C402 pilot reported the crew door blew open in flight; damaging an antenna and degrading air/ground communication. The pilot diverted to make a precautionary landing.

Narrative

I was scheduled to fly round trips from ZZZ1 to ZZZ2 all day. I flew two round trips using IFR flight plans. After completing my second I was concerned about a convective SIGMET which said there was an area of embedded thunderstorms with high tops on my route. I was also concerned about a lack of good alternates. Finally I was concerned that although the visibility was high for the approach in use; the ceiling was at mins for that approach; in ZZZ1 my departure airport. I cancelled the third round trip after speaking to our flight follower/dispatchers at Operations Control. I noticed many more experienced pilots were cancelling their trips. After waiting; the weather in ZZZ1 improved; but not in ZZZ. A senior colleague had just flown from ZZZ to ZZZ2; with the intention of flying to ZZZ; but cancelled due to the same weather concerns I saw once he got to ZZZ1. He then offered to fly the passengers to ZZZ; so that they could catch a bus. A flight was created and all of them took him up on it. Having seen this and still worried about weather; I also cancelled my flight; my last round trip to ZZZ. I also then offered to fly the passengers to ZZZ. A cooler was loaded in the aft. I flew the passengers to ZZZ without event. When I arrived in ZZZ; the ramp agent opened the aft cargo door with the appropriate tool to retrieve the cooler. I felt time pressure as I knew the weather was forecast to go down quite low and be quite windy; with inoperative lights on the lowest approach in ZZZ1. When I went to start the aircraft; the door unlocked light was illuminated. I terminated the start and asked if he could reclose it. This did not turn the light off. I opened and closed the crew hatch; and this did not turn the light off either. The ramp crew member suggested I take the aircraft over to Maintenance; but I said I was happy to fly it; because I was so worried about the weather and created time pressure on myself that didn't need to exist. I was wrong. He was correct. I did see the light go out in my run up briefly; but I chose to fly after it came back on again; this was clearly my; and only my; fault. I quickly checked the weather on ZZZ1 and departed. While I was in cruise I tried to lightly jiggle the crew door handle to see if it would turn out the light permanently; and the crew door blew open. I was surprised; and I wasn't sure if it was still attached. I turned my attention to whether the aircraft was still controllable. It was but with a slight yaw. I tried to advise the ZZZ Controller; however I was not readable. I realize now that was because the door smashed the antenna off. I tried to advise Operations Control on COMM 2 and got through but it was garbled. I squawked 7600. I then realized the door might have hit a flight control surface and squawked 7700. I proceeded to ZZZ1 because I had already been worried about the weather in ZZZ; and felt I was closer and wasn't sure if the situation would deteriorate. I already had obtained the ATIS in ZZZ1. I made straight in XX and landed. I managed to get a taxi clearance after several tries on comm 2. I taxied in and called the Tower on the telephone. This was completely my fault for 'trying to make it work' instead of downing the aircraft. I also rushed for no reason. Company owns many aircraft; and there is a ferry service I could have taken. I apologize for my actions and the trouble. Sorry. In the future I will not try so hard to complete the flights that I waive discrepancies and good advice. I rushed and ignored good advice from experienced team members. I really wanted to 'make it work' and not be 'one of those whiny pilots who writes up aircraft'. If I have the opportunity to fly again; I will write up all discrepancies the first time and take advice from more experienced team members. I will ignore that I don't want to be a hindrance by writing up aircraft; because; as I have demonstrated; the opposite is much worse. I will also fly less; so that I become less complacent. Sorry for all the trouble.