###### Abstract  
This article presents the circularly polarized antenna operating over 28 GHz mm-wave applications. The suggested antenna has compact size, simple geometry, wideband, high gain, and offers circular polarization. Afterward, two-port MIMO antenna are designed to get Left Hand Circular Polarization (LHCP) and Right-Hand Circular Polarization (RHCP). Four different cases are adopted to construct two-port MIMO antenna of suggested antenna. In case 1, both of the elements are placed parallel to each other; in the second case, the element is parallel but the radiating patch of second antenna element are rotated by 180 deg. In the third case, the second antenna element is placed orthogonally to the first antenna element. In the final case, the antenna is parallel but placed in the opposite end of substrate material. The S-parameters, axial ratio bandwidth (ARBW) gain, and radiation efficiency are studied and compared in all these cases. The two MIMO systems of all cases are designed by using Roger RT/Duroid 6002 with thickness of 0.79 mm. The overall size of two-port MIMO antennas is 20.5 mm \(\times\) 12 mm \(\times\) 0.79 mm. The MIMO configuration of the suggested CP antenna offers wideband, low mutual coupling, wide ARBW, high gain, and high radiation efficiency. The hardware prototype of all cases is fabricated to verify the predicated results. Moreover, the comparison of suggested two-port MIMO antenna is also performed with already published work, which show the quality of suggested work in terms of various performance parameters over them. CP Antenna; LHCP; RHCP; MIMO; 28 GHz 2023 1430  
antenna designing [2]. To overcome the aforementioned problem, compact, wideband, high gain, simplified geometry antennas are in demand [3]. Currently, millimeter wave spectrum is utilized to obtain the high data rate transmission, as well as circularly polarized (CP) antennas, which are a good option for stable communication due to their resilience to multipath interference and ability to provide polarization mismatching between transmitting and receiving antenna [4; 5]. Due to this application of CP antenna, the CP antenna are highly used in satellite communication, RFID, and future 5G base station and mobile communications [6]. In literature, numerous CP antenna were designed for lower bands (GPS, ISM, WLAN, and 5 Gsub-6 GHz) [7; 8; 9; 10] and higher bands (mm-wave spectrum) [11; 12; 13; 14; 15; 16; 17; 18; 19; 20; 21]. The CP antennas are also made capable of operating at left-hand side, called Left Hand Circularly Polarization (LHCP), and right-hand side, called Right Hand Circularly Polarization (RHCP). CP antenna offering simultaneous LHCP and RHCP for GPS communication is reported in [7]. The antenna operates at 1.56 GHz and offers an axial ratio of \(<\)1.35. The overall dimension of the antenna is 100 mm \(\times\) 100 mm and has simple geometrical configuration. The two-port MIMO antenna reported in [8] contains overall dimensions of 22.5 mm \(\times\) 50 mm. The reported work operates at 5.6 GHz and offers an axial ratio over 5.37-5.72 GHz. A low-profile CP antenna having simple geometry and loaded with meta-surface is given in [9]. The antenna offers bandwidth of 4.28-6.37 GHz and axial ratio bandwidth of 5.18-6.29 GHz. In [10], the reconfigurable CP antenna is reported for three bands of 2.5, 3.3, and 3.8 GHz. The antenna offers LHCP and RHCP ranging from 2.47-2.56 GHz, 3.2-3.42 GHz, and 3.74-3.83 GHz. In literature, a number of CP antenna are designed for millimeter wave applications as well. In [11], a frequency reconfigurable patch antenna is given for 28 GHz 5G applications. The antenna has simple geometry and a compact size of 19 mm \(\times\) 17 mm but offers narrow axial bandwidth of 1.7 GHz and 1.8 GHz at resonance frequencies of 28 GHz and 38 GHz, respectively. A reflect array antenna for satellite communication is reported in [12]. Although the antenna offers wide axial ratio bandwidth of 2.35 GHz, it has a large dimension of 180 mm \(\times\) 180 mm, as well as complex geometry.