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Introduction
Polarization is one of the most essential and significant features of an antenna in modern wireless communication systems [1]. Polarization of an antenna is the locus traced out by the tip of the electric field vector of the wave radiated or received by the antenna as a function of time. Polarization of an antenna is usually determined from the electric field orientation; it will be either linearly or circularly polarized (CP) [1]. The orientation of the electromagnetic field radiated by a linearly polarized antenna lying in one plane, orthogonal to the direction of electromagnetic wave propagation. For example, a vertically polarized antenna can efficiently transmit and receive only a vertically polarized wave and vice versa, which is known as co – polarization [2]. Due to the reciprocity property of the antenna, the transmission and reception of the antenna are always similar. If a receiving antenna is vertically polarized and a transmitting antenna is horizontally polarized, then it is called cross-polarization [2], which incurs a huge signal loss. In circular polarization the antenna emits electromagnetic energy in a circular spiral pattern which covers horizontal, vertical, and all the planes in-between them. With circular polarization, the orientation of the transmitting and receiving antennas is irrelevant as the circular pattern of the transmitting and/or receiving antenna will always match with pattern of the incoming signal. Among other salient advantages of CP antennas are multipath rejection, which leads to signal interference in linear polarization, mitigation of fading effects and immunity against Faraday rotation effect, which make CP antennas somewhat indispensable components in modern wireless communication systems [2].
In the case of circular polarization, the orthogonal field components are almost of equal amplitude, spatially orthogonal and in phase quadrature. Thus the tip of the resultant electric field vector generates a circular locus while evolving in time.
In linear polarization, Fig. 1(a), the tip of the electric field vector oscillates along the same line, here along the x- or y-axes, thus tracing out a straight line.
Fig. 1.
Electric field vector tracing (a) linear, (b) circular, and (c) elliptical polarizations [1].
[Figure omitted. See PDF]
In the case of elliptical polarization, shown in Fig. 1(c), however, the quadrature-phase difference between the two field components is maintained, but the...