A dielectric structure of an electromagnetic device includes: a first dielectric portion, FDP, having a proximal end, a distal end, and a three-dimensional, 3D, shape having a direction of protuberance from the proximal end to the distal end oriented parallel with a z-axis of an orthogonal x, y, z coordinate system; and a second dielectric portion, SDP, having a proximal end and a distal end, the proximal end of the SDP being disposed proximate the distal end of the FDP, the FDP and the SDP having a dielectric material other than air; wherein the SDP has a 3D shape having a first x-y plane cross-section area proximate the proximal end of the SDP, and a second x-y plane cross-section area between the proximal end and the distal end of the SDP, the second x-y plane cross section area being greater than the first x-y plane cross-section area.