# Bandwidth-Enhanced Circularly Polarized Crescent-Shaped Slot Antenna via Circular-Patch Loading
## 1 Introduction  
With rapid progress in the field of modern wireless communications, there is increasing demand for compact and broadband antennas [1; 2; 3]. The printed-slot antenna is a promising candidate due to its obvious advantages, such as wide impedance bandwidth, low cost, low profile, and ease of fabrication. Moreover, wireless-communication systems with circularly polarized (CP) antennas can provide better orientation-angle flexibility without causing severe polarization mismatch between transmitter and receiver [4; 5; 6; 7]. Therefore, there have been many efforts to design broadband CP slot antennas [8; 9; 10; 11; 12; 13; 14; 15; 16]. For slot antennas, a typical technique for realizing circular polarization is to implement perturbation structures to the signal line and/or slot. For instance, by having an inverted-L-shaped tuning stub attached to the signal line of a coplanar waveguide (CPW), and implanting a pair of grounded strips in the slot, a 3 dB axial-ratio (AR) bandwidth of 30.36% was achieved [8]. In other work [9], two orthogonal metallic strips, one protruding from the ground plane toward the slot center and the other protruding from the signal line, were used to realize a 3 dB AR bandwidth of 36.01%. By embedding a lightning-shaped feed line and a pair of inverted-L-shaped strips, another antenna [10] attained a 3 dB AR bandwidth of 48.82%. A regular hexagonal slot antenna with an L-shaped monopole reportedly reached a 3 dB AR bandwidth of 50% [11]. Broadband CP operation was also obtained using the slots with special shapes, and the resultant 3 dB AR bandwidths reached 58.66% for an S-shaped slot in Reference [12], and 60.87% for an L-shaped slot [13]. In another design [14], a microstrip-fedannual ring slot antenna with a pair of grounded hat-shaped patches was introduced, demonstrating a 3 dB AR bandwidth of 65.65%. A symmetric aperture consisting of two diamond-shaped slots fed by a CPW feedline was reported [15], leading to a 3 dB AR bandwidth of around 67.60%. In Reference [16], a falcate-shaped radiating patch and a circular slot were embedded to reach a 3 dB AR bandwidth of 58.7%. Recently, a CP slot antenna with a 3 dB AR bandwidth of 73.33% was realized in a design that used a sickle-shaped printed radiator excited by a straightforward tapered microstrip feed line [17].  
In this paper, we propose a broadband CP crescent-shaped slot antenna with enhanced 3 dB AR bandwidth, realized by the loading of a circular patch. To achieve a circular polarization operation, a bent feeding line was attached to a 50-\(\Omega\) microstrip feed line. A circular patch was then loaded to a proper position inside the crescent-shaped slot to significantly broaden CP bandwidth. Antenna design was analyzed and optimized using the ANSYS High-Frequency Structure Simulator (HFSS), which is an electromagnetic-wave simulator based on the Finite-Element Method (FEM). The antenna was fabricated on a single substrate; therefore, the fabrication process is easy and cost-effective. A broad 3 dB AR bandwidth of up to 84.40% (1.89-4.65 GHz) was experimentally achieved, which is entirely covered by a measured \(-\)10 dB reflection bandwidth of 92.33% (1.79-4.86 GHz). The achieved CP operating-frequency band of the proposed antenna covers the operating bandwidths of several systems, such as Bluetooth/WLAN (2.4-2.484 GHz), LTE Band 1 (2100 MHz), LTE Band 7 (2600 MHz), and some sub-6 GHz bands used in 5G mobile communications (e.g., 3.4-3.8 GHz in Europe, 3.3-3.6 GHz in China, and 3.4-3.7 GHz in Korea). The following sections describe the antenna design in detail and discuss the experiment results to confirm the feasibility of the proposed antenna.