Couldn't say for sure, but I have a suspicion -> static discharge in the antenna. Worked on Canopy at Motorola. At one time they used an antenna coupling circuit that was highly static sensitive (at the antenna). They reworked the antenna coupling to make it less succeptible. If these cards are not explicitly designed for outdoor use then I'd suspect this. When the antenna coupler pops you get a high insertion loss to the antenna (power going in or out suffers). This seems to fit your symptoms. I'm not a ham, and don't have any suggestions on how to reduce the potential for static discharge in the antenna to pop an antenna coupler on the board. But I'd check if the antenna mount is grounded to the same ground as the radio board.
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