## 1 Introduction
Nevertheless, for frequencies below 60 GHz, there are other alternatives that combine printed circuit boards (PCB), surface mount technology (SMT) and silicon beamformer chips, resulting in a cost-effective solution alternative. As shown in Figure 1, in this configuration the radiating elements are typically placed on one side of the PCB, while the opposite side is employed to mount all SMD devices, so that the chips can be placed very close to the radiating elements, which minimizes feed losses and maximizes the gain-to-noise-temperature (G/T) of the antenna receiver and the effective isotropic radiated power (EIRP) of the antenna transmitter. The proposed 2 \(\times\) 2 circularly polarized antenna Tx array is based on this concept and some examples can be found in the literature [9; 10]. In [11; 12], dual-band phased arrays are presented to cover Tx/Rx satellite communications, interleaving in the same space K/Ka  
Figure 1: Cross-section of the Ka-band antenna array. band radiating elements with Ka-band elements, but introducing new integration problems that they will have to solve and with reduced bandwidths of 4.3% in Tx (29.5-30.8 GHz) and 4.8% in Rx (20.2-21.2 GHz). Focusing on this type of RF beamforming structure, in the literature we find state-of-the-art beamformers which include low-noise amplifiers (LNA), power amplifiers (PA), phase shifters, variable gain amplifiers (VGAs), power combiners, simple pole double throw (SPDT) switches, together with SPI control and even biasing networks all in a single chip, working at frequencies up to 95 GHz, for 4, 8 or 16 channels, and with a high yield [13; 14; 15]. To feed the beamformers array, an RF distribution network is implemented with cascaded Wilkinson dividers so that all chips are fed with a balanced RF signal. On the other hand, regarding the antenna radiating elements, many techniques have been developed to improve the circular polarization radiation bandwidth at the Ka-band, which include air- or substrate-integrated cavity-backed structure [16; 17], stacked patches [18], aperture-coupled patches [19] and multilayered structures [20; 21]. Microstrip antennas are commonly used due to their great advantages in terms of simplicity, versatility and low profile, while they are inexpensive and easy to manufacture [22]. Three different single circularly polarized radiating elements with different beamwidths and working at 30 GHz have been investigated in [23]. The first design implements a classical microstrip patch antenna fed through a U-shaped slot and where the circular polarization is achieved by trimming opposite corners. The second and third use the same radiating element but are inserted in a cavity and enclosed by folded conducting walls, respectively. In [24], an antenna element using aperture-coupled stacked patches and a fork-based feeding network in asymmetric stripline technology, from 17.6 to 30.3 GHz, is presented. Their main advantage resides in the fact of having a single radiating element with a good circular polarization over a great bandwidth. However, this can limit the beamforming performance in a phased array at some frequencies. In this work, a circularly polarized, sequentially rotated, aperture-coupled antenna array with highly integrable, cost-effective silicon beamformers is proposed for SOTM applications. The 2 \(\times\) 2 antenna array presented in this work will be the basic modular unit of a full Ka-band AESA. This paper is organized as follows: Section 2 describes the system architecture, the antenna fabrication technology and the proposed multilayer stack-up. Section 3 shows the experimental measurements of the radiating element, discusses its design and performance, and provides a complete experimental characterization of the 2 \(\times\) 2 antenna array configuration. Finally, conclusions are drawn in Section 4.