In order to facilitate the integration of the offshore surface antenna and reduce the manufacturing cost, the resonance shortening length and omnidirectional radiation performance of a centrally loaded whip antenna are analyzed. By calculating the impedance characteristics of each part of the midload whip antenna, the shortened length of the antenna in the resonance state is obtained; the superimposed field strength of the midload whip antenna to the far field is calculated and its radiation performance is analyzed; the simulation is carried out by using FEKO electromagnetic simulation software. Through the field test, the reflection coefficient and resonant frequency of the antenna before and after shortening are compared, and the directivity of the midload whip antenna and the traditional whip antenna is compared. The simulation and experimental results show that under the preset frequency of 75 MHz, the reflection coefficient of the shortened midload whip antenna decreases by 4.414 dB and 19.09 dB, respectively. The optimal operating frequency is about 68.6 MHz–79.4 MHz, and the bandwidth is 10.8 MHz. Compared with the traditional whip antenna, the midload whip antenna at the radiation null point receives a larger field strength.
In order to analyze the working status of the underwater unmanned vehicle not fully surfaced, the optimal working frequency when the whip antenna radiates the maximum power is given. The input impedance of the antenna on the water is theoretically calculated. It is regarded as the load of the underwater part of the antenna, and the total input impedance of the whip antenna is obtained. The relationship between the antenna radiated power to the external field and the input power is analyzed, and the optimal operating frequency corresponding to the maximum radiated power is determined. Using simulation experiments and actual measurements, the radiated power of the 1 m whip antenna when being immersed in seawater at 0.25 m, 0.5 m, 0.75 m is obtained, and the corresponding optimal working frequency is calculated, which are in good agreement with the theoretical deduction results. The results show that as the depth of the antenna immersed in seawater increases, the power radiated from the antenna to the external field decreases, and the optimal working frequency increases accordingly.
By analyzing a variety of traditional wave spectrum models, combining the actual wavelength and wave surface displacement to compare the electromagnetic field propagation process, the P-M (Pierson-Moscowitz) spectrum is used to theoretically derive the electrical characteristics of the rough sea surface and its influence on the propagation process of high-frequency electromagnetic waves. Based on the bilinear superposition algorithm and Matlab numerical calculation software, the electrical characteristics of rough seas and the theoretical results of electromagnetic wave propagation in rough seas are simulated and verified. The results show that the modeling of rough sea surface based on P-M spectrum has the advantages of high reducibility and stable frequency spectrum.
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