  For decades, a wide variety of observations spanning the radio through
optical and on to the x-ray have attempted to uncover signs of type Ia
supernovae (SNe Ia) interacting with a circumstellar medium (CSM). The goal of
these studies is to constrain the nature of the hypothesized SN Ia mass-donor
companion. A continuous CSM is typically assumed when interpreting observations
of interaction. However, while such models have been successfully applied to
core-collapse SNe, the assumption of continuity may not be accurate for SNe Ia,
as shells of CSM could be formed by pre-supernova eruptions (novae). In this
work, we model the interaction of SNe with a spherical, low density,
finite-extent CSM and create a suite of synthetic radio synchrotron light
curves. We find that CSM shells produce sharply peaked light curves, and
identify a fiducial set of models that all obey a common evolution and can be
used to generate radio light curves for interaction with an arbitrary shell.
The relations obeyed by the fiducial models can be used to deduce CSM
properties from radio observations; we demonstrate this by applying them to the
non-detections of SN 2011fe and SN 2014J. Finally, we explore a multiple shell
CSM configuration and describe its more complicated dynamics and resultant
radio light curves.
