## 1 Introduction  
The Josaphat Microwave Remote Sensing Laboratory (JMRSL), Center for Environmental Remote Sensing (CEReS), Chiba University in Chiba, Japan has been conducting research and development related to ionosphere sensors onboard microsatellites since 2015 [1; 2]. Developments in space technology have motivated scientists and engineers to consider the idea of a small satellite. The evolution and ongoing miniaturization of technology have led to the emergence of new small satellites, such as nanosatellites capable of performing simple and complex missions in outer space [3]. As a radio wave transmitter and receiver, an antenna is a critical part of a satellite system. Many satellite applications in L-band and S-band, such as Global Navigation Satellite Systems (GNSS) and satellite communication systems, require wideband circularly polarized (CP) antennas. The CP antenna is resilient to multi-path interferences, the Faraday rotation effect, and polarization mismatch, especially for small satellites without attitude and stabilization control. The proposed satellite antenna for ionospheric monitoring [4] has a quadrifilar helical antenna [5] with a crossed-dipole, but these antennas are an inappropriate size for nanosatellites. The compact size, lightweight, and simple fabrication method make this antenna suitable for a satellite system. CP printed-slot antennas have been broadly developed to achieve wide 3 dB axial ratio bandwidth (ARBW) fed by a coplanar wave (CPW) and microstrip line. A CPW-fed slot antenna with a lightening feed line and inverted-L grounded strips [6] produced a 3 dB ARBW of 48.8% and 4.2 dBic peak gain, whereas a CPW-fed symmetric-aperture antenna [7] produced a broader 3 dB ARBW of 68% and 4 dBic peak gain. Microstrip-line-fed ring slot antennas have lower bandwidth and gain, such as the square-ring slot antenna [8]. Better gains have been achieved by microstrip-line-fed wide-slot antennas, such as the patch-loaded square slot antenna [9] that produces a 3 dB ARBW of 12% with a peak gain of 5.5 dBic, and the L-shaped microstrip-fed circular-shaped slot antenna [10] that achieves a 3 dB ARBW of 44% with a peak gain of 5 dBic. Gain enhancement with substrate-integrated waveguide (SIW) cavity-backed antenna using \(TE_{410}\)[11] produced a gain of 9.2 dBic with no 3 dB ARBW. Antennas with asymmetrical square-ring slots and a shifted feedline on the \(x\)-axis [12] presented a 3 dB ARBW of 4.3% and a gain of 4.3 dBic. Wider ring slots incorporated with grounded hat-shaped patches and a curved feed line [13] significantly enhanced the ARBW with a 3 dB ARBW of 65% and a peak gain of 5.1 dBic. However, the drawbacks are a weaker structure and a much larger size. The conventional circular-slotted antenna is linearly polarized. Therefore, changing the structure, such as introducing perturbation to the slot, a hybrid coupler [14; 15], and two linked slot rings [16], has been proposed to transform the polarization from linear mode to circular mode. However, these modifications yield narrower 3 dB ARBW than impedance bandwidth (IBW). As such, improving the 3 dB ARBW technique is necessary. Many methods have been investigated for this purpose, including a slanted rectangular slot with a circular patch [17] and a narrow horizontal slot on the equilateral triangular patch antenna [18]. However, these modifications did not improve the impedance bandwidth, axial ratio bandwidth, or gain. The goal of this study was to create a design for enhancing the CP performance of a circular-slotted antenna (CSA) to achieve a wide 3 dB ARBW close to its IBW, and a higher gain. CP radiation was generated by creating two truncations, a simple 50 \(\Omega\) microstrip feedline, and a parasitic patch. The truncations were created by a pair of asymmetrical rectangular slots on the diagonal axis of the grounded patch to generate AR polarization.