Graphical Prediction of Trapped Mode Resonances in Sub-mm and THz   Waveguide Networks
  An analytical method for the visualization and prediction of trapped-mode resonances based on the dimensions of a dispersive microwave network is described. The method as explained is intuitive, easy to implement, and has proven itself to be a useful tool in the avoidance of problems associated with trapped modes prior to fabrication, as well as to correct those problems in designs where this design detail was overlooked. 
Matthew A. Morgan and Shing-Kuo Pan
astro-ph.IM