Investigation of polarization divider in an L-waveguide

Abstract

The polarization divider under study is a taper from an L-waveguide with equal arm to a square waveguide. From the L-waveguide side of divider, two rectangular waveguides for input were joined. Depending on which rectangular waveguide was the input one, the anticlockwise or clockwise polarized wave is excited. The working principle of this polarizer was introduced in [1].

Calculation of the main characteristics of polarizer has been carried out by generalized S-matrix method (GSM) [2], that allows study of devices taking higher-order modes into account. For the consideration of the transformation of modes in the polarizer we approximated irregular L-waveguide by a stepped transition consisting of N regular equal-branch L-waveguide sections. These latter smoothly contract to a square waveguide with keeping the symmetry along the wave propagation direction. The basic elements of this transition arc the junctions of the different cross-section waveguides. Following the GSM vector-potentials of the field in a regular waveguide are expanded in terms of the Fourier series of cross-sectional eigenfunctions, taking into consideration the basis function of the coupling windows. The eigenfunctions and the cut off numbers equal-arm of L-waveguide were calculated by the partial domain method [3], From the computed scattering matrices of the cross-section junctions a complete S-matrix of the waveguide polarizer was computed. This scattering matrix contains all the necessary frequency characteristics: VSWR, isolation, ellipticity coefficient, etc.

The design of three various polarizers was performed based on the above technique. According to calculations, we see that in the working bandwidth in some sections of L-waveguide there exist up to 4 propagation H-modes and E_n mode. Thus, the transmission of energy is performed by not only two fundamental L-waveguide modes, but also by the higher-order propagating H- and E-modes, that degrade the polarizer VSWR and isolation. The results of computations are compared with experimental data. The method of design and optimization of the antenna polarization divider is discussed.