The pilot reported he departed from his private airstrip (850 feet by 70 feet, turf) with a passenger for a local flight. Upon returning for landing, his neighbor was operating a combine in a soybean field near the runway threshold. The pilot extended his traffic pattern to allow time for the combine to move out of the runway approach path. However, upon turning back toward the runway, the combine had not moved as far as he had expected. The pilot stated it was at that point the airplane struck a power line. It subsequently came to rest upside down and was destroyed during the accident sequence. Both the pilot and passenger were able to exit the airplane. They sustained minor injuries. The pilot stated that there were no failures or malfunctions associated with the airplane prior to the accident.