What was found
CLIMBING THROUGH FL240 PRESS WARNING LIGHT ILLUMINATED CABIN DIFF PRESSURE HIGHEST READING WAS 3PSI. AIRCRAFT RETURNED TO KYIP FOR UNSCHEDULED LANDING. UPON INSPECTION OF AIRCRAFT IT APPEARS THAT THE AIRCRAFT SUSTAINED A LIGHTING STRIKE AND THE FWD RADAR ALT ANTENNA WAS MISSING THUS RESULTING IN THE CABIN PRESSURIZATION BEING UNABLE TO MAINTAIN PRESSURE. CORRECTIVE ACTION: REPLACED DAMAGED DEACTIVATED #1 RAT ALT ANTENNA WITH BLANKING PLATE, AND PERFORMED LIGHTING STRIKE INSPECTION, NO DEFECTS NOTED.