Apparatus, including a flexible insulating substrate, having a first side and a second side, rolled about an axis parallel to the substrate. The apparatus also includes a first conducting spiral that is right-handed relative to a normal to the substrate, and a second conducting spiral that is left-handed relative to the normal, formed on the first side of the substrate. The first conducting spiral has a first initial termination and a first final termination, the second conducting spiral has a second initial termination and a second final termination, the spirals have a displacement therebetween, with a preset magnitude so that when the substrate is rolled about the axis the first initial termination aligns with the second initial termination. The apparatus also has a via penetrating the substrate from the first side to the second side so as to interconnect the first initial termination and the second initial termination.
