A C402 ON LEVELOFF AT 5000 FT ASSIGNED ALT THE COCKPIT DOOR SUDDENLY OPENED AND STRUCK TOP OF FUSELAGE. DOOR TOOK OFF TOP ANTENNA RESULTING IN LOST COM.

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ON IFR INTL FLT PLAN TO ZZZZ1 FROM ZZZ2 HAD JUST LEVELED OFF AT ASSIGNED ALT OF 5000 FT AND ON COURSE DIRECT. CREW DOOR OPENED AND HIT TOP OF FUSELAGE (AND REMAINED OPEN). CALLED CENTER TO ASK TO RETURN TO ZZZ2. NO REPLY. CALLED ON EACH RADIO SEVERAL TIMES AND SWITCHED TO PREVIOUS FREQ TO CALL A FEW TIMES. REALIZED DOOR MUST HAVE DAMAGED ANTENNA (IT TOOK IT OFF; I DISCOVERED AFTER LNDG). SQUAWKED 7600. I WAS IN VFR AND NEAREST SUITABLE ARPT WAS ZZZ2/HOME BASE. I TURNED AND DSNDED TO 2000 FT; HEADED NW TO BE SURE NOT TO ENTER CLASS C AIRSPACE AROUND ZZZ. PROCEEDED S OFF SHORELINE AND HEADED TO ZZZ2. GOT GREEN LIGHT GUN SIGNALS AND LANDED AT ZZZ2. ZZZ2 TOLD ME APCH WANTED ME TO CALL MR X. SPOKE TO MR X WHO SAID HE COULD NOT FIND THE PERSON I WAS TO SPEAK TO SO ASKED ME TO EXPLAIN WHAT I DID. WHEN I TOLD HIM; HE SAID 'SOUNDS LIKE YOU DID THE RIGHT THING' AND SAID 'NO MORE WAS REQUIRED OF ME.' HE EXPLAINED THEY JUST NEEDED AN EXPLANATION. I'M FILING THIS JUST IN CASE HE DID NOT PASS THE MESSAGE ON TO THE ORIGINAL PERSON AND APCH THINKS I DID NOT CALL TO EXPLAIN MY ACTIONS. I BELIEVE I TOOK THE CORRECT AND APPROPRIATE ACTIONS AND DID NOT VIOLATE ANY REGS. I THANKED ZZZ2 ON GND (1 RADIO WAS SCRATCHY BUT READABLE ON GND) FOR THE LIGHT GUN SIGNALS. THEY SAID 'NO PROB' SO I PRESUME THEY WERE SATISFIED WITH MY ACTIONS.