1. With the developments of all kinds of modern weapons, long-range BVR (Beyond-Visual-Range) attack, stealth weapon design and low-altitude raid have become prominent subject nowadays. These new technologies have become a major threat when conduct anti-air warfare. As a result, many countries around the world start to develop over-the-horizon radar. 2. The over-the-horizon radar mainly can divide into two groups, the High Frequency Sky Wave Radar and the HF Surface Wave Radar, according to their propagation methods. The former radar uses ionospheric refraction of the sky. The later radar uses diffraction effect from earth surface. They can detect ship and low-flying targets beyond visual range to compensate the blind area of normal radar.