The decreased output is probably due to the fact that the shield is a shorted loop, magnetically coupled to the line, so it acts as a secondary winding of a shorted transformer, weakening the signal in the primary, i.e. the line. To know if there is a real effect linked to the fact that the line is ferromagnetic, it is to replace it by a non-magnetic Cu or Al line, as suggested by Poynt99, and see if the difference is significant.
Classical electromagnetism tells us that there will be very little difference, because the ferromagnetic material here is not a core in the center of the winding but along the conductor, which it is. If the difference is significant, then there is indeed more to investigate.
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