DURING A NIGHT TIME VOR/DME APPROACH FOR LANDING IN FOGGY CONDITIONS, THE AIRPLANE COLLIDED WITH A FIFTEEN FOOT HIGH MOVEABLE IRRIGATION SYSTEM LOCATED IN AN OPEN FIELD APPROXIMATELY TWO AND A HALF MILES EAST OF THE AIRPORT. RADAR DATA INDICATES THAT THE FLIGHT WAS ESTABLISHED ON THE APPROACH AND AT A PROPER ALTITUDE AT THE FINAL APPROACH FIX. TWO RADAR TARGETS AFTER THE FIX INDICATE THAT THE FLIGHT DESCENDED BELOW THE PUBLISHED MINIMUM ALTITUDE AND COLLIDED WITH THE IRRIGATION SYSTEM. POST CRASH INSPECTION DID NOT REVEAL ANY MECHANICAL FAILURES OR MALFUNCTIONS.