2. H. Nakano and K. Nakayama, A curved spiral antenna above a con- ducting cylinder, IEEE Trans Antennas Propag 47 (1999), 3– 8. 3. H.-W. Son, G.-Y. Choi, and C.-S. Pyo, Design of wideband RFID tag antenna for metallic surfaces, Electron Lett 42 (2006), 263–265. 4. M. Hirvonen, P. Pursula, K. Jaakkola, and K. Laukkanen, Planer in- verted-F antenna for radio frequency identification, Electron Lett 40 (2004), 848 – 850. © 2008 Wiley Periodicals, Inc. ULTRA-WIDE BAND MICROWAVE FILTER UTILIZING QUARTER- WAVELENGTH SHORT-CIRCUITED STUBS Mohammad Shahrazel Razalli, 1,2 Alyani Ismail, 1 Mohd Adzir Mahdi, 1 and Mohd Nizar Hamidon 3 1 Department of Computer and Communication Systems Engineering, Universiti Putra Malaysia, UPM Serdang, Selangor 43400, Malaysia; Corresponding author: alyani@eng.upm.edu.my 2 School of Computer and Communication Engineering, Universiti Malaysia Perlis, P.O. Box 77, Pejabat Pos Besar, Kangar, Perlis 01007, Malaysia 3 Faculty of Engineering, Department of Electric and Electronic Engineering, Universiti Putra Malaysia, UPM Serdang, Selangor 43400, Malaysia Received 3 March 2008 ABSTRACT: Five poles quarter-wave short-circuited stubs is designed and developed to support the ultra-wide band (UWB) applications. The filter, with a total size of 41 mm  12 mm operates within 2.7–9.83 GHz, produces a fractional bandwidth of greater than 100%. The filter is fabricated on RT/Duroid 5880 board with thickness of 0.508 mm. Measured results indicate that the filter, consisting of five short-circuited stubs, and a stepped impedance transmission line can cover up to 114% bandwidth within UWB frequency range with insertion loss (S21) better than 1.27 dB. © 2008 Wiley Periodicals, Inc. Microwave Opt Technol Lett 50: 2981–2983, 2008; Published online in Wiley InterScience (www. interscience.wiley.com). DOI 10.1002/mop.23830 Key words: microwave filter; UWB; microstrip; quarter wavelength; short-circuited stubs 1. INTRODUCTION There are a number of research and studies being performed in the recent past to expand bandwidth in the bandpass filter. The demand in high speed communication has led to the design and develop- ment of wide band filters to support the applications such as ultra-wide band (UWB) technology that promises communication speed of up to 1000 Mbps. UWB data signal uses an ultra-narrow time cycle (impulse) to represent its modulation radio frequency. This data signal in time domain (t) could be ranging from 0.1 to 1 ns, thus in frequency domain (f), the data signal has a very large bandwidth of one to tenth of GHz. A number of bandpass filters have been developed to support UWB applications, reported were filters utilizing par- allel coupled-edge, half wavelength open circuited stubs, coplanar waveguide, and one-eight wavelength short circuited stubs [1–9]. Microstrip filter designed based on a circuit model for an optimum short-circuited stub which gives a fractional bandwidth of more than 120% is reported in [2]. The filter in [2] uses a straight 50  line loaded with six short-circuited stubs. We have taken the method presented in [2] as our basis of design, but our approach is to implement a stepped impedance transmission line loaded with only five short-circuited stubs. In this article, we propose a UWB microwave filter utilizing quarter-wavelength short circuited stubs with a stepped impedance transmission line that that can also support the requirement of UWB communication bandwidth of more than 100%. 2. UWB FILTER DESIGN WITH QUARTER-WAVELENGTH SHORT-CIRCUITED STUBS The design is employed from a low-pass Chebychev filter proto- type with 0.1 dB passband ripple. The equivalent circuit for short-circuited stub filter is shown in Figure 1 [1, 10]. The model in Figure 1 is derived from J-inverters by using conventional filter design and the line admittances, Y i,i+1 are given to fulfill the specifications. The separation distance between the stubs are de- noted by l i,j whereas the stub length is given by l i . For a given degree n, where n is the number of poles in a filter, the stub length and separation depend on characteristic admittances, Y i , and trans- mission line admittances, Y i,i+1 . The transmission line admittances, Y i,i+1 can be obtained by using Eq. (1) [1, 10], Y i,i+1 = Y 0 J i,i+1 Y 0  , for i = 1 to n - 1 (1) where J i,i+1 is the J-inverter given by Eq. (2), J i,i+1 Y 0 = hg 0 g 1  g i g i+1 for i = 2 to n - 2 (2) Stub admittances, Y i can be found from Eqs. (3) and (4), Y i = Y n = g 0 Y 0 1 - h 2  g 1 tan + Y 0 N i, i+1 - J i, i+1 Y 0  (3) for i = 1 and for i = n, and (3) Figure 1 Short-circuited stubs filter model TABLE 1 Admittances and Impedances of Stubs and Transmission Lines i Y i (mhos) Y i,i+1 (mhos) Z i (ohms) Z i,i+1 (ohms) 1 0.01037 0.0224 96.43 44.64 2 0.01043 0.0209 95.87 47.84 3 0.01072 0.0209 93.28 47.84 4 0.01043 0.0224 95.87 44.64 5 0.01037 – 96.43 – DOI 10.1002/mop MICROWAVE AND OPTICAL TECHNOLOGY LETTERS / Vol. 50, No. 11, November 2008 2981