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IEEE MICROWAVE AND WIRELESS COMPONENTS LETTERS, VOL. 15, NO. 6, JUNE 2005

Realizing an Electronically Tunable Reflectarray Using Varactor Diode-Tuned Elements Sean V. Hum, Student Member, IEEE, Michael Okoniewski, Senior Member, IEEE, and Robert J. Davies, Member, IEEE

Abstract—Electronically tunable reflectarrays hold significant promise as cost effective architectures for RF beamforming applications. The successful operation of these arrays depends on the phase agility of the cells used to realize the array. This letter describes the design of an electronically tunable reflectarray based on a novel cell architecture that provides a large degree of phase agility. Experimental results demonstrating the beamforming capabilities of the array at 5.8 GHz are presented. Index Terms—Antenna arrays, microstrip arrays, reflectarray antenna, reflector antennas.

I. INTRODUCTION

M

ICROSTRIP reflectarrays [1] have attracted significant interest as alternatives to reflector antennas due to their low profile and light weight. Most research has focused on realizing fixed beam reflectarrays, whereby phasing of the scattered field from the array is achieved by manipulating the physical attributes of the printed elements, e.g., their size [2], length of integrated loading stubs [3], and rotational orientation [4]. However, if the array and, in particular, its elements could be manipulated electronically, a much more versatile reflectarray could be created that is capable of forming dynamic beam patterns. Such a beamforming approach would have many advantages over traditional tunable antenna array architectures, including a major reduction in the hardware required per element, increased efficiency achieved through the use of spatial feeding, and amenability to closed-loop control. Such an antenna could find broad uses in any application requiring adaptable high gain beam patterns and, furthermore, could be mass produced. Tunable reflectarrays require elements whose scattered field phase can be actively adjusted over a broad range. Mechanical approaches have been proposed in [4] and [5], whereby the phase shifting is accomplished via element rotation or displacement of a dielectric rod beneath the element, respectively. However, only a few approaches have been demonstrated in actual reflectarrays. An electronically tunable reflectarray cell was demonstrated in [6] where the scattering phase is controlled by a varactor diode that loads the radiating edge of a patch antenna. Unfortunately only 180 of phase range was achieved with this element, allowing for only limited beam scanning. This problem was recently alleviated using an active cell topology [7] though Manuscript received July 30, 2004; revised September 30, 2004. This work was supported by NSERC, iCORE, TRLabs., and the AIF. The authors are with the TRLabs Department of Electrical and Computer Engineering, University of Calgary, AB T2N 1N4, Canada (e-mail: svhum@ ieee.org). Digital Object Identifier 10.1109/LMWC.2005.850561

Fig. 1.

Electronically tunable reflectarray system.

an array based on this cell has not yet been demonstrated. Recently, it has also been shown that tunable impedance surfaces can be adapted for use as reconfigurable reflectors [8]. In this paper, the design of an electronically tunable reflectarray based on a novel varactor diode-tuned cell is presented. This cell topology exhibits a high degree of phase agility, exceeding that of previously presented tunable reflectarray cells. A fixed-beam reflectarray based on fixed versions of the reflectarray cell (with varactor diodes replaced with discrete capacitors) was recently presented in [11]. This paper describes the next stage in this project which is the development and measurement of a fully tunable version of the reflectarray. II. REFLECTARRAY ELEMENT DESIGN A diagram illustrating the tunable reflectarray system (without the feeding horn) is shown in Fig. 1. The antenna portion consists of microstrip reflectarray (a) connected to an array of digital-to-analog converters (DACs) on a separate board (b) for developing the appropriate voltages across the varactor diodes in the array. The DAC board is in turn controlled by a microcontroller which facilitates communications with a control computer. For many reflectarray implementations, the fundamental mechanism which provides changes in scattered field phase from a reflectarray cell is a change in the resonant frequency

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HUM et al.: REALIZING AN ELECTRONICALLY TUNABLE REFLECTARRAY

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The frequency range over which the scattered phase from the of the centre of the tuning range cell remained within was 92.5 MHz (1.6%). At 5.8 GHz about 3.5 dB amplitude fluctuation in the scattered field about the centre of the tuning range is observed (Fig. 2), and is caused by a considerable series resistance associated with the varactor diode. The effect is more severe at lower frequencies where the frequency is closer to the resonant frequency of the unloaded patch, suggesting that significant power dissipation in the varactor diode is occurring at this point. This can be alleviated by using higher-Q diodes. Overall, the cell geometry was optimized to maintain reasonable balance between phase range, bandwidth, and loss. III. TUNABLE REFLECTARRAY DESIGN AND RESULTS Fig. 2. Measured scattering of the element employing varactor diodes.

of the microstrip patch element. This is usually implemented by adjusting the shape or loading of the patch element in fixed reflectarrays. The proposed reflectarray cell utilizes concepts developed for frequency agile antennas, where the resonant frequency of a patch is modified electronically using varactor diodes [9]. By carefully choosing the size of the patch and the location of the varactor diodes, the phase agility of the patch can be maximized. For the proposed design, a phase-agile reflectarray element was designed using two surface-mount varactor diodes that serially connect two halves of a microstrip patch, as shown in inset of Fig. 1. A DC bias network is co-located with each diode and voltage is supplied by vias connected to the DAC board behind the array. Overall, a varactor diode-based cell topology was chosen to make the cell simple to build compared to other approaches which use active components. The element was originally designed to operate at 5.5 GHz and and realized on a 1.524 mm substrate with . An MCE Metelics MGV-100-21 varactor diode was chosen, with a capacitance of 2.10–0.15 pF between 0–20 V of reverse bias voltage. The cell geometry was designed and optimized with the aid of a finite-difference time-domain simulation of the patch, in an infinite array configuration (parallel plate waveguide (PPWG) technique [10]), and with a mm at 5.5 GHz. Equivauniform cell spacing of lent, lumped element circuit models of the diode under different bias voltages were used in the computations. Experimental verification of the element behavior was performed by placing the cell at the end of a piece of WR-187 rectangular waveguide which approximates the infinite array approach at a nonnormal angle of incidence. Good correlation between FDTD simulations and measured results was observed; interested readers are encouraged to consult [11]. The tunable reflectarray, originally designed to operate at 5.5 GHz, was later migrated to an operating frequency of 5.8 GHz. The element spacing did not change from the original design [11] but the cell dimensions were modified for optimal tuning at 5.8 GHz. The corresponding measured results with 14 mm, 17 mm, and 1 mm are shown in Fig. 2. An excellent tuning characteristic was achieved at 5.8 GHz. It offers a tuning range of 325 while maintaining a reasonably flat phase slope which yields a larger frequency bandwidth.

The tunable reflectarray shown in Fig. 1 was realized using a 7 10 microstrip array built from the elements described in Section II. Previously, the validity of the cell concept was tested by building a fixed-beam reflectarray realized from fixed-capacitor-based cells, which produced good results despite the fact that phase values in the array had to be discretized considerably [11]. In this paper we discuss results from a fully tunable version of the array based on the cell shown in Fig. 1. The array is fed by a feed horn which will eventually be integrated with the reflectarray in an offset-feed arrangement. The DAC board enables each of the elements to be individually addressed, and up to 30 V of diode bias to be developed with 8 b of resolution. Testing of the array was performed in the far field so that the properties of the reflector alone could be studied without having to consider the effects of feed blockage. Initial testing of the array has been performed by characterizing the monostatic radar cross section (RCS) of the array since it can be conveniently compared with theoretical expectations from array theory and the experimental setup only required a single feed horn. It is important to note that this is not the conventional way a reflectarray is scanned since a separate integrated feed horn usually feeds the array. In Fig. 1, the horn feeds the array at broadside . when it is located at Since monostatic RCS measurement were used to characterize the array, the reflective behavior of the reflectarray was best characterized by programming it to produce retrodirective beams. The required element phases were precomputed, and through the characteristic shown in Fig. 2, translated to DAC configurations that were downloaded to the controller. Four different retrodirective beams angles in the azimuthal direction were considered, for . The RCS of the reflectarray a fixed elevation angle of was measured using a network analyzer and normalized with respect to a metal plate having the same equivalent area as the antenna array. The results of this experiment are shown in Fig. 3. The desired retrodirective beams are visible on the right side of the plot; the beams on the left are also expected by array theory but are not of interest. Overall, the beam positions correlated well with the desired beam angle. Some beam pointing error at angles far from broadside is apparent and expected due to the open-loop control scheme used to phase the array. The increased scan loss away from broadside is also expected and a curve illustrating the expected scan loss is included in the plot. This loss results from the response of microstrip elements as well as from the decrease in projected aperture area as the beam is scanned. The peak

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Fig. 3.

IEEE MICROWAVE AND WIRELESS COMPONENTS LETTERS, VOL. 15, NO. 6, JUNE 2005

Measured monostatic RCS of the reflectarray at  = 80 .

values of the monostatic RCS from the various configurations correlate well with this curve. Compared to the peak RCS measured from the metal plate, it can be seen that the reflected beams from the tunable reflectarray have been attenuated significantly. Most of the attenuation of the reflected beam is caused by loss in the cells, as discussed earcase, the cell loss is 2.6 dB; lier. For example, for the other losses at this angle include phase error loss (1.0 dB), scanning loss (0.3 dB), and microstrip element factor loss (0.7 dB) for a total aperture loss of 4.6 dB. Phase error losses originate from: 1) variation in the manufactured patch dimensions (up to %), 2) variation in the varactor diode parameters (up to %), and 3) deviations from the phase characteristic of an element in the actual array compared with the element in the RWG measurement. Phase error loss can be alleviated by employing closed-loop control to adjust cell voltages. This technique was used to compensate for the phase error loss reported earlier by incorporating an optimizer loop between the array hardware and measurement instruments to maximize the measured RCS from the array. When all the losses are incorporated into the RCS calculation the theoretical and measured RCS compare favorably, with a difference of only 1.5 dB between them. This difference is small and expected since actual cell loss could still be higher due to slight variations in the manufactured patch dimensions which have the strongest effect on cell loss. While Fig. 3 shows the overall performance of the array, and demonstrates its ability to electronically form a beam in a requested direction, it does not illustrate how close is the response of the antenna to the theory. We have, therefore, replotted the together with the response measured response for predicted numerically in Fig. 4. Other curves show similar relation to theory. While the overall correlation between the curves is good, the figure provides good illustration of the loss and beam-pointing error. Further, a small specular reflection at is clearly visible in both Figs. 3 and 4, and is due to an oversized ground plane used to realize the tunable array (a 38% increase in area compared to the fixed-beam array). In the next generation of the antenna, better integration of the control electronics will allow us to use smaller ground plane and thus alleviate this effect.

Fig. 4.

Monostatic RCS of one reflectarray configuration at  = 80 .

IV. CONCLUSION An electronically tunable reflectarray has been presented that utilizes a novel cell exhibiting a large degree of phase agility. This array has been used successfully to produce a broad range of retrodirective beam patterns. There are immediate plans to integrate a proximal feed horn with the array so that it can be scanned like a traditional reflector. Losses in the reflectarray cells have been identified as key to improving the aperture efficiency of the antenna and new cell topologies are currently under investigation. There are also plans to implement integrated closed-loop control of the beam pattern so that phase error loss can be avoided in future implementations of the reflectarray. REFERENCES [1] J. Huang, “Microstrip reflectarray,” in Antennas Propagation Soc. Int. Symp. Dig., vol. 2, Jun. 1991, pp. 612–615. [2] D. M. Pozar et al., “Design of millimeter wave microstrip reflectarrays,” IEEE Trans. Antennas Propagat., vol. 45, no. 2, pp. 287–296, Feb. 1997. [3] D. C. Chang and M. C. Huang, “Multiple-polarization microstrip reflectarray antenna with high efficiency and low cross-polarization,” IEEE Trans. Antennas Propagat., vol. 43, no. 8, pp. 829–834, Aug. 1995. [4] J. Huang and R. J. Pogorzelski, “A Ka-band microstrip reflectarray with elements having variable rotation angles,” IEEE Trans. Antennas Propagat., vol. 46, no. 5, pp. 650–656, May 1998. [5] M. E. Cooley et al., “Novel reflectarray element with variable phase characteristics,” in Proc. Proc. Microwaves, Antennas Propagation, vol. 144, May 1997, pp. 149–151. [6] L. Boccia et al., “Application of varactor diodes for reflectarray phase control,” in Proc. IEEE Antennas Propagation Soc. Int. Symp., vol. 3, Jun. 2002, pp. 132–135. [7] L. Boccia et al., “A microstrip patch antenna oscillator for reflectarray applications,” in Proc. IEEE Int. Symp. Antennas Propagation, vol. 4, Jun. 2004, pp. 3927–3930. [8] D. Sievenpiper et al., “Two-dimensional beam steering using an electrically tunable impedance surface,” IEEE Trans. Antennas Propagat., vol. 51, no. 10, pp. 2713–2722, Oct. 2003. [9] P. Bhartia and I. J. Bahl, “Frequency agile microstrip antennas,” Microw. J., pp. 67–70, Oct. 1982. [10] F.-C. E. Tsai and M. E. Bialkowski, “Designing a 161-element Ku-band microstrip reflectarray of variable size patches using an equivalent unit cell waveguide approach,” IEEE Trans. Antennas Propagat., vol. 51, no. 10, pp. 2953–2962, Oct. 2003. [11] S. V. Hum and M. Okoniewski, “An electronically tunable reflectarray using varactor diode-tuned elements,” in Proc. 2004 IEEE Int. Symp. Antennas Propagation, vol. 2, Jun. 2004, pp. 1827–1830.