# A Fundamental Study on Circularly Polarized Double-Loop Antennas with Quasi-Two Sources
## 1 Introduction  
A circularly polarized (CP) antenna [1] has two main advantages: it alleviates signal fading due to multipath propagation, and provides signal insensitivity to the orientation between the transmitting and receiving antennas [2]. These advantages have been leading to modern communication systems, such as mobile, navigation, and satellite applications [3]. Antennas used in the applications are designed based on several fundamental studies, and this study concerns a wideband CP antenna [4] with a simple and compact configuration. It is known that a loop antenna radiates a CP wave with the help of a perturbation gap [5; 6], segment [7], or loop deformation [8]. A recent study has shown that CP radiation can be obtained for a square-loop antenna with quasi-two sources [9], where two loop corners are excited with the same amplitudes and a phase difference of 90 deg, using a branched feedline vertical to the ground plane. This study is a sequel to the previous one [9] and demonstrates the newest findings with respect to a square-loop antenna with quasi-two sources. Special attention was paid to the increased bandwidth of CP radiation. For this, we investigated two concentric double-loop antennas using the moment method [10]. Several studies have attempted to increase the CP wave bandwidth of a loop antenna [5; 6; 7; 11; 12; 13; 14]. The studies are classified into three groups by their loop types: annular single [11; 12], separate dual [5; 6; 13], and concentric double loops [6; 7; 14]. The double-loop type has an advantage over the others due to its compact and simple antenna configuration. Herein, a double-loop antenna has been constructed using driven and parasitic elements, each requiring a perturbation gap [6] or segment [7] for CP radiation. In contrast, the present loops require neither a perturbation gap nor a segment. Instead, pairs of loop corners are connected by lines \(F_{n}\)-\(C_{n}\) (\(n=1\) and \(2\)), as shown in Figure 1, leading to a more straightforward design for CP radiation than a conventional one. Note that there are recent studies [15; 16] whose titles include double loops, but CP radiation is generated by four slots [15] or two probes [16] instead of using CP loops. In addition, recent studies have focused on antenna size reduction at the cost of configuration complexity, while CP loops have simple configurations. A motivation for this study comes from the fact that an array antenna composed of single loops with quasi-two sources exhibits a wider axial ratio bandwidth than non-resonant antenna arrays, such as a compact spiral or curl [17]. If the single loops in the array were replaced with double loops, we could expect a much wider bandwidth than that of the single-loop array. With this expectation, we investigated the use of a double-loop antenna with quasi-two sources in this fundamental study to increase the axial ratio bandwidth compared to that of a single-loop antenna with quasi-two sources. This study is improved compared to the previous related studies [7; 9], as summarized in Table 1. It is emphasized that the novelty of the present study is to clarify the effects of parasitic loop location on CP wave bandwidth for the first time. Furthermore, the present study optimizes the spacing \(S\) between loops for wideband CP radiation. In contrast, the previous study [7] fixed the spacing \(S\) and investigated only the case of an inner parasitic loop. As a result, the present study can attain a 3 dB axial ratio bandwidth of 40%, which is 13 times as wide as that in [7]. Note that the present study uses fewer loop corner connections than the previous one, leading to a simpler configuration.