1. Introduction

The attenuation factors associated with the penetration of an electromagnetic wave into and through a shielding barrier (as predicted by the accepted theory of shielding) are presented. The accepted theory consists of the use of (1) Maxwell’s equations and (2) the analogy between transmission lines (as predicted by wave theory) and the penetration of a wave through a shielding barrier. The use of the analogy is the method of choice by the electronic industry. This method is fully analyzed, where four (4) factual statements are presented that illustrate that the use of the analogy yields highly inaccurate results. A theory of shielding is presented (which is proven to be accurate through test). The theory of the penetration of a wave through a seam or EMI gasketed seam is presented.