Compared to traditional microstrip filters, the dual-mode patch resonator shows attractive features such as its sharp band rejection and compact size. In general, the dual-mode operation consists of two degenerate modes, which are excited by asymmetrical feed lines and some perturbation element(s) on the patch resonators. Moreover, in order to improve the coupling between the resonator and the input/output (I/O) ports, a pair of coupling gaps was adopted widely to enlarge the interface between the feed lines and the patch resonator.1-3 However, the pair of coupling gaps causes more etching uncertainties and insertion loss of the patch resonator than those with only one coupling gap or without a gap.4,5 On the other hand, some methods to reduce the filter’s size are presented. For example, Sung reported that spur-lines etched in a patch resonator are an effective way to make a compact dual-mode filter.6

Based on previous research on spur-lines,4,7 a square-patch dual-mode bandpass filter (BPF) with a novel feeding scheme and meander spur-lines is proposed in this article. By introducing two spur-lines on both sides opposite to the feed lines and removing the gaps on the feed lines, a compact and lower insertion loss dual-mode filter is designed. Its transmission performance and current distribution results are discussed. The design method is also verified by measurements.

Dual-mode BandPass Filter Design

Figure 1 Configuration of the proposed filter.

The basic structure of the proposed dual-mode BPF is shown in Figure 1, where the dimensions are in millimeters. This filter is composed of one square-patch resonator with a pair of orthogonal slots and I/O feed lines. A pair of meander spur-lines is etched in the patch resonator to perturb the fields of the patch resonator. Compared to a straight spur-line,6 a meander spur-line provides more slow-wave effect and occupies a smaller circuit area.7 Without coupling gaps in the I/O ports, a low insertion loss performance is obtained. In order to perturb two degenerate modes, the two slot lengths are not equal and the two feed lines are orthogonal.

Figure 2 Photograph of the designed bandpass filter.

The side length of the patch is designed to be 20 mm and the feed lines are connected to a 50 Ω microstrip line. The slot’s length and width are indicated by S and g, respectively. The difference in length between the two slots is ΛS. The physical dimensions of the meander spur-line are described by b, c, d, g and m. The proposed filter is fabricated on a substrate with a relative permittivity εr = 4.5 and a thickness of 0.8 mm. The center operating frequency is designed to be 2.3 GHz. The physical parameters are chosen as follows: S = 20 mm, g = 0.3 mm, b = 2 mm, c = 0.4 mm, m = 1.3 mm, d = 1.4 mm and ΔS = 0.2 mm. A size reduction of approximately 36 percent is obtained at the same resonant frequency, compared to a conventional dual-mode bandpass filter.6 A photograph of the designed dual-mode bandpass filter is shown in Figure 2.

Results and Discussion

Figure 3 Measured performance of the proposed filter.

Figure 3 shows the measured transmission performances of the designed dual-mode BPF; the measurement was obtained using a HP8722 network analyzer. A center frequency of 2.3 GHz and a 3 dB bandwidth of 23 percent are measured. Note that the two degenerate modes are located at 2.27 and 2.33 GHz, respectively. There are two transmission zeros on both sides of the passband, providing a sharp rejection and selectivity. They are -49.4 and -37.8 dB at the frequencies of 1.85 and 2.58 GHz, respectively. Furthermore, the insertion loss is better than -1.5 dB from 2.22 to 2.40 GHz and the return loss is better than -25 dB from 2.25 to 2.35 GHz. The minimum insertion loss is -0.9 dB at 2.26 GHz including SMA connector loss.

Figure 4 Simulated current distribution at the resonant frequency; (a) phase = 0° and (b) phase = 90°.

The simulated current distribution at the resonant frequency is shown in Figure 4, which is obtained by a commercial full-wave EM simulator, Ansoft HFSS. One port is driven by current in orthogonal phase while the other port is terminated in a 50 Ω microstrip line. Two degenerate modes, related to mode TM010 and TM100, can be observed. Note that the high current density distributions (red area) are located around the ends of the slots. It is interesting to see that the current density distribution is distorted by the meander spur-line, which yields the current flow rerouting.

Figure 5 Simulated insertion loss for different slot lengths.

Simulated insertion loss performances of the proposed filter with different slot lengths are presented in Figure 5. The center operating frequencies of the filter are approximately 2.17, 2.38 and 2.56 GHz for S = 17 mm, 19 mm and 21 mm, respectively. The center operating frequency is changed by 18 percent. Therefore, the simulated results show that the passband can be adjusted greatly by the slot length. Correspondingly, the transmission zeros are obviously controlled by this physical parameter.


In this article, a dual-mode square-patch filter with meandered spur-lines is introduced and verified by measurements. Compared to a traditional dual-mode square patch filter, the proposed filter provides a 36 percent size reduction with a lower insertion loss of -0.9 dB. Interestingly, it is found that the current density distribution can be distorted by meander spur-lines, which yields the current flow rerouting. This compact dual-mode BPF with low insertion loss has a good prospect in microstrip circuit applications.


1. J.S. Hong and S.H. Li, “Theory and Experiment of Dual-mode Microstrip Triangular Patch Resonators and Filters,” IEEE Transactions on Microwave Theory and Techniques, Vol. 52, No. 4, April 2004, pp. 1237-1243.

2. O. Akgun, B.S. Tezekici and A. Gorur, “Reduced-size Dual-mode Slotted Patch Resonator for Low-loss and Narrowband Bandpass Filter Applications,” Electronics Letters, Vol. 40, No. 20, 30 September 2004, pp. 1275-1277.

3. L. Zhu, B.C. Tan and S.J. Quek, “Miniaturized Dual-mode Bandpass Filter Using Inductively Loaded Cross-slotted Patch Resonator,” IEEE Microwave and Wireless Components Letters, Vol. 15, No. 1, January 2005, pp. 22-24.

4. H.W. Liu, C.Z. Qun and L.L. Sun, “Dual-mode Triangular-patch Bandpass Filter Using Spur-lines,” Electronics Letters, Vol. 42, No. 13, 22 June 2006, pp. 762-763.

5. J.L. Li, J.X. Chen, J.P. Wang, Q. Xue, W. Shao, L. Xia and L.J. Xue, “Dual-mode Microstrip Bandpass Filter Using Circular Patch Resonator with Two Transmission Zeros,” Microwave and Optical Technology Letters, Vol. 46, No. 1, 5 July 2005, pp. 28-30.

6. Y. Sung, B.Y. Kim, C.S. Ahn and Y.S. Kim, “Compact and Low Insertion Loss Dual-mode Patch Filter with Spur-lines,” Microwave and Optical Technology Letters, Vol. 43, No. 1, 5 October 2004, pp. 33-34.

7. H.W. Liu, R.H. Knoechel and K.F. Schuenemann, “Miniaturized Bandstop Filter Using Meander Spurline and Capacitively Loaded Stubs,” ETRI Journal, Vol. 29, No. 5, October 2007, pp. 614-618.

Haiwen Liu received his BS degree in electronic systems and his MS degree in radio and remote sensing from Wuhan University, Wuhan, China, in 1997 and 2000, respectively, and his PhD degree from Shanghai Jiao Tong University, China, in 2004. He is currently a professor in the school of information engineering at East China Jiaotong University, Nanchang, China. His main areas of research include radio systems and microwave RF IC design.

Xiaohua Li received his BS degree from the School of Information Science and Technology, Southwest Jiaotong University, in 2006. He is currently pursuing his master’s degree from Southwest Jiaotong University, in collaboration with the Chinese Academy of Science.