Research On Differential Dual-frequency Microstrip Antennas

The rapid development of wireless communications technology has led to the need for fully integrated radio frequency (RF) front-end products,where differential signals are preferable.As a key component of the RF front-end, most of the antennas have a single port.Different from the traditional designs of antennas,the differential signal is fed directly into the two ports of differential antennas.Thus,the differential antennas provide a new way for the fully integrated RF front-end.Moreover, with the fierce competition in wireless communications market, the demand for dual-frequency antennas in wireless communication systems has increased dramatically. In this thesis, the differential dual-frequency microstrip antennas suitable for wireless communications applications are studied. A miniaturized dual-frequency microstrip antenna using stacked configuration is firstly proposed, and the transmission line model of the antenna is also presented. Then, two differential dual-frequency antennas using proximity coupled technique are demonstrated.Finally, a compact differential dual-frequency antenna is presented.The main works in this thesis are as follows:In chapter 2,the basic electrical parameters of antennas are firstly introduced.Then,the operating principle of microstrip antennas (for example, a rectangular patch antenna) is described. Finally, the transmission line model (TLM),cavity model (CM) and the integral equation method (IEM) are analyzed and compared.In chapter 3,a compact stacked dual-frequency antenna suitable for wireless communications applications is designed, and the TLM for the proposed antenna is also presented. Compared with the conventional stacked dual-frequency antenna, the radiating element with a half guided wavelength at the first resonant frequency is distributed on two layers and connected through via holes,thus the dimension of the antenna is reduced effectively. The first resonant frequency is controlled by the dimensions of stacked patches,and the second resonant frequency is mainly determined by the dimensions of upper patch. The rectangular patch antenna is modeled as a section of transmission line;via hole between stacked patches is modeled as an inductance and a capacitor in parallel.The characteristics admittance and radiation admittance of patch antenna, as well as the inductance of via hole are calculated using the same method as these of the existing literature. In particular, the formula for the capacitance of via hole is modified.In chapter 4,two differential dual-frequency antennas using proximity coupled technique (antenna A and antenna B)are designed and fabricated. For antenna A, six symmetrical slits are etched in the non-radiatiing edges of a rectangular patch.The surface current path of the patch is increased by the symmetrical slits,thus the first two resonant frequencies are reduced effectively. For antenna B,a slot is etched in the ground plane.The curved surface current of the patch is guided by the slot, and the same effect can be achieved with that of slits in patch. In addition, a differential dual-frequency antenna with enhanced gain is presented. The radiating patch is split into two parts, and that the back radiation of the antenna is reduced.Therefore,the antenna gain is enhanced.In chapter 5,a compact differential dual-frequency antenna is demonstrated. The miniaturization is realized using the method in Chapter III. The first resonant frequency (f1) is controlled by the dimensions of stacked patches,and the second resonant frequency (f2) is mainly determined by the dimensions of upper patch. Based on the analysis of existing differential microstrip antennas,a compact differential dual-frequency antenna with improved bandwidth is proposed. A reflecting board below the ground plane of the antenna is introduced, and three via holes are used to connect the reflecting board and ground plane.Via holes and reflecting board change the input impedance of the antenna, and two new resonant frequencies are excited. By properly adjusting the locations of via holes, a lower new resonant frequency close to f1 can be excited, and that the bandwidth at f1 is improved.