  Thin-gap Resistive Plate Chambers (RPCs) with a 1 mm gap size are introduced
in the Phase-II ATLAS upgrade. Smaller avalanche charge due to the reduced gap
size raises concerns for signal integrity. This work focuses on the RPC signal
propagation process in lossless conditions, and an analytical study is
implemented for the ATLAS RPC. Detector modeling is presented, and the
simulation of the RPC signal is discussed in detail. Simulated characteristic
impedance and crosstalk have been compared with the measured value to validate
this model. This method is applied to different RPC design geometries,
including the newly proposed $\eta-\eta$ readout scheme.
