A post foundation system is provided. The system has a shaft with a helical disk attached to one end and a post support attached to the other end. The helical disk drives the shaft into the ground as a rotational force is applied to the shaft. A fin section is rotatably coupled about the shaft between its first and second ends. The fin section can have one or more fins extending outwardly from the shaft. The fin section engages the ground to stabilize the post foundation system as the shaft of the post foundation system is driven into the ground.