OK, being a communications tech in the Navy, I can tell you that XM is line of sight. In that you have to be in direct line of the satellite to receive the signal. Hence there is no satellite over Africa where I am now so even if I had a receiver I would not get any reception. But since it transmits in the EHF frequency spectrum I can tell you that it does penetrate porous substances like fiberglass or plastic. In fact on submarines our antennas are covered with a fiberglass dome to protect the actual antenna from sea water. So putting the antenna under the fairing should not cause loss of signal unless the signal was very weak to begin with. As with any antenna the fewer things between it and the transmitter is always better, so there is a slight increase in signal strength with the antenna mounted on top of the 550. I hope this clears things up.