I suspect that the polar diagram is showing a marked reduction in the same direction as the solar panel, or is it an overall reduction?
The signals from the aircraft are "line of sight" so the clearer the antenna's "view", the better the range. Even trees can reduce the range for external aerials so the fact that yours is in your loft and the roof now has the solar panel fitted will make your aerial quite inefficient in comparison to the same aerial mounted outside.
What sort of range are you getting at the moment?