Recently, active integrated antennas have 2.4 GHZ LNA/PA/CIRCULARLY POLARIZED ACTIVE MICROSTRIP ANTENNAS TECHNICAL FEATURE

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1 MICROWAVE JOURNAL REVIEWED TECHNICAL FEATURE EDITORIAL BOARD 2.4 GHZ LNA/PA/CIRCULARL POLARIZED ACTIVE MICROSTRIP S The design, numerical simulation, realization and measurement of 2.4 GHz amplifier-type circularly polarized (CP) active microstrip antennas are presented. A low noise amplifier (LNA) is used for the receiving assembly and a power amplifier (PA) is used for the transmitting configuration. A 2.4 GHz CP square-microstrip antenna using a Lange coupler for two orthogonal feeds is designed. The spectral-domain electric field integral equation (SD-EFIE) technique is employed to accurately determine the microstrip antenna dimensions, input SWR, current distribution and radiation patterns. The semibalanced amplifiers (using PAs or LNAs) are designed and integrated with the microstrip antenna to form a transmitting- or receiving-type CP active microstrip antenna. The microstrip antenna/amplifier modules are fabricated on two FR-4 substrates with the ground planes soldered together. Potential applications of the CP active microstrip antennas include base station antennas for industrial, scientific and medical (ISM)-band cordless telephones or wireless local area networks (WLAN). Recently, active integrated antennas have received much attention as a result of their compact size and lower cost when used in transmitting and receiving systems. 1 Active microstrip antennas that integrate printed patch antennas with active circuits (such as amplifiers) improve the input SWR and bandwidth and deliver high gain performance. The amplifier-type active microstrip antennas use semibalanced amplifier configurations and two orthogonal feeds in a square microstrip antenna to achieve a CP active antenna. 2,3 The integrated RF amplifier is an LNA or PA for receiving or transmitting purposes, respectively. By adding antenna switches, the transmitting and receiving active antennas may be integrated into a transmit/receive (T/R) active antenna module. This assembly can be used for wireless communication systems, such as base station antennas for cordless telephones or WLAN applications in which CP antennas generally are preferred. This article presents the design, numerical simulation, realization and measurement of 2.4 GHz semibalanced amplifier-type CP active microstrip antennas for ISM-band wireless com- S.. LIN AND H.R. CHUANG National Cheng Kung University, Department of Electrical Engineering Tainan, Taiwan, Republic of China T.S. HORNG National Sun et-shen University, Department of Electrical Engineering Kaoshung, Taiwan, Republic of China

2 PORT 1 PORT 2 (a) PA PA 3 db, 90 LANGE COUPLER CP WAVE POINT 1 POINT 2 CP WAVE PORT 1 PORT 2 LNA X 3 db, 90 POINT 2 LANGE COUPLER (b) SUBSTRATE FOAM CONNECTOR TECHNICAL FEATURE Z Z θ LNA POINT 1 L L POINT E y θ X X E x E y E x ING PROBE GROUND PLANE SUBSTRATE ACTIVE CIRCUIT (c) Fig. 1 CP active square patch antennas using a Lange coupler for two orthogonal feeds; the (a) transmit- and (b) receive-type active antenna assemblies and the (c) two-sided patch antenna. Fig. 2 The side view of a multilayer structure with microstrip/striplines and via holes. ε rn ε r2 ε r1 STRIPLINE Z GROUND PLANE VIA HOLE h h h SIMULATION MEASUREMENT FREQUENC (GHz) Fig. 3 A 2.45 GHz probe-fed square microstrip antenna and its computed and measured input SWR. SWR mm y 9.44 mm POINT mm mm TABLE I 2.45 GHz PROBE-FED MICROSTRIP PROPERTIES Center frequency (GHz) Substrate (FR-4) dielectric constant (ε r ) 4.5 Substrate thickness (d) (mm) 1.6 Substrate loss tangent (tanδ) Metal (copper) conductivity (σ) (S/m) Microstrip dimensions (mm) Feed type Feed position (from the microstrip edge) (mm) munication applications (shown in Figure 1). First, a two-stage RF PA (approximately 20 db gain and 20 dbm output power) and a two-stage LNA (23 db gain and 2.8 db noise figure) are designed. The amplifiers are fed through a 90 Lange coupler to two orthogonal feeds of a square microstrip antenna to achieve circular polarization. The SD-EFIE technique 4 is employed to accurately determine the microstrip antenna dimensions, input SWR, current distribution and radiation patterns. The microstrip antenna/ amplifier modules are fabricated on two FR-4 substrates with the ground planes soldered together. The performance of the active microstrip antennas (including input SWR, radiation patterns and gain) are measured and discussed. THE MICROSTRIP AND RF AMPLIFIERS A Square CP Microstrip Antenna Circular polarization can be generated with a 90 hybrid coupler (such as a Lange coupler) by using two orthogonal feeds in a square patch radiator. Two orthogonal linear modes of current distribution on the patch are excited by two orthogonal feed probes with x coaxial line 9.61 a 90 phase difference. The radiation waves from two orthogonal linear modes of the patch radiator produce a CP wave. Note that right- or left-hand CP can be selected by feeding the input signal to port 1 or port 2 and connecting a 50 Ω load to the unused port. To determine the patch antenna dimensions and coaxial feed positions accurately, the SD-EFIE technique 4 is employed for design simulation. Figure 2 shows a multilayer structure with strip lines and via holes that is modeled numerically using the SD- EFIE technique. The general form of the SD-EFIE used to model the multilayer structure 5 is E (i) x,y,z tan = 1 4π 2 where x and y are on the microstrips (i and j identify different interfaces). The subscript tan means the tangential component and the subscript inc denotes the incident field. The functions denote the spectral-domain dyadic Green s function and the current densities, respectively. The dyadic Green s function and the spectral-domain dyadic Green s function are expressed as G = = 1 xxg xx xyg xy xzg xz yxg yx yyg yy yzg yz zxg zx zyg zy zzg zz G uv x,y,z z s 4π 2 G ij and J j Guv k x,k y,z z s e jk x x xs e jk y y ys dk x dk y (2) A numerical solution is obtained using the method of moments. The planar microstrip antenna used here is modeled using the SD- EFIE technique. A design example of a single probe-fed square microstrip antenna at a center frequency of GHz is shown in Figure 3 together with the simulated and measured input SWR. Table 1 lists the G ij k x,k y,z z s J (j) k x,k y,z s e jk xx e jk yy dk x dk y dz s i + E inc x,y,z = 0 (1) tan

3 TECHNICAL FEATURE t = 1/8 T t = 1/4 T t = 3/8 T t = 1/2 T t = 5/8 T t = 3/4 T t = 7/8 T t = 1 T Fig. 4 The CP probe-fed square patch antenna s current distribution at different time steps. Fig. 5 A three-dimensional view of the antenna s x- and y-component current distributions. X-CURRENT t = 1/4 T -CURRENT X-CURRENT -CURRENT t = 1/2 T X-CURRENT -CURRENT X-CURRENT -CURRENT t = 3/4 T antenna s properties. Good agreement between the antenna s comput- Fig. 6 The computed 2.4 GHz CP probe-fed square microstrip antenna s E-plane radiation patterns (φ = 90 ). 90 E θ θ Eφ 90 SCALE: 0 60 db ed and measured performance at the resonant frequency is observed. t = 1 T The microstrip antenna current distribution excited by the two orthogonal feeds with 90 phase difference at various time steps (T = period) is shown in Figure 4. Three-dimensional views of the x- and y-component microstrip antenna current distribution are shown in Figure 5. It can be observed that the x- and y-directional currents have equal magnitudes and a 90 phase difference, which is the condition needed to produce a CP wave in the forward direction of the microstrip radiator. The computed antenna radiation patterns of the vertical polarization E θ and horizontal polarization E φ components are shown in Figure 6 where the circular polarization construction can be observed.

4 TECHNICAL FEATURE RF INPUT GHz 6.2 kω 4 pf 1.0 pf 1.2 kω Fig. 8 The 2.4 GHz two-stage PA. 4 pf 8.2 nf 4 pf 8.2 nf AT BJT Fig. 7 The 2.4 GHz two-stage LNA. 5.6 kω 1.2 kω V cc = 5 V 3 pf AT BJT 7 pf RF OUTPUT TABLE II TWO-STAGE 2.4 GHz LNA PERFORMANCE Frequency range (GHz) 2.40 to 2.47 V cc (V DC) 5 Gain (db) 21.6 Input SWR < 3.6 Output SWR < 1.44 Noise figure (db) 2.6 OIP3 (dbm) 21 TABLE III TWO-STAGE 2.4 GHz PA PERFORMANCE Frequency range (GHz) 2.40 to 2.47 V DD (V DC) 5 V GG (V) 4 Gain (db) 20.5 Input SWR < db compression point (dbm) 22 PAE (%) 30 OIP3 (dbm) 31.4 RF INPUT GHz 2.2 pf 1.0 pf 1.5 kω 4 pf 8.2 nf 4 pf 8.2 nf 6.2 kω 2.2 kω AT BJT 7 pf 6.8 kω V gg = 5 V G 4 pf D CL2 S 2.2 pf V dd = 5 V RF OUTPUT The LNA and PA The two-stage LNA uses HP type AT bipolar junction transistors (BJT), as shown in Figure 7. The design goal is to emphasize low noise figure in the first stage and achieve high gain in the second stage. Conjugate matching is designed between the two amplifier stages. The bias point is V CE = 5 V and I C = 5 ma for the first stage and V CE = 5 V and I C = 15 ma for the second stage. The designed LNA is implemented on an FR-4 substrate (ε r = 4.6 and thickness d = 1.6 mm). Gain is approximately 21.6 db and noise figure is approximately 2.6 db from 2.4 to 2.47 GHz. The LNA s performance is listed in Table 2. The two-stage PA uses a type AT BJT as the driver and a Siemens type CL-2 MESFET as the power stage. Figure 8 shows the PA s circuit diagram. The PA layout is on a similar FR-4 substrate. Each transis- tor is measured first and the input/ output matching networks are designed. Both lumped elements and microstrip lines are used selectively in the matching networks to save space. After each amplifier stage is constructed, measured and tuned to the desired characteristics, the twostage PA is assembled, tested and tuned. The PA s performance is listed in Table 3. The 1 db gain compression point P 1dB is approximately 22 dbm. Gain is approximately 21 db and gain flatness is ±0.5 db. Poweradded efficiency (PAE) is approximately 30 percent and output IP3 (OIP3) is approximately 31.4 dbm. Integrating the Microstrip Radiator with the Semibalanced Amplifiers The CP active microstrip antenna is constructed using semibalanced amplifiers. The 90 hybrid coupler is realized by a Lange coupler. 5,6 This configuration as used in broadband amplifier designs also improves the bandwidth of the patch antenna and reduces the input SWR. A 3 db Lange coupler with a center frequency at GHz is designed on the FR-4 substrate. For

5 TECHNICAL FEATURE (a) SEMIBALANCED LNAs RADIATOR (b) SEMIBALANCED PAs RADIATOR Fig. 9 The realized 2.4 GHz (a) receive and (b) transmit CP active microstrip antennas. the transmitting-type active antenna, two of the designed PAs are fabricated on the same FR-4 substrate as the Lange coupler. The input terminals of the two PAs are connected to ports 2 and 3 of the Lange coupler and the PA outputs are connected to the two orthogonal feeds of the patch radiator. The ground planes of the active circuit board and patch radiator are soldered together. The receiving-type active antenna with two LNAs has the same structure (except a different input and output configuration). Figure 9 shows the layouts of the integrated receivingand transmitting-type CP square microstrip antennas with semibalanced LNAs and PAs on an FR-4 substrate. THE ACTIVE MICROSTRIP S PERFORMANCE MEASUREMENTS The active antenna measurements are performed in an anechoic chamber room as shown in Figure 10. A passive CP microstrip antenna (with the same patch design and without amplifiers) is also shown as a reference. The measured SWR of the two active antennas is displayed with that of a typical single-input patch antenna for comparison. Significant SWR improvement can be observed over a broad bandwidth (SWR < 1.25 from 2.4 to 2.5 GHz or < 1.5 from 2.3 to 2.5 GHz). The measured active antenna patterns are shown in Figure 11 together with those of the reference passive CP patch antenna. It can be seen that the measured results of the passive CP patch antenna agree well with simulation results in the front direction of the patch antenna. ANECHOIC CHAMBER HORN HORIZONTAL POLARIZATION NETWORK ANALZER TRANSMITTING TPE ACTIVE ROTATE 360 SWR (S 11 ) HORN HORIZONTAL POLARIZATION ANECHOIC CHAMBER FREQUENC (GHz) NETWORK ANALZER ACTIVE ROTATE 360 PORT 2 PORT 1 PORT 1 PORT 2 RECEIVING TPE SINGLE PA ACTIVE LNA ACTIVE Fig. 10 The active patch antenna s measurement setup in an anechoic chamber and its results. VERTICAL POLARIZATION E θ HORIZONTAL POLARIZATION E φ θ = ACTIVE 10 WITH 20 db GAIN LNA (db) θ = Z E-PLANE φ = 90 (a) Z E-PLANE φ = 90 (b) Fig. 11 Measured E-plane patterns of 2.4 GHz active CP microstrip antennas with a reference passive CP microstrip antenna; (a) receiving and (b) transmitting types ACTIVE WITH 20 db 20 GAIN PA 270 (db)

6 (Note that in the SD-EFIE approach the substrate plane of the patch antenna is assumed to be infinite.) The axial ratio of the passive CP patch antenna in the forward direction is almost 0 db. As observed, both of the vertical and horizontal polarization pattern levels of the transmitting- and receiving-type active antennas are approximately 20 db higher than those of the reference passive patch antenna. The axial ratio of the active patch antenna is approximately 2 db. The nonzero axial ratio and the non-symmetrical radiation patterns of active antennas may be due to the nonidentical characteristics of the two amplifiers (LNA or PA) in the semibalanced configuration. Also, while the front-to-back gain ratio is greater than 20 db for the passive patch antenna, substantial back lobes are present for the transmitting-type active antenna due possibly to the radiation leakage of the PA and the imperfect ground plane. The performance may be improved if the active antenna is enclosed in a metallic container. TECHNICAL FEATURE CONCLUSION CP transmitting- and receiving-type active patch antennas that operate at 2.4 GHz and integrate LNAs or PAs have been designed, implemented and tested. The 2.4 GHz LNA that uses low cost BJTs has a gain of 23 db and a noise figure of 2.8 db. The PA uses a BJT and a GaAs FET and has approximately 20 db of gain and 20 dbm of output power at 2.4 GHz. The SD- EFIE technique was employed to determine the patch antenna dimensions accurately and perform a simulation of the antenna s radiation characteristics. By using a semibalanced amplifier structure with a 90 hybrid Lange coupler, the active patch antenna both produces a CP wave and achieves a significant improvement in the input SWR and bandwidth. The active antenna gain (transmitting or receiving type), which includes gain from the amplifier, is approximately 20 db higher than that of the reference passive patch antenna. CP active patch antennas can be used for wireless communication systems, such as base station antennas for cordless telephones or WLANs. ACKNOWLEDGMENT The authors would like to thank H.J. Li and R.. Lane of the department of electrical engineering, National Taiwan University, for providing the antenna measurement facility and measuring assistance. A portion of this work was supported by the National Science Council of the Republic of China under grant NSC E References 1. Jenshan Lin and Tatsuo Itoh, Active Integrated Antennas, IEEE Transactions on Microwave Theory and Techniques, TT-42, December 1994, pp T.O. Perkins, Active Microstrip Circular Microstrip Antenna, Microwave Journal, Vol. 30, No. 3, March 1987, pp Homgming An, Active Antenna Uses Semibalanced Amplifier Structure, Microwaves & RF, Vol. 33, No. 12, 1994, pp T.S. Horng, Spectral-domain Analysis Software for Hybrid Microwave Integrated Circuits (SDASHMIC), National Sun et- Shen University, Department of Electrical Engineering, T. Itoh, Numerical Techniques for Microwave and Millimeter-wave Passive Structure, John Wiley & Sons, N, N, G. Gonzalez, Microwave Transistor Amplifiers Analysis and Design, Prentice-Hall, D. Pozar, Microwave Engineering, Addison-Wesley, Sin-ue Lin received his BSEE and MSEE from National Cheng Kung University, Tainan, Taiwan, in 1994 and 1996, respectively. During his MS study, he developed 2.4 GHz transmitting- and receiving-type active microstrip antennas. Since 1998, Lin has been an RF engineer at RF Link System Inc., Hsinchu, Taiwan. His research interests include RF circuit and antenna design for wireless communications. Huey-Ru Chuang received his BSEE and MSEE from the National Taiwan University, Taipei, Taiwan, in 1977 and 1980, respectively, and his PhD in electrical engineering from Michigan State University, East Lansing, MI, in From 1987 to 1988, he was a postdoctoral research associate at Michigan State University s Engineering Research Center. From 1988 to 1990, he was with the Portable Communication Division of Motorola Inc., Ft. Lauderdale, FL. Since 1991, he has been an associate professor with the department of electrical engineering, National Cheng Kung University, Tainan, Taiwan. Tzyy-Sheng Horng received his BSEE from the National Taiwan University, Taipei, Taiwan, in 1985 and his MSEE and PhD from the University of California, Los Angeles in 1990 and 1992, respectively. From 1985 to 1987, he served in the Republic of China air force as an anti-aircraft artillery officer. Since 1992, Horng has been employed as an associate professor in the department of electrical engineering at the National Sun et-shen University, Kaohshung, Taiwan, Republic of China. His research interest is in the area of electromagnetic simulation of microwave ICs.