GHz. The insertion loss difference between the lowest and highest frequency is 9.67 dB. For the applications where such difference is acceptable, the tuning range and bandwidth of the filter are calcu- lated to be 10.8 and 2%, respectively, using the measured S21 data of the fabricated filter shown in Figure 6. The insertion loss of the filter is measured as 6.18 dB at 2.2 GHz which is 4.2 dB lower than insertion loss at 2.34 GHz. In applications where an insertion loss less than 5 dB is needed through the tuning range, tuning range of the fabricated third order tunable bandpass filter is 6.2%. Figure 7 shows the fabricated third order tunable bandpass filter printed on a Rogers RO4003 substrate. Despite the high insertion loss in the lower frequency band, the tuning range of the varactor loaded SRR filter is fairly good and is five times of the filter bandwidth. 3. CONCLUSION A compact planar tunable microstrip bandpass filter using varactor loaded SRRs has been implemented. A reverse-biased diode with voltage dependent variable nonlinear capacitance is used as the varactor. A prototype third order tunable bandpass filter using three coupled resonator with a passband located at around 2.4 GHz is designed, fabricated and tested. The dimension of the third order fabricated tunable filter is 23.8 mm by 24.9 mm, and is less than 1/3 of the resonant wavelength. Therefore the miniaturization factor is about 3.5 compared to a third order half-wavelength coupled line filter. REFERENCES 1. C.T.-C. Nguyen, L.P.B. Katehi, and G.M. Rebeiz, Micromachined devices for wireless communications, Proceedings of the IEEE 86 (1998), 1756 –1768 2. K. Entesari and G.M. Rebeiz, A differential 4-bit 6.5-10-GHz RF MEMS tunable filter, IEEE Trans Microwave Theory Tech 53 (2005), 1103–1110. 3. Y. Ishikawa, T. Nishikawa, T. Okada, S. Shinmura, Y. Kamado, F. Kanaya, and K. Wakino, Mechanically tunable MSW bandpass filter with combined magnetic units, IEEE MTT-S Int Microwave Symp Digest, Dallas, TX (1990), 143–146. 4. A.R. Brown and G.M. Rebeiz, A varactor-tuned RF filter, IEEE Trans Microwave Theory Tech 48 (2000), 1157–1160. 5. M. Makimoto and M. Sagawa, Varactor tuned bandpass filters using microstrip-line ring resonators, IEEE Int Symp Microwave Theory Tech Digest, Baltimore, MD (1986), 411– 414. 6. B.W. Kim and S.W. Yun, Varactor-tuned combline bandpass filter using step-impedance microstrip lines, IEEE Trans Microwave Theory Tech 52 (2004), 1279 –1283. 7. D. R. Smith, W. J. Padilla, D. C. Vier, S. C. Nemat-Nasser, and S. Schultz, Composite medium with simultaneously negative permeabil- ity and permittivity, Phys Rev Lett 84 (2000), 4184 – 4187. 8. K. Aydin and E. Ozbay, Capacitor-loaded split ring resonators as tunable metamaterial components, J Appl Phys 101 (2007), 024911. 9. I. Gil, J. Bonache, J. Garcia-Garcia, and F. Martin, Tunable metama- terial transmission lines based on varactor-loaded split-ring resonators, IEEE Trans Microwave Theory Tech 54 (2006), 2665–2674. 10. J.S. Hong and M. J. Lancaster, Microstrip filters for RF/microwave applications, Wiley, New York, 2001. © 2009 Wiley Periodicals, Inc. A HIGH-PERFORMANCE 94 GHz PLANAR QUASI-YAGI ANTENNA ON GaAs SUBSTRATE Le Huu Truong, Yong-Hyun Baek, Mun-Kyo Lee, Sun-Woo Park, Sang-Jin Lee, and Jin-Koo Rhee Millimeter-Wave Innovation Technology Research Center, Dongguk University, 3 Ga, 26 Pildong, Jung-Gu, Seoul 100 –715, South Korea; Corresponding author: jkrhee@dongguk.edu Received 24 January 2009 ABSTRACT: A high-performance 94 GHz planar Quasi-Yagi antenna on a GaAs substrate is presented. The antenna is fabricated on a 100 m GaAs wafer and has dimension of 1.6  2.6 mm 2 . Experimental results show that this antenna covers a wide bandwidth from 91 GHz to 106 GHz, with the return loss of better than -10 dB. An end-fire radia- tion pattern is formed, a peak gain of 7.1 dBi, and front-to-back ratio of 15 dB at 94 GHz are also achieved. © 2009 Wiley Periodicals, Inc. Microwave Opt Technol Lett 51: 2396 –2400, 2009; Published online in Wiley InterScience (www.interscience.wiley.com). DOI 10.1002/mop. 24628 Key words: Quasi-Yagi antenna; end-fire antenna; CPW-fed; millime- ter-wave antenna; W-band antenna; GaAs substrate 1. INTRODUCTION YAGI-UDA antenna is one of the most common antennas in practice, which was first introduced in Japan by Uda in 1926, and improved later by Yagi. Through its long existence, the classical Yagi–Uda antenna has been applied widely in many applications, such as TV signal received antenna and mobile base station an- tenna. However, at millimeter-wave and submillimeter-wave ranges in which the circuit size is about wavelength, capability of the classical Yagi–Uda antenna is limited due to its feeding diffi- culty and fragility. To overcome these difficulties, a solution was found first by printing a Yagi-like structure in a planar substrate and feed the antenna through a broadband transition using a microstrip line [1] or coplanar waveguide [2]. This structure was improved and applied in an active phased array antenna at X-band [3]. The qualified results showed that it not only has similar performance as the classical one but also can be fabricated easier Figure 7 Fabricated third order tunable bandpass filter. [Color figure can be viewed in the online issue, which is available at www.interscience. wiley.com] 2396 MICROWAVE AND OPTICAL TECHNOLOGY LETTERS / Vol. 51, No. 10, October 2009 DOI 10.1002/mop