###### Abstract  
Broadband antennas with a low-profile generating circular polarization are always in demand for handheld/ portable devices as CP antennas counter multipath and misalignment issues. Therefore, a compact millimeter-wave antenna is proposed in this article. The proposed antenna structure comprises two circular rings and a circular patch at the center. This structure is further embedded with four equilateral triangles at a 90\({}^{\circ}\) orientation. The current entering the radiator is divided into left and right circular directions. The equilateral triangles provide the return path for current at the differential phase of \(\pm\)90\({}^{\circ}\), generating circular polarization. Structural development and analysis were initially performed through the characteristic mode theory. It showed that Modes 1 to 4 generated good impedance matching from 20 to 30 GHz and Modes 1 to 5, from 30 to 40 GHz. It also demonstrated the summation of orthogonal modes leading to circular polarization. The antenna-measured reflection coefficient 1S11 \(>\) 10 dB was 19 GHz (23-42 GHz), and the axial ratio at \(-\)3 dB was 4.2 GHz (36-40.2 GHz). The antenna gain ranged from 4 to 6.2 dBi. The proposed antenna was tested for link margin estimation for IoT indoor conditions with line-of-sight (LOS) and non-line-of-sight (NLOS) conditions. The communication reliability with co- and cross-polarization was also studied under these conditions, and the results proved to be satisfactory. C 2024 24 1569 14 February 2024 23 February 2024 24 1569 14 February 2024 23 February 2  
multipath signals [9]. Also, CP antennas are preferred over linearly polarized (LP) antennas as they provide the solutions to antenna misalignment, polarization change due to multipath, and attenuation to rain and snow. Therefore, broadband and CP can be realized with structures such as a magneto-electric dipole [6], dielectric resonator (DRA) [1], substrate integrated-waveguide (SIW) loop antennas [10], and planar dipole antennas fed by an SIW [11]. However, the antenna profile of these structures is large for IoT handheld or portable devices. Therefore, the pursuit of simple, planar, cost-efficient CP antennas is always in demand. In [12], an E-shaped patch structure whose bandwidth was increased by feeding the signal through an L-shaped probe is proposed. Similarly, in [13], an L-shaped probe was used to feed a signal to an arrangement of six square parasitic elements which were arranged in a \(2\times 3\) fashion. The arrangement of parasitic elements with a suitable distance achieved a broad bandwidth of 7.5 GHz (21-28.5 GHz). Both Refs. [12; 13] were able to achieve broadband with LP but not CP. A metamaterial-based metasurface also aids in attaining wider bandwidth, like in [14]. Here, a circular patch was the main radiating element fed by a coaxial feed. The radiator was optimized by truncating with two semi-circles that were further surrounded by square ring resonators which behaved as a metasurface acting as secondary resonators that enhance the bandwidth. Due to the truncation of the circular patch and surrounding metasurfaces, CP was achieved. In [15], a planar structure with two hexagonal rings and a partial ground is proposed. The electrical length of the outer and inner rings was \((3/4)\lambda\) and \(\lambda\) at 28 and 38 GHz, respectively, resulting in dual resonance with broad bandwidth. A monopole antenna with a grounded coplanar waveguide (CPW) can also achieve wider bandwidth. In [16], a monopole antenna with a ground-CPW is proposed. Here, the CPW had a couple of slots on either side and further along the monopole's two pairs of parasitic stubs. The combination of these resulted in 2 GHz of bandwidth from 27 to 29 GHz. In [17], a conventional approach was used to achieve CP. In this case, the edges of rectangular patch were trimmed, and a signal was fed from the feed line, which was at the bottom layer through via. The structure achieved a wide bandwidth with left-hand CP.