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The reflections of high frequency (HF) radio waves between ionosphere and earth’s surface make long-distance information transmission possible. In this paper, the propagation process of radio signals was analyzed and the ionosphere was simplified. Considering the strength loss of signals that occurs in the travelling process and at the reflection points, two pairs of differential equations and integral equations were established to simulate the strength variations of HF radio waves and noises. A different equation of SNR was also developed, which utilized the failure threshold of signal-noise-ratio (SNR) as a criterion to evaluate the effectiveness of signals. Meanwhile, the pace of SNR attenuation was simulated when reflections happens on calm ocean, turbulent ocean, smooth terrain and rugged terrain.

Water resistance of basic magnesium sulfate (BMS) cement has been studied in this paper. The solubility of main hydration of 5Mg(OH) 2 ·MgSO 4 ·7H 2 O in BMS cement has been analyzed by analyzing ions changes in 5Mg(OH) 2 ·MgSO 4 ·7H 2 O solution. Compressive strength and softening coefficient changes of BMS cement after immersing in water have been tested to evaluate water resistance of BMS cement. And crystal compositions and microstructure of BMS cement before and after immersing in water has been analyzed by XRD and SEM to study the mechanism. These results show that 5Mg(OH) 2 ·MgSO 4 ·7H 2 O possesses a low solubility at room temperature. Basic magnesium sulfate cement displayed an excellent water resistance based on additives in basic magnesium sulfate cement can inhibit the hydration of hard burnt MgO to sheet-like Mg(OH) 2 and reduce crystallization stress in BMS cement.