A common microwave oven is made basically by a a magnetron source (2.45 GHz), a stirrer, a cooking cavity, and a control system. Microwaves are produced from the magnetron source and propagate with waveguides inside the cooking cavity. These microwaves are absorbed from the water which exists inside the food and heat it. The metalic parts of the cavity act as reflectors of the microwaves. The stirrer is used to homogenize spatially the power distribution of microwaves inside the furnace. In the next picture is presented the structure of a microwave furnace.

The window of the oven is used for reflection of microwaves, for heat isolation and also for the direct observation of the food. It is usually double glazed with a metalic net inside it. The metalic net has special coating against the oxidation and appropriate geometry to create band gaps in different angles and increase the number of reflections. The geometry of such a net is shown in the next picture.

By using FDTD simulation and typical values of used coated nets in microwave ovens the reflection and transmission coefficient (S parameters) are calculated for vertical incidence of microwaves. The result is presented in the next picture. It proves that microwaves can propagate outside the oven window if the net is coated.

The coating which is used to protect the net from oxidation is actually against its reflection ability. It is a global problem in the design and construction of microwave ovens.