# A Four-Port Dual-Band Dual-Polarized Antenna for Ku-Band Satellite Communications
## 1 Introduction  
Recently, there has been an increase in the demand for high-speed internet connectivity across large coverage areas, both in the defense and commercial sectors [1; 2; 3; 4; 5]. Satellite communication (SATCOM) Internet is one of the most effective solutions to meet such demands, providing communication and broadcast services to thousands of terminals across a wide geographical area. This potential market is attracting commercial companies like Starlink, OneWeb, and Kuiper, leading to a new race to launch space-based broadband connectivity with SATCOM systems. In general, to ensure an optimal link and high data rate between the satellite and the user, a mechanically scanned or phased-array user terminal is required to track low-Earth-orbit (LEO) satellites (for fixed and mobile users) and geostationary equatorial orbit (GEO) (for mobile users) or medium-Earth-orbit (MEO) satellites. Traditionally, mechanically steered parabolic dishes or waveguide slot arrays have been used for SATCOM antennas due to their low cost and high efficiency [6]. However, these antennas are bulky and have a large swept volume. Although fixed-beam antenna arrays can offer a low-profile solution [7], they still require mechanical scanning both in the azimuth and elevation planes, so they consume a significant volume. A low profile is also achieved by combining electronic steering in elevation with mechanical beam steering in azimuth [8; 9]. However, it still requires a motor with a high torque capacity, which has moving parts and requires extensive maintenance. To overcome the above-mentioned issues, phased-array antennas (PAAs) are preferred, since they can direct their main beam anywhere without requiring physical rotation. This concept involves arranging several small elements in such a way that controls the radiationphase of each element individually. A progressive phase shift is applied between the elements in order to determine the beam pointing angle. Additionally, it also allows the tracking of multiple satellites or objects simultaneously. The main drawback of conventional PAAs is the need for separate transmit (Tx) and receive (Rx) antennas due to their different operating frequencies, which significantly increases the actual occupied space and manufacturing cost. Therefore, it is necessary to combine the Tx and Rx antennas into a single radiating aperture to minimize the overall system volume. However, the design of such a shared-aperture dual-band dual-polarized (DBDP) antenna is challenging. The combination of two orthogonally arranged dual-band single-polarized antennas is the most common method used to realize a shared-aperture DBDP antenna [10; 11; 12; 13; 14]. Nevertheless, this approach has two inherent drawbacks. First, it is difficult to control the two frequency bands independently since they use the same design parameters. Second, a mechanism to distinguish between Tx and Rx operations would also be required. A more efficient solution is to implement four different excitation ports, two for the low-band (LB) element and two for the high-band (HB) element, for the realization of a DBDP antenna with a shared aperture. This approach provides better electrical performance and reduces the manufacturing complexity. Despite these advantages, only a few four-port shared-aperture antennas with DBDP operation have been reported in the literature [15; 16; 17]. In this paper, a four-port shared-aperture DBDP antenna is proposed based on the multilayer printed circuit board (PCB) technology for Tx/Rx Ku-band satellite communication applications. The antenna consists of an LB \(L\)-probe proximity-fed square-ring antenna and a HB stacked-patch antenna, sharing a common radiating aperture. Each antenna has two excitation ports for dual-polarized operation. The proposed antenna is designed and optimized to cover two frequency bands, 10.7-12.75 GHz (Ku-band downlink) for Rx mode and 13.75-14.5 GHz (Ku-band uplink) for Tx mode, with a frequency ratio of approximately 1.2.