A pair of crossed dipoles comprise two electric dipoles nominally orthogonal in orientation and nearly co-centered in space. Crossed dipoles have long facilitated beamforming, direction finding, and polarization estimation. Each dipole measures one distinct Cartesian component of the incident electric-field vector. A pair of crossed dipoles thus constitutes a diversely polarized array capable of distinguishing incident sources based on their polarizations, besides basing on their directions-of-arrival and frequency spectra. One pair of crossed dipoles can suffice to determine the incident electromagnetic wavefront's (bivariate) polarization and the azimuthal direction-of-arrival. For an extensive survey of the relevant literature, please refer to Yuan et al. (2012), Wong et al. (2017, Khan and Wong (2019).If these two constituent dipoles are exactly perpendicular to each other, they would experience no electromagnetic coupling between them. (The mutual coupling would be negligible across two perpendicular dipoles, if the dipoles are fed differentially at their central feed points, due to the symmetry in both the fields and the currents/ voltages of the feed structure. "Differential feeding" here refers to the signals fed to the dipoles' terminals in an opposite but equal way.) However, real-world implementations of the crossed dipoles may deviate from this idealized orthogonality, thereby resulting in mutual coupling between the two dipoles. Please refer to Figure 1, which defines the skew angle φ (i.e., the angular deviation from perpendicularity) between two identical dipoles-One on the z-axis and the other on the y′-z′ plane.A dipole pair's impedance matrix Z is 2 × 2 in size, with entries complex in value. This impedance matrix is also symmetric, centro-symmetric, and persymmetric. Hence, there exist only two distinct complex-value scalars that need to be modeled, namely Z 1,1 = Z 2,2 and Z 1,2 = Z 2,1 . Each above complex-valued scalar may be represented by its magnitude and its complex phase. Therefore, four real-valued scalars need to be modeled. (These mutual-coupling coefficients, {Z i,j , ∀(i, j)}, are independent of the incident electromagnetic field's direction-of-arrival. However, the coupled voltages and the coupled currents depend on both the mutual-coupling coefficients and the incident electromagnetic field's direction-of-arrival, as illustrated in Section 6.)
The "Phenomenological"/"Behavioral" Approach to Model Mutual ImpedanceFor such a pair of skewed or slanted dipoles of equal length: Previous analysis of the concerned antenna electromagnetics has led to knotty mathematical expressions for the mutual impedance: (a) As a six-page equation in Czyz (1957), (b) as an unsolved integral equation in Murray (1933), Baker and LaGrone (1962), Richmond (1970, Richmond and Geary (1975), Han and Myung (2012), Han et al. (2013), or (c) as nested summations in Richmond