Browsing by Subject "Antennas ((Electronic)"

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Open Access
Method of moments analysis of microstrip antennas in cylindrically stratified media using closed-form Green's functions
(2012) Karan, Şakir
Numerical methods based on Method of Moments (MoM) have been widely used for the design and analysis of planar microstrip antennas/arrays and printed circuits for various applications for many years. On the other hand, although the design and analysis of similar antennas/arrays and printed circuits on cylindrical structures are of great interest for many military, civil and commercial applications, their MoM-based analysis suffers from the efficiency and accuracy problems related with the evaluation of the Green’s function representations which constitute the kernel of the regarding integral equations. In this dissertation, novel closed-form Green’s function (CFGF) representations for cylindrically stratified media, which can be used as the kernel of an electric field integral equation (EFIE) are developed. The developed CFGF representations are used in a hybrid MoM/Green’s function solution procedure. In the course of obtaining the CFGF representations, first the conventional spectral domain Green’s function representations are modified so that all the Hankel (Bessel) functions are written in the form of ratio with another Hankel (Bessel) function. Furthermore, Debye representations for the ratio terms are used when necessary in order to avoid the possible overflow and underflow problems. Acceleration techniques that are present in the literature are implemented to further increase the efficiency and accuracy of the summation and integration. Once the acceleration techniques are performed, the resultant expressions are transformed to the space domain in the form of discrete complex images (DCIM) with the aid of the generalized pencil of function (GPOF) method and the fi- nal CFGF expressions are obtained by performing the resultant space domain integrals analytically. The novel CFGF expressions are used in conjunction with MoM for the investigation of microstrip antennas on cylindrically stratified media. The singular terms in mutual impedance calculations are treated analytically. The probe-fed excitation is modeled by implementing an attachment mode that is consistent with the current modes that are used to expand the induced current on the patches. In the course of modeling the probe-fed excitation, the probe-related components of CFGF representations are also derived for the first time in the literature and MoM formulation is given in the presence of an attachment mode. Consequently, several microstrip antennas and two antenna arrays are investigated using a hybrid MoM/Green’s function technique that use the CFGF representations developed in this dissertation. Numerical results in the form of input impedance of microstrip antennas in the presence of several layers as well as the mutual coupling between two microstrip antennas are presented and compared with the available results in the literature and the results obtained from the CST Microwave Studio.
Open Access
Mutual coupling reduction in microstrip antennas using defected ground structures
(2012) Yayan, S. Melikşah
Mutual coupling between microstrip antenna elements (through space and surface waves) has a significant role in the performance merits of the microstrip antenna arrays. In many applications, low mutual coupling levels are desired such as bistatic radar systems where isolation is essential in order not to have any interference between the transmitter and receiver antennas. Furthermore, presence of mutual coupling among the antenna elements can affect the sidelobe levels, beam position and frequency bandwidth of arrays. Mutual coupling among the array elements usually occurs as a result of surface waves and space waves. Mutual coupling through the space waves are very strong if the array elements are very close to each other. However, they die out quickly as the separation between the array elements become larger. On the other hand, although the mutual coupling due to the surface waves are weaker than that of space waves when the array elements are close to each other, they remain as the only coupling mechanism when they are far away from each other, in particular for arrays of microstrip antennas. In this thesis, the main goal is to reduce the mutual coupling between the microstrip antennas resulting from the surface waves by using a defected ground structure (DGS). The DGS is formed by etching either a dumbbell shape or a slotted complementary split ring resonator (SCSRR) to the part of the ground plane that remains between the microstrip antennas along their E-plane direction. It has been observed that although a considerable reduction in the mutual coupling can be achieved, the radiation patterns of the antennas are deteriorated due to a significant increase in the backlobe radiation. Hence, a reflector and a cavity combination is used to decrease the backlobe radiation to a certain level. Finally, to test the DGS in an array environment, the performance merits of a 2×2 microstrip antenna array is investigated in the presence of a dumbbell DGS, where each microstrip is backed with a cavity and a reflector. Based on both the simulations and the measurements, it has been concluded that despite the achieved mutual coupling reduction between the microstrip antennas in the array environment, the far-zone radiation patterns related merits have not been improved.