On the slow wave behaviour of the shielded mushroom structure

The dispersion theory of the shielded mushroom structure is examined using multiconductor transmission line theory (MTL), which in the low frequency limit reveals that a slow, forward wave mode is supported. The theory shows that the degree of the slow-wave effect is controlled by the distance between the shielding conductor and the patch (mushroom) layer, with a significant slowing of the mode possible as the aforementioned distance is decreased. By examining the low frequency modal eigenvector, in conjunction with the dispersion equation, a physical explanation of the mechanism causing the slowing is arrived at. The theory is compared with full-wave finite element simulations, with excellent correspondence between the two observed.