The Constant Permittivity, Constant Loss Tangent Model

It is common for people accustomed to working in the frequency domain (particularly with narrow-band microwave applications) to use a lossy dielectric model with a constant permittivity and a constant loss tangent. Such a model is easy to specify—only two numbers are required. Unfortunately, it is also non-causal.

The special case of a lossless material (zero loss tangent) with constant permittivity is causal, however.