# A Circularly Polarized Antenna Array with Gain Enhancement for Long-Range UHF RFID Systems
## The Proposed Antenna Design
### Sequential Rotated Feed Network  
In order to obtain CP radiation, the SR technique [10] was implemented. A series power divider was designed according to [20] but with SICL technology, and used as the feed network basic element to achieve equal amplitude signals and orthogonal phases as required by the SR method. It was implemented as the cascade of four quarter-wave transformer lines with characteristic impedances \(Z_{1}\), \(Z_{2}\), \(Z_{3}\), and \(Z_{4}\), respectively (see Figure 2a). Since each antenna was designed to exhibit an input impedance of 50 \(\Omega\), the four characteristic impedances of the feed line sections were \(Z_{1}=25\)\(\Omega\), \(Z_{2}=16.7\)\(\Omega\), \(Z_{3}=25\)\(\Omega\), and \(Z_{4}=50\)\(\Omega\), respectively, guaranteeing both impedance-matching and equal power distribution [19]. As the feed signal propagated clockwise, left-handed CP (LHCP) radiation was formed in the \(z>0\) plane.  
One- and two-level SR feed networks were designed with SICL technology and compared: the one-level feed network shown in Figure 2a was implemented with a central series power divider, where each output port was used to feed one patch antenna. On the other hand, the two-level feed network represented in Figure 2b was designed by cascading each output port of the central series divider with another series divider (called an \"element series divider\" hereafter). Each element series divider was used to feed one patch antenna, as shown in Figure 1.  
By employing the one and two levels of SR implemented with SICL technology, two antenna arrays were designed: the first employs four linearly polarized (LP) patch antennas, each of them fed  
Figure 1: Antenna geometry: (**a**) three-dimensional view, (**b**) detail of lateral view for one antenna element, and (**c**) top view.  
by one of the series power divider output ports in Figure 2a (obviously, although each antenna element is LP, the use of the SR feed network guarantees the generation of CP); the second array also employs four patch antennas, but each of them is fed by the four output ports of the element series divider of the two-level SR feed network in Figure 2b, connected to four orthogonal points as can be seen in Figure 1a. Simulated performance of the two antenna arrays were compared in Figure 3 in terms of S\({}_{11}\), AR, and LHCP broadside gain. Simulation results were obtained with the software ANSYS HFSS 15.0.0.  
As expected, the two-level SR feed network exhibits improved performance over the one-level SR feed network antenna array. In particular, S\({}_{11}\) and AR bandwidths widen considerably by applying the second level of SR, as already seen in [14]. Furthermore, since with the one-level SR feed network the antenna elements are LP, while with the two-level SR feed network the antenna elements are CP (thanks to the presence of the element series divider), there is a gain enhancement of about 3 dB in terms of maximum value [21]. Finally, the S\({}_{11}\)/AR bandwidth widening causes a general gain enhancement within the whole bandwidth, yielding to a wider 3 dB gain bandwidth.