Sirius cuts out under every overpass

[TowX]

My old Sirius antenna went bad. Got a new one and this time I installed it in the rear spoiler. Works fine except under every overpass. Now it cuts out a couple seconds after going under, and does so with a noticable electronic click each time. The old antenna mounted on the roof (when it was still working) never did this. Is this a trait of being mounted under a piece of fiberglass or do I need to "reboot" the system.