##### Definition

The simplest way to define a coax in Analyst is with two cylinders of different diameters on the same axis, where a smaller metal cylinder is used for the core and a larger cylinder is the dielectric insulator. See Cylinders for details on drawing cylinders. In this case, conducting impedance boundary conditions are applied to the curved faces of the larger cylinder to represent the shield.

An alternative, but more complex and computationally expensive approach uses three solids: the core, the shield, and the dielectric insulator. The core is a cylinder, and the insulator and shield are pipes, where the inner radius of the dielectric pipe matches the radius of the core cylinder, and the outer radius of the dielectric pipe matches the radius of the shield's inner radius. See Pipes for details on drawing pipes.

First, this is more difficult to set up, involving three structures with a total of five radii. Second, matching radii is critical in this scenario because it can lead to unforeseen issues in the meshing stage, as small numerical differences in the radius definitions can cause unexpected gaps to open up between the dielectric and conductors. If this occurs, you should thicken the dielectric pipe so that it overlaps both conductors, by making the inner radius of the dielectric smaller than the radius of the cylinder and by making the outer radius of the dielectric larger than the inner radius of the metal pipe. The material overlap rules will correctly resolve any material ambiguities. The resulting geometry looks like this:

The only possible advantage of this setup is that it allows for a finite-thickness outer conductor, and a few applications may require that level of representation in the geometry.