Mutual Coupling in Antenna Array

In present wireless communication technology, antenna arrays play an important role, which significantly improves the performance of wireless communication system. An antenna array offers various advantages such as high gain and beamforming ability. However, antenna arrays have some disadvantages especially if the antenna elements have to be placed in a close proximity, it produces significance mutual coupling that can deteriorate antenna performance. Mutual coupling between antenna elements not only affects radiation pattern, it also influences matching characteristics, antenna efficiency and correlation. Thus packing more number of antennas with reduced antenna to antenna spacing, has always attracted research interest for several decades. The antenna researchers are still on their way to find more optimal solutions to improve the packing density while also reducing the mutual coupling between the antenna elements.

Let’s see the mutual coupling mechanism in transmitting and receiving antenna array: In transmitting array, all antenna elements are supposed to be in transmitting mode, which in practical scenario does not happen. Antenna elements start absorbing a part of signal transmitted by closely placed antennas in the array as shown in Figure 1. The coupling path of the antenna elements antenna-1 and antenna-2 of the antenna array in the transmitting mode is shown in Figure 1. Let us consider Figure 1, in which only antenna-1 is excited by a source. The generated power travels towards antenna-1, labelled as 'a', and radiates to the free space labelled as 'c'. Some of the radiated energy is received by the nearby antenna-2, labelled as 'b' and a current is subsequently induced on antenna-2. The induced current on antenna-2 also radiates to the free space, a part of this radiated energy is again received by antenna-1. Such a process will continue indefinitely. If both antenna elements are excited, the total field will be the sum of the radiated field and the field re-scattered from both antenna elements.

In receiving array, all antenna elements are supposed to only receive the signal but this is not the case in practical scenario. The antenna array as shown in Figure 5, receives the incident wavefront coming from the free space, labelled as 'd' strikes antenna-1, and a current is subsequently induced. The induced current in antenna-1 radiates some of the energy to the free space, while the rest of the energy, labelled as 'e' travels towards the load. Part of the radiated energy is received by antenna-2, labelled as 'f'. Similarly, antenna-2 also reradiates some portion of the received energy which is subsequently received by antenna-1. The same process will also happen when the wavefront strikes antenna element-2. If both the antennas receive simultaneously, the total field will also be the sum of the incoming and rescattered fields from both the antenna elements.

The coupling paths of transmitting and receiving antenna arrays are different. Furthermore, due to the fact that the excitations of the transmitting and receiving arrays are different, they have different mechanisms that cause the mutual coupling. As a result, the mutual-coupling problems for an antenna array in the transmitting and receiving modes are in general two different problems, which should be treated differently.

The simplest method to suppress the mutual coupling is to increase the physical separation between the antenna elements. Consequently, the mutual coupling becomes lower, but the size of antenna can subsequently be larger. Several techniques for the mutual coupling reduction has been adopted to improve the isolation between antenna elements. Some of the techniques utilize the defected ground structure (DGS), electromagnetic bandgap structures (EBG), parallel coupled line resonator, metamaterial and polarization convertor.

Mohit Mishra Research: Mutual coupling in antenna array Communication Lab I Email: mohit.mishra@iitg.ac.in