Me, a F-28 Captain, was walking briskly down the jetway and on to the aircraft. I knew that the main entry door on the F-28 was a little lower than on the Boeings but my mind was elsewhere. As I entered the aircraft, the top of my head smashed into door jam and I was knocked backward – landing flat on my back and somewhat dazed. As I came to my senses, I was trying to figure out what hit me when I felt a presence. Looking over my shoulder I saw a jetway full of passengers. They were all looking at me with amazed looks on their faces. I am sure that some were beginning to rethink their travel plans not wanting to be flown by someone who can’t even make it through the front door. Embarrassing to say the least.