Under the conditions of normal incidence and ω ω < 1 the electric field across a metallic grid is continuous, but the magnetic field is not, so a transmission line with an admittance 2 Y ( ω ω ) between the two lines can be used to model the transmission and reflection from a metallic filter. If, for example, three identical grids were stacked, then there would be three admittance shunts in parallel across the transmission line. Using simple transmission line theory, the reflection coefficient Γ Γ ( ω ω ) and transmission coefficient τ τ ( ω ω ) are calculated to be