Rakesh Kumar Yadav et al. / International Journal of Engineering Science and Technology (IJEST) RECTENNAS DESIGN, DEVELOPMENT AND APPLICATIONS *Rakesh Kumar Yadav, **Sushrut Das and *R. L. Yadava, *Department of Electronics & Communication Engineering, GCET, Gr. Noida, U. P., India **Department of Electronics Engineering, Indian School of Mines, Dhanbad, Jharkhand, India [email protected] ABSTRACT The present paper describes the development of rectenna in terms of its applications in Microwave Power Transmission, Harmonic Rejection, CP radiation and ISM band. These rectennas consist of several antennas such as dipole, antenna arrays, slot meander line and rhombic loop antennas along with the rectifying diodes. In some cases more than one rectifying devices have also been used and antenna is found to be act at dual bands. It has also found that rectenna reject the harmonics upto 3rd order and enhanced the performance characteristics. The maximum efficiency about 91% with 1.2 W of input power has been observed if the rectenna is used for microwave transmission. KEYWORDS: Rectenna, Conversion Efficiency, Band Reject Filter, Voltage Standing Wave Ratio, Industrial- Scientific-Medical Band, Microwave Power Transmission. I. INTRODUCTION The rectenna has been a growing area of research in recent years, as the microwave integrated circuit and monolithic microwave integrated circuit technologies became more mature allowing for high level integration. The rectenna termed as rectifying antenna, is combination of an antenna and a nonlinear rectifying element (Schottky diode, IMPATT diode…etc.) where the two elements are integrated into a single circuit. The schematic of a rectenna system is shown in Fig. 1. Such a system is capable to receive and detect microwave power and converts the RF power into dc voltage at high frequencies (THz). Fig.1 Schematic rectenna system. Since the rectenna is a receiving module collecting power from a radiation field, its sensitivity is defined by = ℎ Division of this value with the effective antenna aperture leads to the normally used quantity for detectors. = This characterizes the nonlinear element with its matching. A rectenna is useful as the receiving terminal of a power transmission system where dc power needs to be delivered to a load, through free space, for which physical transmission lines are not feasible. It is also suitable in applications where dc power needs to be distributed to more numbers of load elements that are spatially ISSN : 0975-5462 Vol. 3 No. 10 October 2011 7823 Rakesh Kumar Yadav et al. / International Journal of Engineering Science and Technology (IJEST) distributed. Such power distribution is achieved by the dispersive nature of microwave energy in space, eliminating the need for physical interconnects to individual load elements. The “rectenna” was invented by Brown and has been used for various applications such as the microwave power helicopter and the receiving array for Solar Power Satellite [1-5]. The experiment on the microwave powered aircraft which was conducted in Canada under the project SHARP (Stationary High Altitude Relay Platform), in which the structure of rectenna was evolved from a bulky bar-type to a planar thin-film type. It was found that the weight to power output ratio reduces effectively, and the power conversion efficiency of 85% is observed at 2.45 GHz [6, 7]. In general it is difficult to predict how the rectenna system is optimized for the maximum conversion efficiency. However, there are several theoretical methods to overcome this problem. These methods can be divided into two groups; the first one is to directly simulate the rectenna circuit in time domain [8], whereas the other is to find a closed-form equation which can explain the relationship between diode parameters and the conversion efficiency [9-10]. All these studies were done on 2.45 GHz because it is not strongly attenuated by the atmosphere even under a severe weather condition [11]. The frequency 2.45 GHz is suitable for the application of power transmission between ground-to-ground, ground-to-space and space-to-ground. However, the operating frequency can be increased to allow power transmission for the space-to-space application, over much longer distances with the smaller antenna and rectenna. Fig. 2 shows the plot of rectenna conversion efficiency with the input power. Fig. 2 General relationship between microwaves to dc power conversion efficiency and input power. The main objective with the design of rectenna is to obtain high conversion efficiency, and there are two approaches to achieve this goal. In first maximum power is collected and delivered to the rectifying diode, while in the second one harmonics generated by the diode are suppressed, which re-radiate from the antenna as power lost. In order to increase conversion efficiency by the first method, several broadband antennas, large antenna arrays and circularly polarized antennas have been designed and experimented. The broadband antenna enables relatively high RF power to be received from various sources while the antenna array can increase incident power delivered to the diode by enlarging antenna aperture and antenna gain. Antenna array is an effective means to increase the receiving power for rectification. However, a trade-off arises between the antenna size and the radiation gain. The circularly polarized antenna offers power reception with less polarization mismatch. However in second method, the rectenna consist of an LPF between the antenna and the diode, as well as an additional LPF on the dc output side of the diode. The main reason for the rise in the efficiency was the improvement in the diode and circuit construction for high input power levels. Several operating frequencies of the rectenna have been considered and investigated. Components of microwave power transmission have traditionally been focused on 2.45 GHz and recently moving up to 5.8 GHz, which has a smaller antenna aperture area than that of 2.45 GHz. Both frequencies have comparably low atmospheric loss, cheap components availability, and reported high conversion efficiency. That is, the rectenna is capable of very high conversion efficiencies (~90%) in optimal circumstances and hence very suitable for automotive radar applications. Therefore in present paper authors describe the progressive development of rectenna, during last two decades, focusing its importance in microwave power transmission, harmonic rejection, CP radiation, dual frequency and high efficiency and ISM band applications. ISSN : 0975-5462 Vol. 3 No. 10 October 2011 7824 Rakesh Kumar Yadav et al. / International Journal of Engineering Science and Technology (IJEST) II. MICROWAVE POWER TRANSMISSION The receiving rectifying antenna (rectenna) is one of the main components of microwave power transmission systems. In order for such systems to operate cost efficiently in land or spaced-based locations, the conversion efficiency from microwave to dc of the rectenna must be high. In year 1992, J.O. Mcspadden, T. Yoo and K. Chang designed a 35 GHz rectenna using a microstrip patch antenna and 29% of measured efficiency was found with 120 mW input power. The measured efficiency versus input power for a 100Ω load resistance is shown in Fig.3. Fig. 3 The power conversion efficiency of the 35 GHz rectenna measured with the waveguide array simulator. They also designed a frequency selective surface using an equivalent circuit model and tested to pass 2.45 GHz with insertion loss of 0.3 dB [12]. It also rejects the second harmonic of 4.9 GHz. The frequency response of the gridded square FSS array is shown in Fig. 4. The 10 dB of attenuation occurred at 4.9 GHz on a small rectenna array whereas the conversion efficiency was decreased by less than 1%. Fig. 4 Calculated and measured frequency response of the gridded square FSS array. After 6 years, K.M. Strohm, J. Buechler and E. Kasper designed a rectenna on high resistivity silicon substrate using SIMMWIC technology [13]. The microstrip rectenna with two Schottky diode and single diode is shown in Fig 5, where the rectenna are combined with a CMOS preamplifier mounted as a multichip module next to the rectenna. They found the amplification of 32 dB while maximum sensitivity of the detector circuit including pre-amplification is 1600 mV/mW.cm-2 at 94.6 GHz. The frequency response and radiation pattern are plotted in Fig. 6 and Fig. 7, which show a 3 dB BW of 1.6 GHz. ISSN : 0975-5462 Vol. 3 No. 10 October 2011 7825 Rakesh Kumar Yadav et al. / International Journal of Engineering Science and Technology (IJEST) Fig. 5 Photo of the microstrip rectenna. (a) Two branches antenna with two Schottky diodes in series (b) Antenna with single diode Fig. 6 Output voltage of the rectenna/CMOS MCM versus frequency (S = 4.7 µW/cm2) Fig.7 H-plane radiation pattern of the packaged rectenna/CMOS module. In year 2000, L.W. Epp et al. [14] proposed a compact rectenna capable of producing a 50 V output suitable for driving mechanical actuators. They describe the circuit that allows the output of multiple rectenna elements to be combined in order to step up the output voltage. For each of the two orthogonal polarizations (dual linear and circular) independent circuit is used. By suitable choice, the output voltage is twice over that of the single polarization case. These are used in the next generation space telescope to eliminate wiring between actuators and provide true mechanical isolation. By proper diode placement and adding resistance dc isolation pads, they found that a series combination of 9 patch element would combine the output of 18 diodes for dual polarization. For actuator application this allow for large output voltage for low incident power density. Fig.8 shows that by increasing load resistance the required 50V output can be obtained for a low incident power density of 6.3 mW/cm2.