12. Y.-J. Park, S.-B. Cho, K.-H. Kim, and D.-G. Youn, Development of an ultra-wideband ground penetrating radar (UWB GPR) for nondestruc- tive testing of underground objects, IEEE Antennas Propagat Soc Symp Dig 2 (2004), 1279 –1282. 13. U. Uschkerat, Comparing UWB GPR measurements and simulation of simple shaped buried targets, 2 nd Int IEE Conf, Detection of Aban- doned Land Mines, 1998, pp. 41– 44. © 2006 Wiley Periodicals, Inc. COMPACT MICROSTRIP LOWPASS FILTER USING A MEANDER-LINE STRUCTURE Guang Yi Su, Chio Nang Chan, Un Leng Tam, and Kam Weng Tam University of Macau Faculty of Science and Technology Wireless Communication Laboratory University of Macau, Macau Received 29 December 2005 ABSTRACT: A novel compact microstrip lowpass filter using N-turn meander line structure is proposed. This low pass filter provides a transmission zero near the passband edge and low passband inser- tion loss. Both the cutoff frequency and transmission zero can be easily controlled by changing the meander line dimensions to ease the filter design. © 2006 Wiley Periodicals, Inc. Microwave Opt Technol Lett 48: 1339–1341, 2006; Published online in Wiley Inter- Science (www.interscience.wiley.com). DOI 10.1002/mop.21619 Key words: filter; meander; low-pass filter; microstrip filter INTRODUCTION The compact and high-performance microstrip lowpass filter is in high demand for modern communication systems and its usage can be for spurious-response suppression. Various types of lowpass filters have been developed to meet the above goal in the past few decades. Traditional designs are the open-stub lowpass filters and stepped-impedance ones. But their main drawback is their large circuitry, which is due to the intrinsic half-wavelength resonator used. In fact, the filter size is exces- sive if the lowpass filter is designed at a lower microwave- frequency regime. To tackle this problem, some filters using photonic bandgap (PBG), defect ground structures (DGS), stepped impedance, and asymmetric structure have been pro- posed [1– 4]. To further reduce the filter-circuitry size, a new group of microstrip lowpass filters utilizing the slow-wave effect were recently developed, and they achieve size reduction as compared to the traditional structures [5, 6]. Similar to these slow-wave filters, a novel compact microstrip lowpass filter using a meander-line structure is presented in this paper. The meander-line sections offer the lowpass-filter slow-wave ef- fect, as well as control of both cutoff frequency and trans- mission zero in order to facilitate the filter design. The perfor- mance of this new filter is validated by simulation and experiment. FILTER DESIGN Figure 1(a) shows the proposed symmetric meander-line low- pass filter, which is composed of six meander sections con- nected by a microstrip transmission line. The larger meandered pattern located in the upper and lower parts of the proposed filter offers shunt capacitance, as the folded lines can have large coupling capacitance. To tune this capacitance, additional stubs are used to connect two adjacent patterns. The smaller mean- dered pattern connected between the input and output terminals acts as a series inductance. Thus, a lowpass response is yielded. A slow-wave characteristic can also be realized by increasing the shunt capacitance and series inductance, leading to size reduction of the filter. Four open stubs are added to the loca- tions near the two feeding blocks of the filter, which behave like shunt inductors so that a transmission zero can be generated and fine tuned. Moreover, two 50 feedings are added at the filter input and output ports. The lowpass response is critically tuned by the proposed structure parameters, as depicted in Figure 1(a). For the mean- der sections, the number of meander line turns (N), as defined in Figure 1(b), varies the cutoff frequency and transmission zero of the lowpass filter, and these two frequencies are distanced to lower frequency for larger N. Similar phenomena are observed for the meander-line height ( A). Due to the slow-wave charac- teristics of the proposed lowpass filter, significant size reduc- tion is anticipated when compared with the conventional stub lowpass filter. To explore the effect due to N and A, an example lowpass filter is designed with a cutoff frequency of 2.5 GHz Figure 1 Simulation (- - - -) and measurement (____) results of the proposed lowpass filter DOI 10.1002/mop MICROWAVE AND OPTICAL TECHNOLOGY LETTERS / Vol. 48, No. 7, July 2006 1339