At the beginning of a flight for the purpose of dropping parachutists, the pilot reported that everything was normal for the start, taxi, and take off. When the airplane reached an altitude of 150 feet agl, the engine lost power. The pilot set-up for a forced landing in a field with the engine producing partial power and regaining full power, then back to partial power, until touch down. During the landing roll, the airplane collided with a fence. The carburetor icing chart indicates that the flight was being operated in serious icing at cruise and climb power. The pilot had not utilized carburetor heat for the flight.