# Design of Typhoon Detection Downcast Device Based on Four-Arm Helical Antenna Structure
## 1 Introduction  
With the intensification of global climate change, the frequency and destructive power of typhoons--a form of tropical cyclone--are on the rise, posing an increasingly serious threat to human social and economic activities. The occurrence and development of these extreme weather phenomena and the changes in their paths are affected by a variety of complex factors, making the prediction of their intensity and movement paths a major challenge in the field of meteorology. Although the accuracy of typhoon track prediction has significantly improved in recent decades with the help of scientific and technological advances, especially the rapid improvement in remote sensing technology and data processing capability, there is still significant uncertainty in the prediction of typhoon intensity. The precise prediction of the intensity change of a typhoon approaching or making landfall is especially difficult, which directly affects the effective implementation of disaster prevention and mitigation measures. Early studies relied mainly on ground observations, aircraft reconnaissance, and satellite remote sensing, which provide valuable data for typhoon research but also have many limitations. For example, although reconnaissance aircraft can directly enter the eye of a typhoon to conduct measurements, this method is costly and involves safety risks; furthermore, while satellite remote sensing can provide large-scale observations, the timeliness and accuracy of the obtained data cannot meet the demands of real-time monitoring. Therefore, the development of droppable sounding devices for high-precision, real-time monitoring of the internal structure of typhoons has become an important direction in current research. Sounding instruments dropped from a designated location at high altitude have been used to measure and transmit real-time typhoon data to ground receiving stations. The United States, Germany, the Soviet Union, Finland,and France have all carried out research and development on sounding instruments and ultimately put them into use [1]. A schematic diagram of the blimp sounding procedure is shown in Figure 1.  
A droppable sounding system consists of a sounding device, a release mechanism, a platform signal-forwarding device, and a ground signal-receiving device. The sounding device is divided into a slow descent device and a meteorological sensing node. The slow descent device enables the sounding device to fall down smoothly and slowly and stably obtain data from the interior of the typhoon for as long as possible. The meteorological sensing node contains the temperature, pressure, and humidity sensors; the localization and wind measurement module; processing unit, transmitter; and power supply of the sounding device [2]. In 1943, the U.S. Air Force first successfully observed an offshore typhoon by means of a droppable sounder, obtaining accurate wind, temperature, pressure, and humidity contour-line data of the typhoon's internal structure [3]. After decades of development, Finnish droppable sounders have become the most representative sounding products in the world, although they are also very expensive [4]. For example, the RD93 sounder from Vaisala Finland can transmit its detected temperature, barometric pressure, humidity, and toilet paper in the 400 MHz band; weighs only 420 g; and has a high accuracy of 0.1 m/s [5]. The GTS1 digital sounder developed by China in the 1990s was highly accurate and fast, had anti-interference capabilities, and was close to the pinnacle of this technology at that time [1]. In 2009, China performed airborne typhoon observation using droppable devices for the first time [6]. In August 2020, the Detection Center of China Meteorological Administration (CMA) and the Meteorological Bureau of Hainan Province, jointly with Chengdu Aircraft Manufacturing Company (CAMC), successfully detected typhoon Morakot and collected the reference data through the YINGLONG-10 UAV.