# A Circularly Polarized Antenna Array with Gain Enhancement for Long-Range UHF RFID Systems
## 1 Introduction  
Wireless communication technologies, nowadays, advance and diffuse quickly, and they are continuously evolving towards smaller devices with fewer energy requirements. This evolution is recently driven by the concept of \"Internet-of-Things\" (IoT), which aims at the development of ultra-dense networks, even if energy consumption is still an issue. In this sense, radio frequency identification (RFID) is a well-consolidated technology and a good candidate for the implementation of some IoT devices [1]. However, today's RFID solutions are still limited in terms of communication range. In the case of a monostatic RFID, the maximum achievable communication range \(r\) in an ideal case of line-of-sight propagation (and under the hypothesis of perfect antennas alignment and no polarization mismatch) is governed by the link budget equations (expressed in dB) [2]:  
\[\left\{ \begin{array}{c}P_{forward}=P_{EIRP}+G_{tag}+20\log_{10}\!\left( \frac{\lambda}{4\pi r}\right)\geq P_{th,tag}\\ P_{back}=P_{forward}+G_{reader}+G_{tag}+20\log_{10}\!\left(\frac{\lambda}{4\pi r} \right)+10\log_{10}\!\left(M\right)\geq P_{th,reader}\end{array} \right., \tag{1}\]  
where \(P_{forward}\) and \(P_{back}\) are the forward link received power (from reader to tag) and the backward link received power (from tag to reader) in dBm, respectively; \(P_{EIRP}\) (dBm) is the effective isotropic radiated power (whose maximum value is fixed by communication standards), \(G_{reader}\) and \(G_{tag}\) (expressed in dBi) are the reader and tag antenna gains, respectively; \(M\) is the modulation factor; \(P_{th,tag}\) and\(P_{th,reader}\) are the sensitivities (expressed in dBm) of the tag and the reader, respectively; and \(\lambda\) is the wavelength. One possibility to increase the communication range is the reduction of the two sensitivities, which is a natural process driven by the technological evolution. Furthermore, the use of semi-passive or active tag devices (which can improve the tag sensitivity or increase the factor \(M\)) is also beneficial [1; 3], but requires an increased amount of energy. Finally, the capability to radiate circular polarization (CP) [4] for immunity to polarization mismatching and resistance to multipath effects [5] is of fundamental importance for maximizing the communication range. From Equation (1), however, it is clear that for long-range RFID communication, high-gain antennas have to be employed. In particular, not only is the reader antenna gain \(G_{reader}\) fundamental to extend the backward link range, but its enhancement can contribute significantly to a transmitting power reduction (since \(P_{EIR\;P}=P_{tx}+G_{reader}\)) without affecting the forward link range. Many studies have been conducted to achieve high-gain RFID reader antennas which can cover the universal ultra-high frequency (UHF) RFID band [6; 7; 8; 9]. A maximum CP gain of 9.3 dBic is demonstrated in [7] with a single antenna configuration and the use of a structure with a high profile (35 mm). To further increase the antenna gain, an array structure can be employed. In addition, the sequential rotation (SR) technique [10] has also been used effectively in many different cases because of its capability to radiate CP and provide high gain and wide bandwidth [11; 12; 13]. An example of SR antenna array for high-gain RFID readers is reported in [11], which exhibits a gain of about 12.3 dBic, but with a relatively large size of \(590\times 590\times 1.524\) mm\({}^{3}\) (\(1.8\times 1.8\times 0.005\,\lambda^{3}\)). Similar or higher-gain antennas can be obtained at the expense of an increased design complexity, such as a complex feed network [12] and a metasurface [13]. In this manuscript, a \(2\times 2\) SR antenna array with high-gain characteristics for long-range UHF RFID systems is developed. Similar to [14] (where the objective was, however, the feed network optimization for achieving wideband axial ratio (AR) with \(\leq\)1 dB performance), a two-level SR feed network using series power dividers was implemented.