In an active array antenna, the phase of an element is realized by a digital phase shifter. For a circularly polarized (CP) radiation, CP elements are typically used that are generally excited by planar hybrids or other type of polarizers. Polarizers have high RF loss particularly at high frequencies. It is possible to generate CP radiation by using properly phased rotating linearly polarized (LP) elements [1]. The latter configuration is attractive for an active array because the phase shifters that are controlled independently can realize the desired phase distribution. Thus, the polarizer layer can be completely eliminated. However, the configuration may have a lower gain as compared with a similar array of CP elements [2]. Gain improvement of such arrays has been proposed in [3]. However, the bandwidth characteristics of an LP elements' array for CP radiation has not been reported in the open literature. In this paper we show that for large element spacing a rotating LP elements' array (Fig. 1(a)) becomes 50% less aperture efficient than a CP elements' array (Fig. 1(b)). On the other hand, for small element spacing, the difference in aperture efficiency between the two arrays is negligibly small. Consequently, for small element spacing, the rotating LP elements' array is likely to have more overall efficiency than the CP elements' array because the latter has the additional RF loss due to polarizer circuitry. Furthermore, it is found that in the array environment the return loss bandwidth of an LP elements' array is narrower than that of a CP elements' array. In order to have a complete characterization of the two arrays, we present numerical results based on a full wave Floquet MoM analysis [4,5] of slot-fed patch elements. Advantages and disadvantages of the two arrays are discussed.
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