Atmospheric ducts are special super-refractive atmospheric structures that can cause over-the-horizon propagation of electromagnetic waves. Different types of atmospheric ducts have different influences on electromagnetic wave propagation. Owing to the complex marine atmospheric environment, different types of atmospheric ducts often occur together. When evaluating the performance of an electromagnetic system near the sea surface, the combined influence of various atmospheric ducts should be considered comprehensively. In this paper, the statistical distribution of atmospheric ducts over the northern South China Sea is analyzed using sounding data and reanalysis data. This paper uses the parabolic equation model to analyze the propagation characteristics of microwaves near the sea surface in the presence of both surface and evaporation ducts. It is found that compared with cases where only one type of atmospheric duct is considered, a hybrid atmospheric duct structure can capture more microwave energy at a lower receiving height. At an antenna height of 5 m, the path loss begins to fluctuate beyond a propagation distance of 50 km, with the maximum fluctuation reaching about 15 dB. Microwave propagation characteristics at different microwave frequencies and antenna heights are also simulated and analyzed.

The evaporation duct (ED) is generated by the evaporation of seawater and can be an influential factor of electromagnetic (EM)-wave propagation. Rainfall also affects atmospheric factors and EM-wave propagation. However, the distribution of the ED and path loss (PL) during rainfall has rarely been reported. This paper analyzes the distribution of the atmospheric factors and ED in the South China Sea (SCS). The results show that the evaporation duct height (EDH) in the area of rainfall is generally lower. The effect of the ED on the over-the-horizon (OTH) propagation reaches 0.69 dB km⁻¹ on average, which is 4.3 times stronger than the maximum rain attenuation (0.16 dB km⁻¹) when the rainfall is less than 5 mm h⁻¹. In the SCS, a 53 km long OTH link was established between Donghai Island and Jizhao Bay to observe the PL. The measurement results show that the nearly saturated relative humidity (RH) leads to a high PL. The results also show that the change in the direction of the sea–land breeze causes a 42.4 dB decrease of PL by transferring the moist patches. Rainfall has an attenuation effect on OTH propagation in ED, mainly owing to the high RH.

Evaporation duct is formed over the sea surface owing to the evaporation of seawater, and the increase in the evaporation duct height (EDH) will lead to the sharp decrease in the profiles of atmospheric modified refractivity near sea surface. Evaporation duct reduces the path loss (PL) of electromagnetic (EM) wave propagation, which has a significant effect on the performance of radar and communication systems. The trapping ability of evaporation duct fluctuates greatly within a single day; however, the effect of the diurnal variation of evaporation duct with measured PLs has seldom been investigated. In this paper, the diurnal variations of measured PLs of three over-the-horizon links are recorded by the developed EM propagation systems for two days in the northern part of the South China Sea. Combined with the data of meteorological observation and reanalysis results, the diurnal variation of evaporation duct is investigated through numerical simulations. The results demonstrate that the measured PLs exhibit strong diurnal variation with a fluctuation about 50 dB. Besides, the EDH shows a good correlation with the variation of PLs: in one day, during 06:00-08:00, the measured PLs are high, and the EDH is low; during 17:00-19:00, the measured PLs are low, and the EDH is high.

Evaporation ducts (EDs) significantly affect marine communication signals. The proficient understanding of ED characteristics is crucial for enhancing the quality of marine communications. We conduct a high-speed, over-the-horizon communication link between the moving vessel and stationary shore over the South China Sea. Meanwhile, we propose an evaporation duct height (EDH) inversion method based on the communication signals to sense EDs. The packet loss rate, path loss (PL) and air-sea paraments are observed during the measurement. Our results showed that the S-band communication signals propagated for 44 km (2.3 times the line-of-sight) in the ED at a bit rate of 40 Mbps and a packet loss rate of 0%. A range of 70 km (3.7 times the line-of-sight) was reached with a bit rate >30 Mbps and a packet loss rate <1.5%. The EDH inversion method using communication signals showed great potential with a root-meansquare error (RMSE) of 2.6 m, while compared with the predicted EDH. An interesting find is that the temporally and spatially nonuniform ED may cause PL to decrease with the range increases. The combination of communication and channel sensing in EDs can provide real-time propagation conditions utilizing original communication signals and then improve the quality of communication.

An evaporation duct is a persistent phenomenon that is formed by evaporation over the sea surface, and influences marine communication and radar detection through its trapping effects. A cold wave is cold air that rapidly intensifies and intrudes a region with strong winds, a dry air mass and precipitation, which affects the air-sea interactions and eventually affects the propagation of electromagnetic waves over the sea. However, there are few studies of over-the-horizon propagation in evaporation duct during a cold wave. In this paper, the path loss and atmospheric factors of two over-the-horizon propagation links over the Bohai Strait and Yellow Sea were observed during a cold wave in November 2021. The cold wave caused path loss to decrease over 35.9 dB and 44.2 dB respectively at links over the Bohai Strait and Yellow Sea, as the evaporation duct height simulated from the measured data increased 5.9 m and 7.6 m. According to the sensitivity analysis, high wind speed was the main factor promoting seawater evaporation during the cold wave, while a decrease in air temperature had the opposite effect. These effects of cold wave should be taken into consideration in marine communication and radar detection systems to enable the optimal use.