The pilot reported that he struck a post on take-off and broke off the main landing gear wheel. The pilot proceeded to Hillsboro, Oregon where an emergency landing was made on a paved runway with the landing gear still extended. The stub caught in the runway causing the aircraft to veer onto the grass on the wing. The pilot reported that he had trimmed the aircraft for a short field take off and checked the windsock before starting his take off roll. Lift off was about 1200 feet down the 2400 foot grass runway. During initial climb the aircraft seemed to settle a bit but indicated airspeed was moving from 70 to 75 mph. Beyond the departure end of the runway was a 5 foot high dike and beyond the dike a 4 foot post. During climb out a 'loud bang' was heard. A visual check disclosed the right main landing gear wheel was missing. The landing gear was left in the down position and the aircraft was flown to Hillsboro, Oregon where a landing was made. The right wheel was found at the departure airport.