###### Abstract  
This article presents a technique for transforming the polarization of a pyramidal horn antenna by adding multiple layers of frequency-selective surfaces in front of the aperture, in order to rotate the direction of the electric field. Thus, two orthogonal components with the same magnitude, phase-shifted by 90\({}^{\circ}\), are generated. Each frequency-selective surface consists of skewed \(\lambda/2\) dipoles. Compared to other similar structures, our antenna system combines the field radiated from the horn aperture with the field scattered by parallel frequency-selective surface structures spaced on the same principle as that for designing a Yagi-Uda antenna array. The proposed horn antenna with multiple frequency-selective surfaces can be used as a feed element for a parabolic reflector antenna for maritime satellite communication systems in the X-band or in the lower part of the Ku band, or as part of a sensor for finding the direction of arrival of a wave, in order to orientate an antenna system. The concept was successfully validated on the basis of simulation and measurements. The proposed technique provides a close to unit axial ratio together with a 3 dB increase in gain compared to the conventional horn antenna, at low manufacturing costs. antenna radiation measurements; circular polarization; frequency-selective surface; multiple layers; pyramidal horn antenna; space communications; Yagi-Uda antenna 2022  
A low-cost, general-purpose pyramidal horn antenna can be easily converted into a CP antenna if a linear-to-circular polarization converter is placed in front of the aperture. Recently, frequency-selective surfaces (FSS) [14] or metasurface-based converters [15; 16; 17; 18; 19] have become an important research direction. There are many studies on horn antennas with phase compensation by FSS aperture loading in order to achieve a high gain and a narrow beam [20; 21]. Single-layer FSS structures are usually used to filter the signal in a certain frequency band [22; 23; 24]. Conversely, multiple layers of FSS are used to change the polarization of the antenna and to increase its gain [25; 26; 27; 11; 28]. In [16], a polarization converter based on a multilayer non-resonant FSS was proposed. The size of the unit cell was much smaller than the wavelength. To generate enough phase difference between two electric field vectors with different directions, the FSS structure included a three-layer patch with truncated corners and a two-layer grid line, etched on the four dielectric slabs. This structure was efficient, but its complexity may have an impact on the manufacturing costs. As in [11; 17], the polarization converter presented in [18] was placed at a certain distance away from horn aperture. Compared to the installation of a polarization converter inside the feeding waveguide, loading a polarization converter in the horn aperture [19] may effectively reduce the overall, vertical size of the antenna and mounting on the antenna is easier. This article presents an alternative method for transforming antenna polarization, basically by adding multiple layers of frequency-selective surfaces in front of the pyramidal horn aperture, in order to rotate the direction of the electric field by 45\({}^{\circ}\) and consequently, to form two orthogonal components with the same magnitude and phase shifted by 90\({}^{\circ}\). Adding an FSS multilayer structure also increases the gain of the antenna. As opposed to other similar approaches, frequency-selective surfaces are much simpler and consist of skewed, half-wave linear dipoles, since they provide a good response in polarization. Compared to other multiple-layer structures, the FSS layers were spaced so as to operate on the principle of a Yagi-Uda antenna array. As a result, the radio waves emerging from each element are transmitted with such delays that the individual fields add up constructively in the main direction of radiation, and destructively in the opposite direction.