In this paper, we report on extending a theoretical framework based on Gaussian Beam Mode Analysis for modelling standing waves in receiver systems coupled to submillimetre wave telescopes. This analytical technique includes a full electromagnetic description of corrugated detector horns, used as a standard feed horn in the THz frequency range. In previous papers we reported on the underlining theory and described some important examples including reflections between a feed horn and telescope secondary mirror and also reflections between a horn and a piano-convex lens ,. As the theory uses a full multi-moded scattering matrix description within the horn, which can then be transformed to equivalent free space modes, multiple reflections between the detector, located at the back of the horn, and any arbitrary surface in the optical path can be accurately analysed. We present an experimental validation of the model, comparing predicted standing wave patterns occurring between two corrugated horns to laboratory measurements, working in a frequency range around 0.1THz.