프랙탈 구조를 갖는 고변환효율 2.45 ㎓ 렉테나의 설계 및 제작

Abstract

Wireless power transmission (WPT) has received significant attention demonstrating efficient RF to DC conversion capability for directive high power transmission applications. Rectifying antenna (rectenna) consisting of antenna and rectifying circuit can convert ambient RF energy to usable DC power. Over the last century, the development of rectenna for space solar power transmission (SSPT) as well as WPT had great achievement with specific functions and applications. Among various types of antenna used in rectennas, microstrip patch antennas are gaining popularity owing to their low profile, light weight, and low cost of manufacture using modern printed circuit technology. The miniaturized patch antennas of rectenna are advantageous for various applications such as RFID tagging systems, sensor batteries or capacitors, WLANs, WiMax, cognitive radio systems, etc. The effective techniques to shrink down the antenna size are: use of high permittivity substrates, use of magnetic substrates, increase in electrical length, reactive loadings, short circuits, and superstrates. However, many of these techniques have complicated patch structures. Beside these techniques, application of fractal geometry is introduced for size reduction. Fractal refers to the class of geometrical shapes composed of multiple iterations of a single elementary shape. Fractal antennas can be designed in many shapes such as sierpinski triangle, koch curve, grid spiral array etc. The self similar property of fractal is useful to design multi frequency antennas such as sierpinski gasket and space filling property of fractal is useful to design small antennas such as sierpinski carpet. The proposed rectenna consists of fractal antenna and rectifier circuit. Fractal geometry is applied to conventional antenna for optimization of its shape in order to make its size smaller than its fundamental counterpart. A simple space filling fractal, sierpinski carpet is etched on the edge fed microstrip patch antenna. The iteration is performed up to 3rd order. The antennas are simulated in finite element method of Ansoft HFSS (ver. 12). The size reductions of 13.8%, 20.0%, and 20.4% are observed after 1st, 2nd, and 3rd iterations respectively. Since 3rd iteration dimension and outputs are similar to the 2nd iteration, three antennas: generator, iteration 1, and iteration 2 are fabricated. The 2nd order iterated fractal is integrated with rectifier circuit to develop an efficient rectenna. Return loss and Smith chart of rectenna are measured using network analyzer (E8362B). Radiation pattern measurement is performed using antenna far field measurement system in anechoic chamber. The simulated and measured output illustrates better return loss, fine impedance matching and higher gain at 2.45 ㎓. One can observe a small difference between measured and simulated data in return loss and impedance. Deviations are partly due to inaccuracies in antenna manufacturing process. The linear gain (10.52 ㏈i) horn antenna is used as a transmitter in rectenna measurements at 2.45 ㎓. The output DC voltage of the rectenna is measured with a voltmeter. Measurement of output DC voltage, DC power is carried out for various input power levels. The maximum rectification efficiency of 57% is obtained for input power level of 20 ㏈m having 30 ㎝ of distance between transmitter and receiver. Hence, it is observed that the increment of iteration order of fractal antenna leads to a higher degree of miniaturization. Also, it is perceived that a high degree of complexity in the structure of the antenna is not required for miniaturization of patch antennas using fractal geometry. The measured results presented indicate that rectenna can work as RF recyclers with an output power level enough to be efficiently stored and reused, in our case specifically working in 2.45 ㎓ ISM band.