The effects of adaptive genetic algorithms (AGAs) and defected ground structures (DGSs) on performance optimization of tapered microstrip filter are investigated. The proposed structure achieves an ultra wide stopband with high attenuation within a small surface area, as well as 45% smaller size, in comparison with conventional filters. The parameters of the filter are optimized using in-home AGA code. In the proposed AGA algorithm, the crossover and mutation probabilities are adaptively changed according to the value of individual fitness. Then by utilizing the proposed DGS, a compact S-band lowpass filter with ultra-wide spurious free window is obtained. The proposed filter achieves an insertion loss of 0.8？dB from DC up to 4？GHz and 21？dB rejection in the stopband from 4.3 up to 60？GHz. The fabricated and measured results exhibit good agreement with the simulated results. They demonstrate that combining AGA and DGS yields best possible response for this group of filters. 1. Introduction In practice, high-performance microwave filters with minimized size and weight are playing an important role for design and fabrication of high-efficiency miniaturized microwave systems. Recently, due to high demand for broad band services, design of such miniaturized wide and ultra-wide stopband filters for interference cancellation (by means of out of-band signal suppression) is gaining more and more attention [1–4]. Nonuniform transmission lines (NUTLs) play an important role in microwave circuits. Applications include impedance transformation and matching, filters, and directional couples. The nonuniform line is traditionally analyzed in the frequency domain [5–7]. Genetic algorithms are widely employed in various fields such as optimal engineering designs. They have been successfully applied to finding the global optimum in a variety of unimodal domains. Parameter control methods are classified as deterministic and adaptive. Deterministic systems employ fixed, predefined parameters for GA. On the other hand, adaptive control uses feedback from the search process to find out how the parameter values change [8, 9]. Many researchers have proposed and demonstrated electromagnetic bandgap (EBG) microstrip structures to achieve compact and wide frequency stopband [10–12]. Also recently, DGSs have become one of the most interesting areas of research in modern communication systems [13–16]. The DGS was first proposed by Kim et al. [17]. The microstrip line with DGS patterns in the ground plane has the stopband characteristics due to the equivalent effective inductance of
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