554 IEEE ANTENNAS AND WIRELESS PROPAGATION LETTERS, VOL. 8, 2009 A Small Printed Quasi-Self-Complementary Antenna for Ultrawideband Systems Lu Guo, Student Member, IEEE, Sheng Wang, Xiaodong Chen, Senior Member, IEEE, and Clive Parini, Member, IEEE Abstract—A novel and simple ultrawideband (UWB) printed quasi-self-complementary antenna is presented. The proposed antenna, which is fed by a microstrip line, has been demonstrated to provide an ultrawide 10-dB impedance bandwidth with satisfactory radiation properties. It also features both physically and electrically small dimensions, 16 mm 25 mm in a physical size and 0 24 in an electrical size, respectively. The key parameters that affect the performance of the antenna are investigated. A good agreement is achieved between the simulation and the measurement. Index Terms—Printed circuit board (PCB) antenna, self-complementary antenna, small antenna, ultrawide band (UWB) antenna. I. INTRODUCTION INCE the approval by the Federal Communication Commission (FCC) of a 3.1–10.6-GHz frequency band allocation to the ultrawideband (UWB) radio technology, there have been considerable research efforts dedicating to this promising technology worldwide . Consequently, as a critical part of the entire system, UWB antennas have been receiving increasing interest from both the academia and industries. Recent UWB antenna designs mainly focus on miniaturized antennas due to their ease of integration into space-limited systems –. Among those miniaturized UWB antennas, self-complementary antennas ,  have illustrated their promising prospects with broadband characteristics. Recently, our group has developed a printed quasi-self-complementary antenna that is fed by a 50– coplanar waveguide without using a matching circuit . The antenna exhibits an ultrawide 10-dB impedance bandwidth as well as reasonable radiation properties. However, it presents a relatively large physical size that still could be a challenge for integration into space-limited systems. In this letter, a very small-size printed quasi-self-complementary antenna fed by a microstrip line is presented. A triangular slot is inserted on the ground plane to enhance the impedance matching of the antenna. It has been shown that the optimal design of this type of antenna can offer an ultrawide impedance S Manuscript received January 13, 2009; revised February 16, 2009. First published March 24, 2009; current version published June 24, 2009. The authors are with the Department of Electronic Engineering, Queen Mary, University of London, London E1 4NS, U.K. (e-mail: [email protected] qmul.ac.uk; [email protected]; [email protected]; [email protected]). Color versions of one or more of the figures in this letter are available online at http://ieeexplore.ieee.org. Digital Object Identifier 10.1109/LAWP.2009.2018711 Fig. 1. Geometry of the proposed microstrip line-fed quasi-self-complementary antenna. bandwidth with satisfactory radiation properties. Key parameters that influence the performance of the antenna are investigated. Most importantly, the proposed antenna features both physically and electrically small dimensions, 16 mm 25 mm in an electrical size, respectively. in a physical size and II. ANTENNA DESIGN AND PERFORMANCE The proposed small printed quasi-self-complementary antenna is shown in Fig. 1. A half-circular disc with a radius of and its complementary magnetic counterpart are printed on the different side of the dielectric substrate (in this letter, the FR4 substrate of thickness mm and relative permittivity was used). and represent the length and the width of the dielectric substrate, respectively. In addition, a triangular notch is cut on the ground plane in order to improve the impedance matching. and denote the width and height of the triangular notch, respectively. The simulations are conducted using the CST Microwave Studio package, which utilizes the finite integration technique for electromagnetic computation . 1536-1225/$25.00 © 2009 IEEE GUO et al.: QUASI-SELF-COMPLEMENTARY ANTENNA 555 Fig. 4. (color online) Simulated (blue line) and measured (red line) radiation patterns with the optimal design at 3.07 GHz. (a) E-plane. (b) H-plane. Fig. 2. The prototype of the proposed microstrip line fed quasi-self-complementary antenna, (a) front side and (b) back side. Fig. 5. (color online) Simulated (blue line) and measured (red line) radiation patterns with the optimal design at 9.31 GHz. (a) E-plane. (b) H-plane. Fig. 3. (color online) Simulated (blue line) and measured (red line) return loss curves of the proposed printed quasi-self-complementary antenna. A prototype of the proposed printed quasi-self-complemm, mentary antenna with optimal design—i.e., mm, mm, mm, mm, mm, mm—is constructed in the Antenna Measurement Laboratory at Queen Mary, University of London (QMUL), U.K., as shown in Fig. 2. The return losses are measured by using a HP 8720ES network analyzer, and the radiation pattern measurements are performed inside an anechoic chamber. Fig. 3 displays the simulated and the measured return loss curves of the proposed antenna. The measured 10-dB return loss bandwidth is from 2.86 to 10.7 GHz, while in simulation, from 2.8 to 11.3 GHz. The measurement confirms the UWB characteristic of the proposed printed quasi-self-complementary antenna, as predicted in the simulation. It is also noticed that the antenna size is not only physically small, but also electrically at 2.86 GHz. small, only Fig. 6. Simulated peak gain of the proposed antenna. The measured and the simulated normalized radiation patterns at 3.07 and 9.31 GHz are plotted in Fig. 4 and 5, respectively. The patterns obtained in the measurement are close to those in the simulation. It is observed that the H-plane patterns are reasonable over the entire operating bandwidth. The simulated peak gain of the proposed antenna is plotted in Fig. 6; it is seen that a satisfactory gain level is achieved through the whole band. 556 IEEE ANTENNAS AND WIRELESS PROPAGATION LETTERS, VOL. 8, 2009 W =6 L = 4:8 Fig. 7. Simulated return loss curves for different widths of the ground plane mm and mm. with W = 16 L = 4 :8 W = 16 W =6 Fig. 8. Simulated return loss curves for different widths of the triangular slot mm and mm. with III. EFFECTS OF DESIGN PARAMETERS It has been shown in the simulation that the operating bandwidth of the printed quasi-self-complementary antenna is primarily dependent on the width of the antenna , the width , and the height of the triangular slot of the triangular slot . These parameters should be optimized for maximum bandwidth. A. The Effect of Antenna Width Fig. 7 plots the simulated return loss curves with different 15, 16, 20, and 24 mm) when is antenna widths ( fixed at 6 mm and at 4.8 mm, respectively. It can be seen that the return loss curves vary significantly and exhibit varis changed, the ious shapes for the four different . When first resonant frequency does not change much; however, the higher resonant frequencies vary dramatically, leading to the variations of the operating bandwidth of the antenna, as shown in Fig. 7. It is also interestingly noticed that when the width increases, the impedance matching at the lower band degrades. This is due to the fact that the antenna will more and more resemble a genuine self-complementary structure with the extension of the width . This results in the antenna impedance approaching more to 188.5 and consequently the mismatch of the input impedance to the microstrip line. Basically, the slotted microstrip line serves as a built-in transformer between the self-complementary structure and the SMA. The optimal anmm. tenna width is found to be at B. The Effect of the Width of the Triangular Slot The simulated return loss curves with mm and of 16 mm for different triangular slot optimal antenna width widths are presented in Fig. 8. It is observed in Fig. 8 that the return loss curves change substantially with the variation . When increases from 2 to 6 mm, the impedance of matching of the antenna gets better; however, the higher edge of from the bandwidth decreases. With the further increase of 6 to 8 mm, the impedance matching of the antenna becomes Fig. 9. Simulated return loss curves for different heights of the triangular slot with mm and mm. worse ,and the higher end of the bandwidth keeps reducing. The mm. optimized triangular slot width is found to be at C. The Effect of the Height of the Triangular Slot Fig. 9 shows the simulated return loss curves for different triangular heights when mm and mm, respectively. It is observed that the impedance matching is also greatly dependent on the height of the triangular slot. When is equal to 4.8 mm, the 10 dB bandwidth covers an ultrawide frequency band; when decreases to 2.8 and 0.8 mm, the lower edge of the bandwidth increases, and the impedance matching mm becomes worse. When rises to 6.8 mm, the for lower edge of the bandwidth reduces; however, the impedance matching for midband is degraded. IV. CONCLUSION A novel and small UWB printed quasi-self-complementary antenna is investigated in this letter. A prototype with the op- GUO et al.: QUASI-SELF-COMPLEMENTARY ANTENNA timal design has been fabricated, and its electrical performances have been examined. The antenna demonstrates an ultrawide impedance bandwidth with satisfactory radiation patterns. It also exhibits both physically and electrically small dimensions, in an electrical 16 mm 25 mm in a physical size and size, respectively. The critical parameters that affect antenna behaviors have been analyzed. The results show this antenna is a good candidate for UWB applications. ACKNOWLEDGMENT The authors would like to thank J. Dupuy of the Department of Electronic Engineering, QMUL, for his help in the fabrication and measurement of the antenna. The authors would also like to acknowledge Computer Simulation Technology (CST), Germany, for the complimentary license of the Microwave Studio package. 557 REFERENCES  L. Yang and G. B. Giannakis, “Ultra-wideband communications: An idea whose time has come,” IEEE Signal Process. 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