In contrast to mechanical scanning and electronic scanning, frequency diverse array (FDA) provides a new mode of scanning by using a small frequency offset across the array elements. But there exists an inherent contradiction in FDA without transmit weights since range resolution is proportional to the frequency offset while time illuminating at a particular target is inversely proportional to the frequency offset, so the accumulated energy may be not enough to detect a weak target. This paper proposes employing time-dependent transmit weights to break the contradiction and scan the scene effectively. The effectiveness of the proposed algorithm is verified by simulation results.