STUDY OF A SLIT CUT ON A MICROSTRIP ANTENNA AND ITS APPLICATIONS Xue-Xia Zhang 1 and Fan Yang 1 1 Department of Electronic Engineering Tsinghua University Beijing, 100084, P.R. China Recei ed 5 January 1998 ABSTRACT: The slit cut on a microstrip antenna is studied in this paper, and a multiport series inductance network is established for analysis. Based on this study, two slit applications are proposed: to adjust the resonant frequency, and to make a small microstrip antenna. Experi- ments agree well with the theoretical analysis. 1998 John Wiley & Sons, Inc. Microwave Opt Technol Lett 18: 297300, 1998. Key words: slit; multiport series inductance network; microstrip antenna INTRODUCTION The microstrip antenna is now widely used. It has many advantages, such as low profile, light weight, low production cost, conformal nature, etc. But once it is fabricated, its features are fixed and difficult to adjust. Some methods have been proposed to solve this problem. In addition, the method for cutting a slit on the microstrip patch proposed in this paper is an effective one. This method is to cut a slit at an adequate position and of an adequate length. The method used to make the slit is very easy: one only needs to draw a slit on the metal patch using a sharp knife as shown in Ž. Ž. Figures 1 a and 3 a . The slit has the effect of decreasing the resonant frequency. Because of this effect, the slit has two applications. First, when the resonant frequency shifts and is greater than the initial design, cutting a slit can adjust the resonant frequency to the expected frequency. Second, an H slit can effectively minimize the size of the microstrip an- tenna from a half wavelength to nearly a quarter wavelength. ANALYSIS AND EXPERIMENTS Modeling of the Trans erse Slit. The main mode of a mi- crostrip patch antenna is the TM mode, so it can be treated 10 as an open circuit resonator. The current flows along the length direction. Thus, the resonant frequency can be ob- tained as follows: Ž. kL 1 e where k is the wavenumber in the substrate and L is the e equivalent length of the current flow path. When a slit is cut on the patch, it cuts off the flow path of the electrical current. The current has to flow around the slit, and the equivalent length of the current flow path gets longer. Thus, the resonant frequency will decrease. This effect can be described in terms of an equivalent series Ž. inductance, as shown in Figure 1 b . The multiport network model is used for analysis 1 . To calculate the effect of the slit carefully, a series slit network model is established. The two patches are linked together Ž. through this multiport slit network, as shown in Figure 1 c . Since the slit is only a line, the effect of the slit can be modeled by inserting a transverse magnetic wall of zero thickness into the microstrip patch. Wheeler’s equivalent volume concept 2 is applied to calculate the effect of this magnetic wall. From 3 , we know that the value of the inductance on a microstrip line can be calculated as follows: 2 L l s 0 1 Ž. 2 ž / h 4 W' where l and W' are the equivalent length of the slit and the 1 width of the microstrip line. Considering the difference between the transmission line and the antenna, the formula should multiply a factor of 12 because, on the transmission line, the current has one direc- tion, while on the antenna, the current has two directions. Thus, the inductance can be calculated by the following formula: 2 h l 0 1 Ž. L . 3 s ž / 8 W' Considering the electromagnetic field distribution along the slit, an inductance distribution is needed to simulate the field around the magnetic wall. Here, we adopt a linear distribution. From Figure 1, we can see that at the center of the slit, the effect of the slit is larger than at the edge. So the susceptance at the center is smaller than at the edge. Thus, the susceptance can be calculated as Y 2 i s i 1,2,..., N Ž . 2 NN 1 Ž. Y 4 i Ž . Y 22 N 1 i s i N 1,...,2 N Ž . 2 NN 1 1 Ž. Y 5 s j 2 fL s where 2 N is the port number of the network, and i is numbered from the bottom to the top. Using the segmentation method 4 , the impedance of the slit microstrip patch antenna can be calculated as 1 Ž . Z Z Z Z Z Z Z in pp pq qq rr l qp 1 i j Ž. Y Z 6 i l ij 0 i j where Z is the input impedance of the antenna, Z , Z , in pp pq Z , and Z are the impedance matrixes of the patch which qp rr can be calculated according to 5 , and Z is the element of l ij matrix Z describing the action of the slit. l Ž. From Eq. 6 , the resonant frequency can be obtained. Some experiments were carried out to verify the multiport slit network model. Different frequency bands and different Ž. positions of the slit were used in the experiment, Figure 2 a shows a microstrip antenna whose initial resonant frequency is 2.59 GHz. The upper two curves represent the results when the slit is cut on a place which is a quarter of the length to the edge, and the lower two curves represent the results when Ž. a slit is cut on the middle of the patch. Figure 2 b shows a microstrip antenna whose initial resonant frequency is 913 MHz. From the figures, we can see that when the slit length- ens, the frequency decreases more, and when the slit is cut on the middle of the patch, the frequency decreases the most. MICROWAVE AND OPTICAL TECHNOLOGY LETTERS / Vol. 18, No. 4, July 1998 297