Modal analysis of a MIMO antenna for sensor networks

Multiple antennas have emerged as an important technology for increasing the efficiency of wireless communication systems. multiple-input multiple-output (MIMO) systems, offering significant capacity gain, can be used to maximise the transmission channel throughput or to decrease the channel fading. However, multiple antennas are not easily integrated in mobile devices or sensors, because the space available is usually limited. Moreover, in order to lead high performances, good isolation and decoupling between antennas are required. Another drawback is that multiple antennas require multiple RF devices for signal processing that are very expensive. To suppress mutual coupling several methods were proposed in the available literature. In [1] the distance between MIMO antennas array was increased mounting them at the corner of a PDA. In [2] several parasitic elements were employed between MIMO antenna array, while in [3] band-notch slots on a ground plane were etched. In this paper we propose a new MIMO antenna that consists in a metallic ring fed at four ports with alternate phase. The alternate phase of the feeding leads to the excitation of orthogonal modes and allows the implementation of the MIMO antenna. The antenna has been analysed using the Theory of Characteristic Modes [4], which is really appropriate to design multimode antennas because of the physical insight it provides.