A coil assembly for reducing variations in characteristic impedance includes a winding section having a first surface and a second surface on the opposite side of the winding section from the first surface, a plurality of first protrusions provided on the first surface, and a plurality of second protrusions provided on the second surface. These protrusions are identical in shape to each other and are arrayed linearly on their respective surfaces so that the first protrusions are offset from the second protrusions. Two conducting wires are wound between neighboring protrusions such that one wire contacts one of the neighboring protrusions, while the other wire contacts the other neighboring protrusion.