  Superconducting resonators and transmission lines are fundamental building
blocks of integrated circuits for millimeter-submillimeter astronomy. Accurate
simulation of radiation loss from the circuit is crucial for the design of
these circuits because radiation loss increases with frequency, and can thereby
deteriorate the system performance. Here we show a stratification for a
2.5-dimensional method-of-moment simulator Sonnet EM that enables accurate
simulations of the radiative resonant behavior of submillimeter-wave coplanar
resonators and straight coplanar waveguides (CPWs). The Sonnet simulation
agrees well with the measurement of the transmission through a coplanar
resonant filter at 374.6 GHz. Our Sonnet stratification utilizes artificial
lossy layers below the lossless substrate to absorb the radiation, and we use
co-calibrated internal ports for de-embedding. With this type of
stratification, Sonnet can be used to model superconducting
millimeter-submillimeter wave circuits even when radiation loss is a potential
concern.
