It sounds to me as though the carpet is acting as a sort of screen to block whatever was causing the poor reception. Possibly electrics wiring or even plumbing pipework. Alternatively it could be acting as a giant reflector and rather than the radio waves passing through or being attenuated by the original flooring, your new carpet is containing them allowing the radio antenna to receive a stronger signal. I wonder what your new carpet or underlay is made from. Interesting.