Mutual coupling reduction in microstrip antennas using defected ground structures

Abstract

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.