In challenging situations, for example when clipping the beam through apertures, true free-space wave
propagation can be used, which now also belongs to many ray tracing software packages.
Full absorption of the incident field occurs when the surface impedance of the cell is matched to the free-space wave
The terms [Z.sup.1.sub.1] and [Z.sup.2.sub.1], (9), are the free-space wave
impedances and are calculated with the measured reflection coefficients from measurements 1 and 2.
In addition, LWA support a fast wave on the guiding structure, where the phase constant [beta] is less than the free-space wave
An antenna is usually defined as the structure associated with the region of transition between a guided electromagnetic wave and a free-space wave
. On transmission, the antenna receives energy from a transmission line and radiates it into space; on reception, the antenna gathers energy from an incident wave and sends it down the transmission line.