## 1 Introduction  
Circularly polarized (CP) antennas have advantages in resisting rain attenuation, suppressing multipath interferences, and receiving arbitrary linear polarization direction. Therefore, CP antennas have been widely used in the field of wireless communication and global positioning [1; 2; 3]. Furthermore, dual-band antennas minimize the requirement of using two antennas for two bands. The dual-band dual-CP antenna, which keeps polarization of the two bands opposite to each other, further enhances the isolation between two ports [4; 5]. The antennas, realized by using printed circuit board (PCB) technology, is an attractive alternative owing to the benefits of low profile, inexpensive cost, and high conformality [6]. In [7], dual-band, dual-CP antenna arrays were implemented with single-layer PCBs. Ref. [8] presented a 2 \(\times\) 2 magneto-electric dipole antenna array that realizes dual-band, dual-CP operation over a wide bandwidth. In [9], a 4 \(\times\) 4 dual-band, dual-CP antenna array using an artificial magnetic conductor (AMC) ground and having a unidirectional radiation pattern was proposed. However, dielectric losses of those PCB-based antennas are non-negligible in the K- and Ka-bands, resulting in higher insertion losses and lower radiation efficiencies, especially for large-scale antenna arrays. Compared with PCB-based antennas, full-metal antennas have advantages of low loss, high efficiency, and large power capacity. Slotted waveguide antennas have been demonstrated with high gains and efficiencies in the millimeter-wave band [10; 11; 12]. However, the involvement of high-precision computer numerical control (CNC) fabrication process significantly increases the cost. Additionally, potential fabrication error leading to the formation of air gaps can result in field leakage, which decreases the efficiency of the antenna. To reduce the sensitivity to fabrication error and to simplify the CNC machining process, gap waveguide (GW) technology has been introduced and developed. The operating principle, fabrication, and applications of GW components have been explored in various studies [13; 14; 15; 16; 17]. GW-based antennas can be fabricated into different layers and then assembled without the need for high precision welding, which significantly reduces the implementation cost [18]. Various GW-based antennas with full metal construction have been reported to provide high efficiencies and high gains [19; 20; 21; 22; 23]. In [19], a GW-based, 4 \(\times\) 4 Ka-band CP array is presented with a maximum efficiency of 98.0% and a maximum gain of 22 dB. In [20], an 8 \(\times\) 8 Ka-band dual-polarized array based on GW technology was proposed. The measured gain and efficiency of the antenna are over 27.5 dB and 75%, respectively. This dual-polarized design was then combined with a hybrid coupler feeding network to realize dual-CP operation [21]. A few published works can be found on GW-based high-efficiency antennas with dual-band characteristics. In [22], a K-/Ka-band linearly polarized (LP) antenna array was proposed. The antenna provides a 25.5 dB gain with efficiency over 84% in K-band, and a 29.5 dB gain with efficiency over 88% in Ka-band. So far, dual-band, dual-CP antennas with metallic GW structures have rarely been reported. It is worth mentioning that a very preliminary approach to a 2 \(\times\) 2 dual-band, dual-CP array antenna was proposed and numerically studied in [22]. However, the limited aperture size and small unit number results in a low directivity. The current work aims to develop a much more evolved version of that concept by addressing technical challenges of increasing the aperture size of the GW-based dual-CP array, finding a suitable fabrication process, and comprehensively investigating the antenna array's performance through both simulations and experiments. In this paper, a GW-based, 4 \(\times\) 4 dual-band, dual-CP antenna array is presented.