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Summary of Reconfigurable Antennas for Wireless and Space Applications

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Satellite and Mobile Communication Course Course Number: 361-2-5931 Lecturer: Professor Arnon Shlomi Article Summary Assignment "Reconfigurable Antennas for Wireless and Space Applications" By Christos G. Christodoulou, Fellow IEEE, Youssef Tawk, Steven A. Lane, and Scott R. Erwin, Senior Member IEEE Proceedings of the IEEE 100, no. 7 (2012): 2250-2261 Assignment by: 1. Introduction A reconfigurable antenna (RCA) is an antenna that is able to be formed, or bent. From this definition one can deduce the significance of such antenna to wireless communication. RCA will allow adaptation, additional functionality and more versatility.

Therefore, RCAs, with the ability to radiate more than one pattern at different frequencies and polarizations, are necessary in modern telecommunication systems. The article discusses the different reconfigurable components that can be used in an antenna to modify its structure and function. These reconfiguration techniques are either based on the integration of radio-frequency micro-electromechanical systems (RF-MEMS), PIN diodes, varactors, photoconductive elements, or on the physical alteration of the antenna radiating structure, or on the use of smart materials such as ferrites and liquid crystals.

All of the above techniques redistribute the antenna currents and thus alter the electromagnetic fields of the antenna’s effective aperture. Therefore, enabling the antenna to enhance its bandwidth, change it operating frequency, polarization, and radiation pattern. 2. Reconfiguring Techniques Six major types of reconfiguration techniques are used to implement reconfigurable antennas, as indicated in Fig. 1. Here I shell focus on two, electrical and optical RCAs. RCAs can be classified into four different categories. a) frequency RCA; (b) radiation pattern RCA, for this category, the antenna radiation pattern changes in terms of shape, direction, or gain; (c) polarization RCA; and (d) combination of the previous categories. There are several advantages in using reconfigurable antennas. (a) Ability to support more than one wireless standard. Hence, it minimizes cost and volume requirements, simplifies integration and offers good isolation between different wireless standards; (b) lower front-end processing.

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Therefore, there is no need for front-end filtering and there is a good out-of-band rejection; (c) best candidate for software-defined radio. Thus, has the capability to adapt and learn and can be automated via a microcontroller or a field programmable gate array (FPGA); and (d) multifunctional capabilities. Consequently, can change functionality as the mission changes, can act as a single element or as an array and can provide narrow- or wide-band operation. However, there are disadvantages for adding tunability to the antenna behavior. a) the design of the biasing network for activation/deactivation of the switching elements which add complexity to the antenna structure; (b) increase in the required power consumption due to the incorporation of active components which augments the system cost; (c) generation of harmonics and inter modulation products; and (d) need for fast tuning in the antenna radiation characteristics to assure a correct functioning of the system. Figure 1: Techniques to achieve RCAs 2. 1.

Electrically RCAs The ease of integration of such switching elements into the antenna structure has attracted antenna researchers to this type of RCAs despite the numerous issues surrounding such reconfiguration techniques. These issues include the nonlinearity effects of switches, and the interference, losses, and negative effect of the biasing lines used to control the state of the switching components on the antenna radiation pattern. RF-MEMS: The antenna shown in Fig. 2 is a reconfigurable rectangular spiral antenna with a set of RF-MEMs switches, which are monolithically integrated and packaged onto the same substrate.

The antenna is printed on a PCB substrate and fed through a coaxial cable at its center point. The structure consists of five sections that are connected with four RF-MEMS switches. The spiral arm is increased by discrete steps as integer multiplications of the length of the first segment of the rectangular spiral. It is increased following the right-hand direction to provide right-hand circular polarization for the radiated field. The location of switches is determined such that the axial ratio and gain of the antenna are optimum at the frequency of interest.

Based on the status of the integrated RF-MEMS, the antenna can change its radiation beam direction [2]. Figure 2: (Left) a radiation pattern RCA. (Right) fabricated prototype with the biasing line 2. 2. Optically RCAs An optical switch is formed when laser light is incident on a semiconductor material. This results in exciting electrons from the valence to the conduction band and thus creating a conductive connection. The linear behavior of optical switches, in addition to the absence of biasing lines, compensates for their lossy aspect and the need for laser light to activate them.

Integrated Laser Diode: Optically RCAs can be implemented by integrating laser diodes directly into the antenna substrate. A copper piece is attached to the back of the antenna ground, as shown in Fig. 3. This piece has a minimal effect on the antenna radiation pattern since it has a small depth and a smaller width and height as the antenna ground plane. The laser diodes are activated via a current driver to generate the required output optical power. An example of this type of reconfigurable antenna is shown in Fig. 3a. The antenna top layer is the radiating patch while the bottom layer represents the antenna ground plane.

Two silicon switches (S1 and S2) are included to allow the antenna to tune its resonant frequency. To activate the silicon switches, laser diodes are integrated within the antenna substrate by attaching a small copper piece to the ground of the antenna, as shown in Fig. 3b. Two holes are drilled throughout the substrate in order to allow the light from the laser diode to be delivered to the silicon switches. These copper pieces are also used as a heat sink for the laser diodes [3]. Figure 3: (a) optically RCA. (b) Laser diode integration with copper fixture, back layer. (c) Prototype, to layer . 3. Smart Materials RCAs Antennas are also made reconfigurable through a change in the substrate characteristics by using materials such as liquid crystals or ferrites. The change in the material is achieved by a change in the relative electric permittivity or magnetic permeability. In fact, a liquid crystal is a nonlinear material whose dielectric constant can be changed under different voltage levels, by altering the orientation of the liquid crystal molecules. As for a ferrite material, a static applied electric/magnetic field can change the relative material permittivity/permeability. . Satellite Applications The need for dynamic space applications has led to the realization of RCAs for satellite communication. In such systems, it is necessary to reconfigure the antenna radiation pattern to serve a new coverage zone, limit fading in rainy areas, and maintain high data rate at all possible frequency bands of operation. E. g. an antenna structure for satellite applications generates an elliptical beam ranging from 10. 95 to 14. 5 GHz using an 85-cm aperture. Using a rotational and zooming mechanism, the antenna can tune its radiated beam from a "small ellipse" of 2. 3°X3. ° to a "large ellipse" of 6°X9° [4]. Reconfiguration in space has also been achieved through the use of deployable antennas. These antennas change their shape from compact, small structures to large blooming antennas in space. The objectives are to realize high gain and high directivity, which are primarily determined by the size of an antenna aperture. The antenna itself can be reconfigurable to cover several frequency bands as the mission of the satellite changes. 4. Summary Reconfigurable antennas were divided into electrically, optically, physically, and smart-material-based tunable structures.

Christodoulou et-al expect future smart reconfigurable antennas to be completely multifunctional and software controlled with machine learning capabilities that can detect changes in their RF environment and react accordingly. Moreover, the merging of deployable and reconfigurable antennas will open new frontiers in the design of antennas for space communications. 5. References 1. Christodoulou, Christos G. , Youssef Tawk, Steven A. Lane, and Scott R. Erwin. "Reconfigurable Antennas for Wireless and Space Applications. " Proceedings of the IEEE 100, no. 7 (2012): 2250-2261. 2. won Jung, Chang, Ming-jer Lee, G. P.

Li, and Franco De Flaviis. "Reconfigurable scan-beam single-arm spiral antenna integrated with RF-MEMS switches. " Antennas and Propagation, IEEE Transactions on 54, no. 2 (2006): 455-463. 3. Tawk, Y. , J. Costantine, S. E. Barbin, and C. G. Christodoulou. "Integrating laser diodes in a reconfigurable antenna system. " In Microwave & Optoelectronics Conference (IMOC), 2011 SBMO/IEEE MTT-S International, pp. 794-796. IEEE, 2011. 4. Roederer, Antoine G. "Antennas for Space: Some Recent European Developments and Trends. " In Applied Electromagnetics and Communications, 2005. ICECom 2005. 18th International Conference on, pp. 1-8. IEEE, 2005.

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