# Design of Typhoon Detection Downcast Device Based on Four-Arm Helical Antenna Structure
## 1 Introduction
In October of the same year, the Shanghai Typhoon Research Institute of China Meteorological Administration (STRI of CMA) and Sichuan Tengdun Science and Technology Company (SSTC) also utilized the TD328 UAV to observe Typhoon Morakot and detected a large amount of data before the landfall of Typhoon Nangka [7]. Since 2017, the China Meteorological Administration (CMA) has taken the lead in organizing the  
Figure 1: Schematic representation of airship sounding (color). prototype development of a new BeiDou intelligent sounding system and has committed to upgrading the meteorological sounding service from L-band radar sounding to the BeiDou sounding observation system [8]. The meteorological sounding system based on BeiDou satellite navigation was put into service in 2023, and the vertical observation capability of the atmosphere was significantly improved [9]. In April 2024, the first BeiDou sounding observation system in Yunnan Province was completed at the Simao National High Altitude Meteorological Observation Station in Pu'er City and entered into the commissioning stage [10]. In summary, the continued development of droppable sounding systems for accurate observation of internal typhoon data is of great significance to the development of meteorological detection systems. However, the existing deployment method is to carry receivers, receiving antennas, sounders, droppable devices, etc., on airships, which not only occupy a large space but are also limited by the weight of the fuselage, which severely restricts the maximum number of sounders and the airtime of the airships and ultimately leads to a limited detection effect. Droppable devices and sounders mounted on flight equipment are often constrained in terms of size and weight, so achieving smaller, lighter equipment and more accurate detection results has become a joint research effort. In this context, the Typhoon Tracking and Detection Instrument (TTDI), a major national scientific research instrument development project in China, was proposed with the aim of improving the real-time monitoring of the internal structure of typhoons and their environmental conditions by means of advanced meteorological detection technology, thereby providing more accurate data support for typhoon intensity prediction. The project utilizes an airborne blimp as an airborne platform equipped with an advanced droppable device and a sounding instrument, the shape and internal structure of which are shown in Figure 2. The device directly enters the core area of the typhoon for observation and real-time transmission of key parameters such as temperature, humidity, barometric pressure, and GPS positioning. This study focuses on the lightweight and integrated design of the droppable typhoon detection system and on improving the overall performance and application efficiency of the detection system through innovations in the housing structure and antenna design of the sounder. At present, the mainstream weather sounders on the market are mainly cylindrical in shape, with a length of about 20 cm, as shown in Figure 2. As the four-armed helical antenna is also cylindrical, the detection device not only has a light weight and low volume but also provides stable and accurate data transmission. This is due to the outer wall of the traditional release device being replaced with a four-armed helical antenna with optimized design parameters, with the receiver and the sounder on the upper and inner parts of the release device, respectively, and the use of materials such as EPS foam and a PVC plastic cylinder. This study details the design principle of the four-arm spiral antenna, as well as parameter estimation and a performance test carried out at 403 MHz frequency. Simulation verification of the antenna design was also carried out using Ansys HFSS 2021 software, and the feasibility and validity of the design scheme were also confirmed through a physical test. The structure of the remainder of this paper is outlined as follows.