Low-Profile Wideband Circularly Polarized Complementary Antenna and Arrays for Millimeter-Wave Communications

Low-profile millimeter-wave (mm-Wave) arrays using wideband complementary element combining circularly polarized (CP) electric and magnetic sources are proposed in this article. A pair of centrosymmetric printed $ {\Gamma }$ -shaped strips serve as a CP electric source, and a modified narrow slot with two etched centrosymmetric stubs can be treated as a CP magnetic source. The combination of them can provide a wide 3 dB axial ratio (AR) bandwidth. Operating mechanism of the proposed CP radiating element is investigated and provided. Two differently constructed CP arrays, i.e., a sequentially rotated $4\times4$ CP array and a full-corporate fed $8\times8$ CP array, are then designed based on the proposed CP radiating element, to confirm the expandability of the proposed CP radiating element. Prototypes of these two arrays were fabricated and measured. According to the measured data, the sequentially rotated fed $4\times4$ CP array exhibits an impedance bandwidth ( $\vert S_{11}\vert < -10$ dB) better than 57.1% (i.e., 25–45 GHz), a 3 dB AR bandwidth better than 31.4% (i.e., 29–40 GHz), and a maximum gain of 16.4 dBic, while the corresponding results for the full-corporate fed $8\times8$ CP array are 25.8% (i.e., 29–37.5 GHz), 19.1% (i.e., 29.8–36 GHz), and 24.3 dBic. These competitive performances together with merits, such as compact size, low profile, low cost, and direct integration ability, make these two arrays promising candidates for mm-Wave communications.

I. Introduction

Urgent demands of data transmission rate will keep rising up with numerous high data transmission applications appearing in modern life and large amounts of devices being connected to the network. Some remarkable techniques, such as multiple-input–multiple-output (MIMO) and orthogonal frequency-division multiplexing (OFDM), have been successfully used in the former generation of mobile communications to increase the spectrum efficiency, thus achieving higher data rate [1], [2], [3]. Compared with these aforementioned techniques, increasing the channel bandwidth should be one of the most effective ways to increase the wireless data transmission rate according to Shannon’s theorem. Unfortunately, it is impractical to furtherly increase the channel bandwidth in the regime lower than 6 GHz due to the shortage of frequency spectrum resource. In the announced document by the Third Generation Partnership Project (3GPP), millimeter-wave (mm-Wave) band has been included as FR2, and the commercialization of 5G mm-Wave communications has been in accelerated progress following the deployment of 5G sub-6 GHz communications [4]. Terahertz (THz) band has also been regarded as a potential candidate for the next-generation communications, i.e., 6G [5]. However, there is still a long way before THz techniques become ubiquitous. Undoubtedly, mm-Wave band should be the most promising one for high-speed wireless communications before the THz techniques become mature and the price goes down [6]. Except for the utilization of mm-Wave band, the construction of space–air–ground–sea integrated network has been put forward to support the communication for anybody at anytime and anywhere in the 6G era [5], [6], and a conceptional illustration of the scenario is shown in Fig. 1. Satellite communications that can provide wide coverage serves a significant role in the space–air–ground–sea integrated network. To cope with the possible severe attenuation caused by the atmosphere, the atrocious weather, as well as the polarization mismatching, circularly polarized (CP) wave is widely used in satellite communications [7]. Consequently, mm-Wave CP antennas and arrays that can produce CP waves in a wide frequency band have become a hot topic in the field of mm-Wave antennas and arrays in recent years [8], [9], [10], [11], [12], [13], [14], [15], [16], [17], [18], [19], [20], [21], [22], [23], [24], [25], [26], [27], [28], [29], [30], [31], [32], [33], [34], [35], [36], [37], [38], [39], [40], [41]. Fig. 1.

Conceptional illustration of the space–air–ground–sea integrated network in the 6G era.