As a further examination, we compare the loss factor as pre- dicted by Eqs. 2 and 4 as a function of the phase error in Figure 3. It emerges that the validity of Eq. (2) is restricted to a phase error of about 0.5. 3. VERIFICATION OF THE NEW GAIN FORMULA However the new approximation [4] proposed in this paper for the loss factor is in conformity with the correct gain behavior, how does it improve the gain prediction? To verify this, the gain predicted by Eq. (1), with Eqs. 2 and 4 employed for the loss factor, are compared with measured gain values for a Ku-band conical horn in Figure 4, and for a K-band conical horn in Figure 5. The measured data are obtained from the DTU-ESA Spherical Near-Field Antenna Test Facility at the Technical University of Denmark. The 1 uncertainty values for the measured values are 0.06 dB for both the Ku-band and K-band horn. It is seen that the expression presented in this paper leads to an improved comparison with measurement. 4. CONCLUSION An approximate loss factor expression useful in conical horn gain estimation was proposed. This expression leads to the correct prediction of the conical horn gain behavior. It also results in an improved comparison with measured gain data in comparison with the existing formula. ACKNOWLEDGMENT The authors would like to thank Dr. Sergey Pivnenko, DTU-ESA Spherical Near-Field Antenna Test Facility at the Technical Uni- versity of Denmark, for providing the measured data. REFERENCES 1. A.P. King, The radiation characteristics of conical horn antennas, Proc IRE 38 (1950), 249 –251. 2. P.K. Dube and S.P. Singh, Far-field patterns of metal and dielectric-wall conical horn antennas, Microwave Opt Technol Lett 36 (2003). 3. T. Milligan, Modern antenna design, 2nd ed., Wiley, 2005. 4. C.A. Balanis, Antenna theory: Analysis and design, 3rd ed., Wiley, 2005. 5. T. Milligan, Universal patterns ease circular horn design, Microwaves 20 (1981) 84. © 2007 Wiley Periodicals, Inc. DESIGN OF COMPACT CPW BANDPASS FILTERS WITH WIDE STOPBAND Li-Jing Lin, 1 Min-Hua Ho, 2 and Wei-Qin Xu 1 1 Electronic Engineering Department, National Changhua University of Education, 50007 Changhua City, Taiwan 2 Graduate Institute of Communication Engineering and Electronic Engineering Department, National Changhua University of Education, 50007 Changhua City, Taiwan Received 12 September 2006 ABSTRACT: Two compact bandpass filters composed of the lumped elements in various coplanar waveguide (CPW) forms have been pre- sented in this study. The filters were designed to operate at the fre- quency of 2.45 GHz. The behaviors of these filters can be simulated by the equivalent lumped-element circuit models with each element realized by the corresponding CPW discontinuity. Simulation and measurements were found to be in good agreement. Most attractive of all, the im- proved-design filter has achieved about a 363% (9.04 GHz) stopband bandwidth under a 20 dB out-of-band rejection, and the measured passband’s minimum insertion loss is around -1.24 dB with 10.84% 3-dB bandwidth. © 2007 Wiley Periodicals, Inc. Microwave Opt Technol Lett 49: 973–976, 2007; Published online in Wiley Inter- Science (www.interscience.wiley.com). DOI 10.1002/mop.22299 Key words: CPW; lumped element; compact design; wide stopband 1. INTRODUCTION Recently, the coplanar waveguide (CPW) structure has been widely used in the design of filters, antennas, and others in present communication systems. Many applications based on CPW struc- tures have been presented and successfully used in microwave systems, e.g., the technologies of satellite systems, mobile com- munications, and monolithic microwave-integrated circuits (MMICs) [1–3]. Especially due to the uniplanar feature of CPW in the design of MMICs, it becomes a primary structure implemented in connections between circuit blocks because of several advan- tages such as ease in series and shunt connection without via hole, insensitive to substrate thickness, lower phase velocity variation with frequency or impedance, lower crosstalk and parasitic radia- tion, low dispersion effect, and simple fabrication. In most of the microwave systems, filters are essential compo- nents with large quantity used in the systems, and play the impor- tant roles in affecting the system performance. Besides, bandpass filters (BPFs) should exhibit several characteristics such as com- pact size, high selectivity, and wide stopband to enhance the performance of wireless communication systems in both the pass- band and the stopband. In the past few years, several researchers have presented some CPW filters in the literatures [4 – 6], but their Figure 5 Gain of a K-band conical horn. L = 9.50 cm and D = 4.67 cm. *Measured Gain, – –  –– – – Eq. (2), and ––– Eq. (4) Figure 1 Layout of the prototype CPW BPF DOI 10.1002/mop MICROWAVE AND OPTICAL TECHNOLOGY LETTERS / Vol. 49, No. 4, April 2007 973