Electromagnetic signal propagation is influenced by the material or conductor used, with specific characteristics leading to attenuation at certain frequencies. Waveguides act as high pass filters, allowing propagation only above a cutoff frequency determined by their geometry and dielectric medium. Conductors like copper experience losses due to finite conductivity and the skin effect, which forces AC currents to the surface and limits performance at higher frequencies. At terahertz (THz) frequencies, copper's skin depth becomes very thin, resulting in significant attenuation and making it unsuitable for THz signal transmission. For deeper understanding, references such as "Field and Wave Electromagnetics" by David Cheng provide valuable insights.