# A Ka-Band Circular Polarized Waveguide Slot Antenna with a Cross Iris
## 1 Introduction  
Millimeter-wave communication systems support a higher data rate and larger capacity. Millimeter wave frequency has therefore been widely used in various applications such as 5G mobile, radar, satellite, military, and automotive communications [1]. Because mm-wave signals can be easily distorted by atmospheric interference such as fog or dust, waveguide antennas have been considered among the most appropriate for mm-wave applications due to their high power handling, high gain, stable performance and low transmission loss [2, 3]. The polarization of antennas is one of the important factors in determining the performance of the communication. For linear polarization antenna, if the polarizations of a transmitter and a receiver do not match each other, communication performance deteriorates. Meanwhile, circular polarization avoids polarization misalignment as the electric field rotates, and it reduces multipath fading with flexibility in orientation the angle between the transmitted receiver [4, 5]. Therefore, circular polarized (CP) waveguide antennas in particular are widely used in modern satellite communication systems due to their advantages of enhancing the channel stability and reliability [6, 7]. Several techniques have been introduced to realize the waveguide CP horn and aperture antennas such as using a printed circuit board with four printed hook shaped arms [7], loading two metallic arms with a 45\({}^{\circ}\) inclination angle [8], and inserting a meta-surface polarizer or chiral metamaterial transformer [6, 9]. However, there are some limitations in applying them to various applications since these antennas are relatively bulky and heavy. In addition, they are not suitable for array systems because a large space is required for polarizers to be installed [10]. Therefore, such antennas are not appropriate for satellite communication systems requiring a high gain array structure with compact size. To overcome these drawbacks of bulk and mass, various types of circular polarized waveguide slot antennas have been studied [11, 12, 13, 14, 15, 16, 17]. Waveguide inclined slot, crossed slot, and v-type slot antennas have been widely used for CP array antennas to generate circularly polarized waves with high gain [13, 14, 15, 16, 17]. However, there are few reports on either wideningthe axial ratio bandwidth of the waveguide slot antenna at the millimeter-wave band region or the design of waveguide slot CP antennas [16; 17].  
This paper proposes a Ka-band circularly polarized waveguide slot antenna with a cross iris. To achieve a circular polarization characteristic, a hexagonal shaped cavity is used [18]. By mounting a cross iris, the circular polarization performance of the proposed antenna is enhanced. The proposed antenna provides a \(-10\) dB reflection coefficient bandwidth covering from 31.51 GHz to 39.21 GHz. The 3 dB axial ratio bandwidth is 6810 MHz (31.72 GHz-38.53 GHz). The antenna peak gains are 9.08 dBi at 33.5 GHz and 7.33 dBi at 37.6 GHz. The boresight gains of 7.87 dBi and 5.76 dBi are achieved at those two frequencies, respectively.