Under the assumption that the wire jacket/spacer material has a negligible effect on the electromagnetic fields, and that the line is suspended in air so that ϵr≈1, the phase veloci...Under the assumption that the wire jacket/spacer material has a negligible effect on the electromagnetic fields, and that the line is suspended in air so that ϵr≈1, the phase velocity vp for a parallel wire line is approximately that of any electromagnetic wave in free space; i.e., c.