# Dual-Frequency Linear-to-Circular Polarization Converter for Ka-Band Applications
## 1 Introduction  
In recent years, wireless sensor networks (WSN) have attracted attention in the wireless communication domain for several applications such as medical and military surveillance, localization, smart homes, smart cities and monitoring. Circularly polarized (CP) antennas are often used in these applications [1; 2; 3; 4] due to their ability to improve channel performance through alleviating multipath interference, low absorption losses and signal attenuation. CP can be generated in different ways depending on the kind of antenna. In this paper, circular polarization was generated by illuminating a planar polarization converter with a linearly polarized (LP) wave. With this approach, the converter and the antenna can be designed independently, thereby avoiding a complicated feeding network typically used in classical CP antennas. Planar polarizers can be divided into two categories: reflection-type circular polarizers [5; 6; 7; 8] and transmission-type circular polarizers [9; 10; 11; 12; 13; 14; 15]. The transmission-type circular polarizers are more common and have been widely investigated in the literature.  
The K/Ka frequency bands allocated in Europe for both civil and military satellite applications are 27.5-31 GHz (Ka-band) for the up-link case and 17.7-21.2 GHz (K band) for down-links. This type of link requires a high isolation between transmission (TX) and receiving (RX) channels that is obtained by assigning orthogonal polarizations to the two bands. As is well known, the use of a planar shared aperture antenna able to cover both bands can be a great advantage [16]. However, the single aperture configuration requires that the antenna, except when operating on the two TX and RX bands, has to be able to manage the dual polarization requirement. In this case planar Dual-band Linear to Circular Polarizers (LCPCs) are of much help. Dual band LCPCs are more difficult to design compared with single-band LCPCs. This is mainly due to mutual effects between the dual-band components. Some examples of circular polarizers have been presented in recentliterature [13; 17; 18; 19; 20; 21; 22; 23] for the dual-band case. While the solutions presented in [13; 22; 23] have a limited AR bandwidth at both bands, the configurations described in [17; 18; 19; 20; 21] have high insertion losses. In addition, several solutions available in the recent literature [21; 22; 23] are not able to provide orthogonal polarizations at the two bands.  
In this paper, we propose a dual-band linear-to-circular planar polarization converter based on a multilayer printed circuit board (PCB). The proposed converter can generate right-handed circularly polarized (RHCP) waves at the Ka-band while left-handed circularly polarized (LHCP) waves are generated at the K-band. It should be noted that opposite polarization may be generated by flipping the polarizer with respect to one of its sides. The elementary cell is formed by six substrate layers separated by five foam spacers. An equivalent circuit model based on transmission lines is proposed using a first-pass design that is then refined by full-wave simulations.  
The paper is organized as follows: Section 2 describes the working principle of the proposed elementary cell and introduces the equivalent circuit used in the design process; simulated and measured results are presented in Sections 3 and 4, respectively; conclusions are finally discussed in Section 5.