**3. The Transmission-Line Matrix (TLM) method for guiding structures**

The transmission-line matrix (TLM) method was developed by P. B. Johns and his co-workers in the 1970s [17]. Its theoretical foundations are based on Huygens's model of wave propagation. The TLM method is very attractive and flexible to analyze electromagnetic field problems and has received increased recognition for full-wave analysis of arbitrary-shaped guiding structures [15]. Through the years, several nodes have been developed in TLM for structured and unstructured meshes in two and three dimensions. We will focus on the symmetrical condensed node (SCN), which is the most widely used for 3D structures.
