna. The fields radiated from that antenna can then be linked to a second HFSS project containing a radome. The linked combination allows analysis of the fine details of the antenna with the macro behavior of the larger radome.

Figure 2 depicts a single element in the 1 × 4 array of broadband tapered slot antennas. Each element consists of a Vivaldi antenna, microstrip multiplexers, low-noise and power amplifiers, and phase shifters. As shown in Figure 2, the Vivaldi antenna has an exponentially tapered slot that provides ultrawide bandwidth at gigahertz frequencies, linear polarization with high cross-polarization isolation in the principal planes, and low side lobes. Through the use of slotline-to-stripline transitions with baluns, VSWRs of less than two can be achieved over several octaves.

The feed network is a four-channel full-duplex system consisting of multiplexers, a Tx/Rx amplifier module, and a phase shifter, as shown in Figure 3. Microstrip multiplexers were selected as a low-cost, easy-to-fabricate alternative to conventional circulators. The top portion of both multiplexers and the Tx/Rx module is for the outbound transmit wave, and the bottom half is for the inbound receive wave. There are four channel paths in each multiplexer. Two paths are for the two transmit frequencies (10 GHz and 19 GHz), and two paths are for the two receive frequencies (12 GHz and 21 GHz). This full-duplex design supports simultaneous transmission and reception.

Each of the four filters in the multiplexer uses a coupled-line resonator approach with a common number of sections. The dimensions of the sections are modified to tune each filter for its center frequency and eliminate parasitic, higher-order harmonics. The filter resonator lengths are adjusted parametrically in the circuit simulator for the appropriate bandpass center frequency (10 GHz, 12 GHz, 19 GHz or 21 GHz). Spurious passbands are eliminated by adjusting the width of the microstrip transmission lines and the gap spacing between them. Traditional microwave circuit simulation with distributed component models are used for this task.

Once the four filters were constructed, the remaining components comprising the multiplexer (tees, 908 bends, transmission lines, etc.) were added to complete the structure. Circuit simulations of the multiplexer are then conducted to provide rapid confirmations of center frequency and insertion loss. Final design simulations were conducted in a planar method of moments (MoM) solver to capture the complex coupling behavior among all circuit components.

The next step is the design and simulation of the Vivaldi antenna array. The design of a single Vivaldi element is distilled into two parts. The first is the optimization of the balun and slotline, and the second is the optimization of the taper profile. Initial balun dimensions were selected based on previously published recommendations made independently by Dan Schaubert and Richard Lee. The balun and slotline design was refined in HFSS by parametrically solving for balun dimensions followed by real-time tuning.

The initial taper profile is achieved by linking HFSS with circuit simulation using Dynamic Link. Figure 4 illustrates how the taper is deconstructed into a cascade of transmission lines with varying slot width. The number of segments, the transmission line width, the length, and the gap between can be quickly parameterized in HFSS. Each segment is uniform along its length so a ports-only solution was used to create a W-element circuit model to be used in circuit simulation. The final segment can be coupled to a 377 resistor to represent the free-space impedance, or coupled to a radiation boundary in HFSS. The latter approach is used in this example.

Once an optimum profile was achieved with the cascaded network, the coupled line gap distances were curve fitted into a taper profile equation. This equation was fed into HFSS using a user-defined primitive (UDP). The final stack-up included two copper ground planes separated by an RT/Duroid 5870 dielectric.

With the final, single-element design, four copies were made and assembled into an array. The array was then tested with a series of pushed excitations whose behavior is known. A phase sequence of {0°, 60°, 120°, 180°}, for example, is known to yield a beam angle of about 22°. The results shown in Figure 5 confirm the array is well behaved when fed with ideal excitations.

Next, the array is integrated with the Tx/Rx module of Figure 3, so that simulations of a fully coupled antenna system can be performed. The Dynamic Link and Pushed Excitations methods were used to examine the antenna radiation performance while including the feed network.

In one test case simulation, each of the Tx/Rx circuits was fed a pushed excitation consistent with a 22° beam angle. In addition, each element was fed amplitude and phase variations to simulate measured manufacturing tolerances garnered from a prototype array[2]. Results for ideal pushed excitations were compared to this test case. The result of the variations between the two cases was the appearance of side lobes at lower frequencies and a 2° shift in the scan angle. This comparison illustrates how the impact of manufacturing tolerances can be tested prior to fabrication.

The final goal is to integrate the antenna system with a fixed-wing aircraft. Data Link technology is used to efficiently solve this computationally intensive project (Figure 6). The antenna system and the radome were each constructed as separate HFSS projects. Each component project is solved in HFSS individually and assembled by data linking the sources and targets in daisy-chain fashion. The source (antenna array) and target (radome) projects were data linked so that the antenna array is the radiation source for the radome target project. Pushed excitations fed to the Tx/Rx circuits were used to achieve a 22° scan angle. Results from the Vivaldi array are then used as the source for the radome project. The fields so produced inside and outside the radome are consistent with a 22° beam scan.

As project simulation times decrease and model-building procedures become more automated, the value proposition for simulation will become more significant. This will be especially true for microwave systems, such as the radar application discussed here. The two design flow examples presented for phased-array antennas and feed network systems (one for manufacturing tolerances of a 1 × 4 Vivaldi array and feed network; the other for an array integrated within the radome of a fixed-wing aircraft, along with its observed field-radiation behavior) reveal that simulations can provide detailed insight into system-level behavior when circuit and EM simulations are intelligently integrated together.

1. T.-Y. Yun, C. Wang, P. Zepeda, C.T. Rodenbeck, M.R. Coutant, M. Li, and K. Chang, “A 10-21 GHz, Low-cost, Multifrequency and Full-duplex Phased-array Antenna System,” IEEE Trans. Antennas and Propagation, vol. 50, pp. 641-650, May, 2002.

2. C. Chang, C. Rodenbeck, K. Chang, and M. Coutant, “A Four-channel Full-duplex T/R Module for Multifrequency Phased-array Applications,” Microwave Journal On-line, April, 2005.

Michael Miller is an application engineer with Ansoft. He has more than 20 years experience in antenna design, the design and application of tailored EM materials, and EM scattering prediction. He has held principal engineering positions at Boeing, Northrop-Grumman and Lockheed-Martin. He has an MSEE from Arizona State University. Miller may be reached at mmiller@ansoft.com.

Arien Sligar attended Oregon State University, where he received his B.S. and M.S. in electrical engineering in 2004 and 2006, respectively. In 2006, he joined Ansoft as an applications engineer focusing on RF/high-frequency tools. Sligar may be reached at asligar@ansoft.com.

Steve Rousselle joined Ansoft in 1999 where he served as a technical director. He has created several courses for Ansoft HFSS, has published several articles on EM simulation, and has presented numerous courses and seminars on signal integrity and EM simulations with Ansoft products. Prior to Ansoft, he worked at Northrop Grumman (TRW) and Delphi Automotive as an antenna-system design engineer. He holds BSEE and MSEE degrees from Michigan Technological University. Rousselle may be reached at srousselle@ansoft.com.

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