COMPACT DUAL-POLARIZATION ANTENNA FOR A MOBILE DEVICE (03-Sep-2008)

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Microstrip antennas have been used in wireless communications for many years due to their low profile, lightweight, low-cost characteristics and compatibility with mounting structures. Recently, studies on microstrip antennas have focused on frequency reuse and polarization diversity of the two-orthogonal polarizations to double the capacity of communication systems and reduce the multi-path fading of received signals in land– based mobile-communications. Moreover, dual-frequency microstrip antenna arrays, often realized through a multiplayer architecture, have gained considerable interest. However, there have been some inherent challenges in the design and architecture of dual-polarized dual-frequency band microstrip antenna arrays. Conventionally, a dual-polarized microstrip antenna is realized by feeding a patch at the two orthogonal edges. This feeding approach requires two feeding networks for two individual polarization components, respectively. But, it is very difficult to find enough space to accommodate two sets of feeding networks if a dual-polarized array is employed within a limited allowable space, such as in a handset device. Hence, strong mode coupling and high cross polarization will occur. The problem will become more severe if one wants to integrate active and passive circuits into the feed-networks. Furthermore, if a dual-frequency operation for the above dual-feed dual-polarized array is realized by multilayered architecture, the size and complexity of the array will be further increased. To overcome the above problems, it is important to create novel miniaturized dual polarized dual-frequency arrays. Three new dual polarization, planar antenna designs that satisfy these requirements are given below. 1. Compact Single-Fed Dual-Polarized Dual-Frequency Band Microstrip Antenna Array A novel single-fed dual-polarized dual-frequency band microstrip antenna array is proposed. It consists of two stacked sub-arrays with different element sizes. Instead of introducing two feedings and complicated feeding networks, the dual polarized dual-frequency band array uses only one feeding, which significantly reduces the complexity of design and fabrication, as well as cross polarization levels, and also effectively avoids feeding couples in the two feeding networks. With the above attractive features, the proposed structure provides a unique inexpensive solution for high performance dual-polarized dual-band antennas for wireless communications. The single-fed dual-polarized dual-frequency microstrip antenna array structure is shown in Fig.1. The antenna consists of two sub-arrays, and they are etched on the opposite two sides of a PCB. The bottom sub-array is over a common ground plane. Each sub-array has four identical square patches (A1, B1, C1 and D1, or A2, B2 C2, and D2) and the patches are electrically connected through two pairs of parallel strips of the widths 1 w , 2 w and lengths 1 d , 2 d . By adjusting these design parameters, the antenna can be designed to operate at dual-frequency bands. Since the two sub-arrays have a common ground plane and they are fed with a coaxial probe at the center, the inverted antenna forms a two-stacked inverted F-shaped array structure. The two connecting strips cross each other so as to suppress the cross polarization and to enhance mode isolation. All strips and patches are arranged along the X or the Y axial directions to realize orthogonal dual-polarizations. The connecting strips not only operate as feeding networks for the four patches but also work as radiating elements. Since the direction of the current will be reversed after it passes one half-wavelength, two pairs of parallel strips and a 2 by 2 patch sub-array can produce in-phase current distributions in each polarization direction if the dimensions of the strips and of the patches are properly chosen. Therefore, high gain radiation becomes possible. 2. Compact Dual-Polarization Multi-Loop Antenna A dual-polarization planar antenna design is given below. Instead of introducing two feeds or complicated antenna geometry, the antenna uses only one feed to excite symmetrically the four rectangular loops (A,B,C,D) and their connection strips ( T,L,M,N and P) to form dual-polarizations, which reduces the possibility of the cross talk between the two dual-polarization components. The simple rectangular loop structure makes it easy to analyze and adjust the current and near-field distributions, thus simplifying the task of antenna design based on requested antenna specifications. Additionally, due to the planar strip structure, the antenna does not need to occupy any additional space. The antenna has high gain and high efficiency. The said advantages make the proposed antenna structure a unique inexpensive solution to achieve pure dual-polarization and good antenna performance at the same time. The dual-polarization antenna structure is shown in Fig.2. The antenna consists of four groups ( A, B, C, D) of loops and connection strips ( T, L, M, N and P) etched on a side of a PCB. If one wants to get a directional radiation, a metal reflector needs to be attached on another side of the substrate. In order to get orthogonal dual-polarization radiation components, all strip sections are arranged along the X or the Y axial. The feed is located at the centre of a common strip P so as to get symmetrical excitation, and hence reduce cross talk between dual-polarization components. The loop sections and connection strips in each polarization are arranged into a four- element array for high gain. Since the direction of the current will be reversed after passage through a half-wavelength due to standing wave distribution along the strip, the side length of each array element is equivalently divided into three parts, which results in an in-phase current distributions on all strip elements in an array in the X-axial or the Y axial if the length of loop is considered as one wavelength. Therefore, high gain radiation becomes possible. 3. Compact Dual-Polarization Multi-Loop Antenna . The proposed dual-polarization antenna structure is shown in Fig.3. The antenna consists of loop strips etched on one side of a PCB. These loop strips can be regarded as the combination of the four electrically connected rectangular loop strips (two A loops and two B loops). If one wants to get a directional radiation, a metal reflector needs to be attached on another side of the substrate. In order to get orthogonal dual-polarization radiation components, all strip sections are arranged along the X or the Y axis. The feed is located at the centre of a common strip so as to get symmetrical excitation, and thus reduce cross talk of the dual-polarization components. The loop sections in each polarization are arranged into a three- element array. The direction of the current will be reversed after it passes through a half-wavelength due to standing wave distribution along the strip. Therefore, each side length of the square loop is equivalently divided into three parts, which causes in-phase current distributions on all strip elements in an array in the X-axial or the Y-axial, when the length of a loop is considered as one wavelength. Thus, high gain radiation becomes possible.