Respiratory motion is a fundamental challenge for thorax MR imaging, particular at ultra-high fields. Recent in-vivo studies have shown that the coil's time-dependent scattering matrix (S-Matrix) can be used for respiration gating to minimize those artefacts. Here, we present electromagnetic simulations of the respiration-induced changes of the transmit RF coil’s S-matrix. Analysing the impact of breathing patterns (conventional, chest, abdominal) illustrates the combined effects of chest/diaphragm motion. The type of respiratory motion and RF coil geometry, position and element type are shown to impact the S-Matrix with implications on coil performance, respiratory triggering, power monitoring and RF pulse design.