I'm just a Ham radio operator here, not a pilot, but I know radio. VHF radio is very particular about line-of-sight, and anything between the antennas can attenuate signals significantly, just as can anything metallic within the close proximity of the antenna. There is also a lot of attenuation unless the antennas are parallel in plane with each other, as much as 20dB, which could do what you describe when you're banked heavily. You might want to speak with others who use your exact type of A/C and see how their antennas are configured as well as if they experience similar issues. I'm guessing that it's mostly caused by your antenna placement and even a small movement will cure things or make them markedly worse- in which case you move it the other direction.