Double‐layer substrate integrated waveguide structure with variable fractional bandwidth

The propagation characteristics of double‐layer SIW structures have been investigated in this paper. On the basis of the theory of resonance cavity and SIW, the simple theoretical formulas for the lower and upper frequencies in the passband have been found. The theoretical analyses have indicated that these two frequencies have much dependence on the width of SIW and the length of the coupling slot, the fractional bandwidth of the double‐layer SIW structures can be controllable. Agreement between the simulation data and the calculated results from the formulas can be observed.