Portable wireless applications employing ultrawideband (UWB) frequency coverage require antennas that are small in size but capable of the broad frequency ranges in such systems. By using a quasiself- complementary structure with coplanar-waveguide (CPW) feed and tapered radiating slot, a miniature antenna was developed measuring only 18 x 16 x 1.6 mm for ease of installation in portable wireless devices. By means of discovering an optimized curvature of the radiating element and the ground plane, the antenna achieves a total bandwidth of 2.3 to 14.6 GHz with flat gain from 2.3 to 12.3 GHz. Simulations show that the optimization approach is effective in producing an antenna with wide impedance bandwidth, good return loss, and flat gain over wide operating bandwidths. Simulations were performed with two well-known commercial computer-aided- engineering (CAE) tools, with both simulation programs showing close agreement to measured results.

Since the United States Federal Communication Commission (FCC) designated the unlicensed frequency band from 3.1 to 10.6 GHz for low-power UWB wireless use,1 a great deal of electronics firms have pursued product development in that area. UWB technology offers numerous advantages over more narrowband technologies, including the capability of transferring high data dates with low spectral power density and low complexity and low cost in the transceiver systems. As part of UWB system development, planar monopole antennas have received considerable attention owing to their attractive merits, such as large impedance bandwidth, ease of fabrication and acceptable radiation properties. To reduce the size of planar UWB antennas, a number of researchers proposed microstrip or CPW feed structures.1-12 Compared to microstrip-fed antennas, CPW-fed designs offer the convenience of having the feed line and slots on one side of the printedcircuit- board (PCB) substrate.2 To obtain wide bandwidths, a variety of different optimum metal radiation patch geometries were developed, including a fork shape,1 an elliptical shape,3 a square shape,4 a spade shape,5 a circular shape,6 or with some modifications made to the radiation patch. In ref. 7, for example, one quarter of a circuit was removed from the two upper portions of a rectangular patch. In ref. 8, an UWB monopole antenna was presented consisting of a printed half-circle disk and a rectangular patch with two steps and a circular slot. Unfortunately, all of these UWB antennas were relatively large, limiting the possibility of their integration into portable wireless devices.

More recent UWB antenna design efforts have focused on miniaturizing the structures of these antennas.9-12 Among those designs include self-complementary antennas with very promising performance and features.11,12 In ref. 11, for example, a very small-size printed quasiself- complementary antenna with relatively wide bandwidth was presented. A triangular slot was inserted on the ground plane to enhance the impedance matching of the antenna. This type of antenna can yield an ultrawide impedance bandwidth with reasonable radiation properties. But the antenna is fed by a microstrip line making it difficult to integrate into a monolithic UWB chip. In ref. 12, a G-shaped quasi-self-complementary antenna with a tapered radiating slot was presented with a wide frequency bandwidth from 3 GHz to more than 12 GHz. Although the antenna was small in size and fed by a CPW line, its metallic patch and ground plane surface area were large, establishing a lower-frequency limit on the design.

The current article reports on a miniature CPW-fed antenna based on the quasi-self-complementary structure proposed in ref. 12. To reduce the patch and the ground plane surface size, 10 ellipses were used in shaping the radiating element and the ground plane. By using this large number of ellipses, it is possible to form a smooth curved surface. This allows more flexibility in the antenna design in terms of optimization and reduces the lower-frequency constraint, allowing the antenna to achieve a fractional bandwidth of more than 110 percent. The size of this new antenna is smaller than the quasi-self-complementary antennas reported previously.11,12 All CAE simulations performed on this miniature antenna were based on the use of the High-Frequency Structure Simulation (HFSS) electromagnetic (EM) simulation software from Ansoft and Microwave Studio from Computer Simulation Technology (CST). The measured performance for the new antenna agreed closely with the independent simulation results from these two commercial full-wave solvers. This report will also present radiation patterns and gain characteristics for the miniature UWB antenna.

The basic concept for the miniature UWB antenna stems from the design work on the quasi-self-complementary antenna in ref. 12. The geometry of that basic design was modified step by step to optimize performance and achieve an overall smaller structure. The current density of the proposed new version of the quasi-self-complementary antenna at 3 GHz is shown in the plot of Fig. 1. The current distributions for Regions A and B of the antenna are high compared to other parts of the radiating element and the ground plane over the frequency range of interest.

Due to the relatively low surface current densities for other parts of the antenna, these areas can be cut in the form of elliptical curves. To reduce the size of the antenna, further modification was carried out on Region C. Four ellipses were used to shape the top radiating element and the ground plane. By using these ellipses in shaping these areas it is possible to form a smooth curved structure. As a result, the geometrical parameters of these regions play an important role on impedance matching performance and certainly affect the lower frequency limit of the bandwidth. The new quasi- self-complementary structure and tapered radiating slot make it possible to achieve good broadband impedance matching with this modified quasi-self-complementary antenna.

The miniature quasi-self-complementary antenna was optimized by making a compromise between size and return loss, as shown in Fig. 2. The antenna is fed by a 50-O CPW line printed on a 1.6-mm-thick FR-4 substrate with relative dielectric constant of 3.0. The CPW line width W2 is 3.5 mm for an impedance of 50 O. The final antenna size of 18 x 16 mm is somewhat smaller than the quasiself- complementary antenna reported in ref. 12, at a size of 19 x 16 mm. The antenna is positioned in the x-y plane with normal direction parallel to the z-axis. The antenna's geometry was optimized for S11 return-loss performance of better than 10 dB over its entire operating frequency range of 3.1 to 10.6 GHz. Optimization was performed with the aid of the HFSS EM simulation software from Ansoft. The final antenna geometry parameters are D1 = 18 mm, D2 = 16 mm, L1 = 5 mm, L2 = 15.3 mm, L3 = 1.7 mm, L4 = 1 mm, L5 = 4 mm, L6 = 8.48 mm, W1 = 7.5 mm, W2 = 3.5 mm, W3 = 3 mm, g = 1 mm, H = 1.6 mm, R1 = 3.25 mm, R2 = 5.65 mm, R3 = 2.02 mm, R4 = 5.65 mm, R5 = 1.5 mm, R6 = 4.24 mm, R7 = 0.5 mm, and R8 = 1.875 mm.

All simulations were carried out with the Ansoft HFSS and CST Microwave Studio commercial CAE software simulation programs. The simulated return loss of the antenna is given in Fig. 3. A satisfactory agreement was observed between the two independent full-wave solvers. The results of the proposed antenna from ref. 12 are also shown for comparison. It can be observed that the lower-frequency limit was reduced by using the optimized curvature of the radiating element and the ground plane. In the modified design presented in this report, the total impedance bandwidth for a better-than-10-dB return loss spans 2.3 to 14.6 GHz, which covers the full FCC-approved UWB bandwidth and achieves a fractional bandwidth of more than 110 percent compared with the results of ref. 12.

Figure 4 shows the simulated radiation patterns in the Eplane (x-y plane) and H-plane (y-z plane) at 3 and 8 GHz, respectively. It can be observed that the proposed antenna has a dipole-like radiation pattern (co-polarization) in the E-plane and a directional radiation pattern (cross-polarization) in the H-plane. The simulated gain of the proposed antenna is illustrated in Fig. 5. The plot shows predicted gain that extends beyond the original design bandwidth of 3.1 to 10.6 GHz, with usable gain past 15 GHz and below 3 GHz. From the HFSS simulation, it was found that the gain variation is less than 2 dB from 2.3 to 14.6 GHz.

In summary, based on work previously presented in ref. 12, a modified quasi-self-complementary antenna was developed with the intention of achieving broad frequency coverage in a miniature structure suitable for use in portable wireless devices. With the goal of reaching better than 10-dB return loss, a wide impedance bandwidth of about 12.3 GHz, from 2.3 to 14.6, was achieved with the compact antenna design. Besides a good agreement between using Ansoft HFSS and CST Microwave Studio, the antenna's fairly uniform radiation pattern and gain flatness indicate the applicability of the designed quasi-self-complementary antenna for UWB applications. The design is smaller than previously reported UWB antennas of this type, with the well-behaved gain response that makes it a candidate for UWB applications in portable electronic devices.

ACKNOWLEDGMENTS
The authors would like to thank the National Nature Science Foundation of China for financially supporting this research under No. 60776021.

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