Forward and backward waves: three definitions and their interrelation and applicability
Institute of Radio Engineering and Electronics, Russian Academy of Sciences, ul. Mokhovaya 11, Moscow, 125009, Russian Federation
The three known property-specific definitions for forward and backward waves in various media and waveguides are reviewed. Criteria by which to distinguish between these waves according to their definitions are introduced. It is shown that using these criteria
simultaneously can yield different or even opposite conclusions in some cases. When and where these criteria and the above
definitions apply is discussed using electromagnetic waves and waveguides as an example.