radio reception reduced when using a roof box?

[Hoppy1970]

Hi,
I have a 2008 Passat 140 diesel estate. No complaints so far except last year when I loaded up the roof box the radio reception wasn't the greatest. It's something I hadn't noticed before. The previous year I can't remember any issues with it. I thought the aerial might be trapped under it but seeing as I can't see it I'm presuming it's someplace else within the car? I should add I also had a maxxraxx bike rack last year too compared to the previous year, but when I'm only using the bike rack it's fine.
Any ideas?

[PFJones]

Hi, I've never heard of this problem before. Have you tried using a car radio signal booster?