Scott Santarelli

Military antenna design using simple and competent genetic algorithms

Over the past decade, the Air Force Research Laboratory (AFRL) Antenna Technology Branch at Hanscom AFB has employed the simple genetic algorithm (SGA) as an optimization tool for a wide variety of antenna applications. Over roughly the same period, researchers at the Illinois Genetic Algorithm Laboratory (IlliGAL) at the University of Illinois at Urbana Champaign have developed GA design theory and advanced GA techniques called competent genetic algorithms-GAs that solve hard problems quickly, reliably, and accurately. Recently, under the guidance and direction of the Air Force Office of Scientific Research (AFOSR), the two laboratories have formed a collaboration, the common goal of which is to apply simple, competent, and hybrid GA techniques to challenging antenna problems. This paper is composed of two parts. The first part of this paper summarizes previous research conducted by AFRL at Hanscom for which SGAs were implemented to obtain acceptable solutions to several antenna problems. This research covers diverse areas of interest, including array pattern synthesis, antenna test-bed design, gain enhancement, electrically small single bent wire elements, and wideband antenna elements. The second part of this paper starts by briefly reviewing the design theory and design principles necessary for the invention and implementation of fast, scalable genetic algorithms. A particular procedure, the hierarchical Bayesian optimization algorithm (hBOA) is then briefly outlined, and the remainder of the paper describes collaborative efforts of AFRL and IlliGAL to solve more difficult antenna problems. In particular, recent results of using hBOA to optimize a novel, wideband overlapped subarray system to achieve -35 dB sidelobes over a 20% bandwidth. The problem was sufficiently difficult that acceptable solutions were not obtained using SGAs. The case study demonstrates the utility of using more advanced GA techniques to obtain acceptable solution quality as problem difficulty increases.

Array aperture design using irregular polyomino subarrays

This paper presents data and a procedure for the use of irregular subarrays in planar wideband arrays. Using element-level phase shifters and time delay at subarray ports, the procedure substitutes polyomino subarrays in place of conventional rectangular subarrays to eliminate quantization lobes. The paper focuses on the following details of the design: the choice of subarray size for specified bandwidth, the selection of subarray type, and the tradeoff between bandwidth and peak sidelobe level. These items of the design procedure are detailed for arrays with 4- and 8-element subarrays.

Experimental validation of a partially-overlapped, constrained-feed network for Limited-Field-of-View (LFOV) applications

This paper presents an experimental verification of the partially-overlapped, constrained- feed network first introduced in (Mailloux, 2001). Unlike (Mailloux, 2001), we chose to operate the network in a LFOV mode to significantly reduce the amount of measured data. A GA was used to optimize the complex weights of the network, resulting in a system that is capable of at least 25-dB sidelobe suppression over angular bandwidths as large as 11.54deg. Furthermore, at least 30-dB sidelobe suppression is achievable for bandwidths as large as 8.05deg. One of the major benefits of this system is that the weights at both the constituent-beam and subarray levels can correct for phase errors within the network. Since it is difficult (and/or expensive) to maintain consistent phase at this frequency, this self-calibration capability provides a big advantage.

Design of realistic phased array patch elements using a genetic algorithm

Presently most wide band array elements are complicated 3-dimensional structures which are difficult and costly to manufacture. Therefore we explore planar patch elements which offer a simplified geometry with potentially reduced weight and cost and are suitable for conformal applications. We use a genetic algorithm to design a patch element in a unit cell of an infinite periodic array and model the feed with the usual δ-gap voltage source. However, since this feed is a non-realizable mathematical concept, we also explore the antenna performance achieved with several realistic feeds. For broadside arrays we obtain a bandwidth B ≈ 4:1 both with the δ-gap and with the realistic feeds. For arrays with scan angles out to 45°, the initial bandwidth B ≈2:1 with the δ-gap is reduced to B ≈ 1.7:1 with a differential coaxial feed, whereas the initial bandwidth is maintained with a GA-optimized custom feed. The element profiles range from 0.13 λL to 0.17 λL, where λL denotes the wavelength at the low end of the operational band.

Applying EGO to large dimensional optimizations: a wideband fragmented patch example

Efficient Global Optimization (EGO) minimizes expensive cost function evaluations by correlating evaluated parameter sets and respective solutions to model the optimization space. For optimizations requiring destructive testing or lengthy simulations, this computational overhead represents a desirable tradeoff. However, the inspection of the predictor space to determine the next evaluation point can be a time-intensive operation. Although DACE predictor evaluation may be conducted for limited parameters by exhaustive sampling, this method is not extendable to large dimensions. We apply EGO here to the 11-dimensional optimization of a wide-band fragmented patch antenna and present an alternative genetic algorithm approach for selecting the next evaluation point. We compare results achieved with EGO on this optimization problem to previous results achieved with a genetic algorithm.

Parameter variation with array size for broadband arrays of irregular polyomino subarrays

This paper has investigated the array parameters for arrays of irregular polyomino subarrays used to introduce time delay for large planar phased arrays, and compared these parameters with those of arrays using rectangular subarrays. It is shown that the gain reduction is only about 0.1 dB compared to rectangular subarrays and about 0.4 dB compared to an array with time delays at each element. Finally, it is shown that the array of irregular subarrays has no evident quantization lobes (quantization lobes do not decrease with array size), but has a number of low residual sidelobes that are shown to continually decrease with array size.

Genetic design and optimization of military antennas

Genetic and evolutionary optimization techniques have been used in military antenna research and design at many levels, ranging from electrically-small antenna element design to broadband applications and array-pattern control. In this paper, we summarize in-house work in these areas, conducted at the Antenna Technology Branch of the Air Force Research Laboratory Sensors Directorate. In particular, we highlight areas where differences in modeling and simulation techniques have proven crucial in avoiding premature convergence and obtaining a valid optimal solution.

Mutual Coupling Effects and Their Compensation in a Time-Delay Scanned Array of Irregular Subarrays

A well-known array broadbanding technique employs rectangular subarrays to introduce time delay into phase scanned arrays. This practice results in large quantization lobes that grow with scan and frequency offset. Previous studies have shown that in the absence of mutual coupling, the use of irregular subarrays can eliminate these quantization lobes without significantly increasing the average sidelobe level. However, there has remained some concern that the phase discontinuity between subarrays would, through mutual coupling, further distort the aperture distribution and increase array sidelobes and reflections. This letter addresses the effect of mutual coupling in arrays of time-delayed subarrays and shows that the pattern distortion caused by mutual coupling in bandwidth systems is relatively small compared to the quantization lobes and can be completely eliminated at any one frequency (for “single mode” radiating elements) by selecting the proper incident voltage to compensate for the coupling. After compensation, any remaining sidelobes are due to the subarray phase discontinuity and are identical to those with mutual coupling neglected.

Hybrid chromosome design for genetic optimization of a fragmented patch array antenna

Chromosome design has been shown to be a crucial element in developing genetic algorithms which approach global solutions without premature convergence. The consecutive positioning of parameters with high-correlations and relevance enhances the creation of genetic building blocks which are likely to persist across recombination to provide genetic inheritance. Incorporating positional gene relevance is challenging, however, in multi-dimensional design problems. We present a hybrid chromosome designed for optimizing a fragmented patch antenna which combines linear and two-dimensional gene representations. We compare previous results obtained with a linear chromosome to solutions obtained with this new hybrid representation.