Is it possible to generate a small DC power from Radio waves?If yes then how?
Yes, it is possible. In fact, crystal radios work on this basis. With a reasonable antenna and a powerful enough AM station, they can drive a high impedance headphone to well-audible volume using only the power received by the antenna.
The basic trick is to us a transformer to step up the low RF voltage picked up by the antenna. Antennas tend to be rather low impedance, like 75-300 Ω. The output of the transformer would be at a higher voltage, although also at a higher impedance. The higher voltage is important so that the forward drop of a diode is small compared to the voltage, and therefore doesn't loose too much power when rectified.
In a crystal radio, the secondary of this transformer had a capacitor accross it to form a tuned circuit that resonated at the desired radio station's frequency. This capacitor was adjustable so that different radio stations could be received.