## 1 Introduction  
With the rapid development of information, communication equipment such as wireless communication, satellite navigation, and radar systems are widely used, making the electromagnetic environment increasingly complex. Especially in emergencies such as natural disasters, accidents, or terrorist attacks, once the communication infrastructure is damaged or overloaded, the stability of the communication system is greatly challenged. This may lead to communication interruptions or delays, severely affecting rescue and emergency response operations. Therefore, in complex electromagnetic environments or emergency situations, emergency communication systems play a crucial role in ensuring communication stability and providing network emergency support [1; 2; 3]. The airborne relay communication system is essential to the emergency communication system. The airborne platform, by carrying real-time information distribution or airborne forwarding payloads, can support the real-time distribution of information, relay signals, and expand the coverage of communication, featuring wide coverage, low latency, high mobility, and strong practicality, effectively enhancing the deployment capabilities of the communication system in emergencies such as major events, natural disasters, accident disasters, and social security incidents [4; 5; 6; 7]. In the airborne relay communication system, to adapt to the characteristics of the low altitude of the airborne platform and the limited payload, and at the same time to improve anti-interference performance and reduce polarization mismatch, the design of the antenna needs to consider miniaturization, high gain, wide beam or half-space beam coverage, dual circular polarization (CP), etc. [8; 9; 10]. Moreover, it is better for the antenna to operate at the K-/Ka-bands, which can directly adapt to existing satellite communication terminals, ensuring emergency communication when satellite communication is limited. In satellite communication systems, multi-beam antennas or phased arrays are commonly used to realize beam coverage or beam scanning functions for specified areas to achieve dual-CP and wide-area coverage [11; 12; 13; 14]. However, in airborne relay communication systems, multi-beam antennas or phased arrays are difficult to apply: multi-beam or phased array antennas often contain multiple antenna elements, along with complex control or feeding networks, which would bring additional weight, power consumption, and technical complexity. Therefore, traditional multi-beam antennas and phased arrays are not suitable for airborne platforms with limited payload and power resources. Adopting a miniaturized wide-beam single-beam antenna structure, combined with time-division multiple access technology, is an effective solution for the antenna system of the airborne relay platform. In view of this, this paper designs and proposes an antenna structure suitable for airborne relay communication systems. The antenna can operate at the K-band or the Ka-band, with dual-CP, a wide beam width, and features such as high gain and miniaturization. The main contributions of this paper are as follows:  
* This paper proposes a wide-beam dual-CP antenna that operates at the K- and Ka-bands, respectively, using a stepped septum polarizer to achieve dual-CP radiation at the K- and Ka-bands, and the beam width is broadened by loading a parabolic metal housing. * This paper optimizes the septum polarizer for the antenna element and uses a side-fed coaxial-waveguide transition as the input structure of the septum polarizer, effectively reducing the impact of feeding structure errors on the performance of the septum polarizer. * Extensive simulations and measurements were conducted on the two designed wide-beam dual-CP antennas, and the results show that the antennas have the characteristics of dual-CP, wide-beam coverage, and miniaturization. The structure of this paper is as follows. Section 2 introduces related works. Section 3 describes the design details of the antenna.