## 1 Introduction  
In the last few years, the attention on software-defined radar (SDR) has grown exponentially. As a matter of fact, this approach offers a series of advantages that goes from digital beamforming to hardware flexibility [1]. The possibility to drive an active electronically scanning array (AESA) with an SDR platform [2; 3; 4; 5] would pave the way for a fully digital radio-frequency (RF) front-end with unprecedented capabilities in terms of reconfigurability, re-use of hardware, and modularity.  
Recently, SDR platforms have been configured and tested to drive passive bistatic radar (PBR) systems based on linear arrays to perform detection and tracking functions. These works [6; 7; 8; 9] were mainly focused on the software implementation of the calibration network and of the digital signal processing (DSP) architecture. However, in order to fully exploit the potentialities of the SDR approach, design flexibility has to be ensured at the hardware level. Antenna elements that allow for dynamic polarization and beam steering are thus required, especially when used in large planar AESA, to reach the high gain and high coverage requirements of radar tracking systems.  
In order to respond to the needs of design flexibility and high performance, inspired by the recent works on multifunction phased array radar (MPAR) [10; 11; 12], we propose here an attractive unit cell fed with two ports that can be independently controlled in amplitude and phase, and that is capable of providing low cross-polarization and high isolation levels without using additional techniques, such as decoupling networks. In order to corroborate the antenna performance, a 4 \(\times\) 8 array demonstrator is fabricated whereas the gain and reflection coefficient of a 4 \(\times\) 6 array are measured, showing good agreement with the expected theoretical results for the single radiating element in a periodic environment.  
This preliminary demonstration paves the way for targeting the development of larger AESA panels, thanks to the modularity of the proposed sub-array architecture. Specifically, we will also discuss here the system requirements of a 40 \(\times\) 40 planar AESA capable of scanning a very directive beam over a wide angular range with controlled sidelobes in C band. Efficient analytical and numerical methods for the realization of a Tseng-Cheng beamforming network [13] and the evaluation of the main radiating properties are proposed to easily interface the AESA with an SDR platform. This latter investigation reveals that the proposed demonstrator looks suitable for achieving the ambitious system requirements of the 40 \(\times\) 40 AESA.  
The paper is organized as follows. In Section 2, we describe the targeted antenna performance of the AESA and present analytical methods for both the design of the digital beamformer and the evaluation of the AESA radiating features. In Section 3, the antenna element is described and designed to ensure polarization flexibility, low cross-polarization, and high isolation. In Section 4, the fabrication of a 4 \(\times\) 8 prototype and experimental validation of a 4 \(\times\) 6 demonstrator is shown. Finally, conclusions are drawn in Section 5.