As part of an ongoing programme of research conducted at Liverpool University into the characterisation of building materials using nonintrusive test methods, a wide band TEM horn facility has been developed for the reconstruction of electromagnetic properties by direct inversion of the reflected signal from the material. This employs an algorithm based on a novel microwave network theory approach in conjunction with a numerical optimisation technique. This paper summarises the measurements undertaken on various combinations of materials for the validation of this method, along with a detailed description of the microwave network theory adopted within the optimisation model.