In the preceding example, it was shown that if one wants to design a narrowband/single-frequency absorber using a mushroom-type unit cell, then the lossless AMC simply needs to be tuned so that it resonates at the desired frequency and then to introduce loss in the system using lumped resistors. Consequently, it becomes obvious that if absorption at multiple frequencies is desired, then the mushroom unit cell needs to be appropriately modified so that it exhibits a multi-resonant response or multiple phase zero crossings. In order to introduce additional resonances in a mushroom AMC, usually some structural modification is required, such as an FSS shaped differently than a square patch.
In the present study, an alternative approach is proposed to introduce the additional resonances. In particular, given the mushroom configuration defined in the previous subsection, we introduce a larger unit cell that consists of four metallic patches, as shown in Fig. 4.21a. The advantage that this offers is that more degrees of freedom can be added and thus more flexibility to the system by applying the transverse non-uniform lumped element loading scheme shown in Fig. 4.21b. Essentially, this unit cell configuration allows us to accommodate two different AMC resonances: one frequency is controlled by the green capacitors, shown in Fig. 4.21b, while the second is controlled by the set of dark blue capacitors. It needs to be emphasized here that the proposed approach is solely based on lumped capacitor loadings, and therefore the port reduction design methodology is directly applicable in this case.
Figure 4.21 (a) Multi-frequency absorber unit cell configuration. (b)
Transversely non-uniform lumped element loading scheme for the realization of the multi-frequency absorber.
Figure 4.22a shows the reflection coefficient phase of the multifrequency AMC when Ca = 1.49 pF and Cb = 0.71 pF. As expected, the structure exhibits two distinct resonances around 2.9 GHz and 4.5 GHz, respectively. Subsequently, in order to create absorption at these frequencies, the structure was additionally loaded with lumped resistors equal to Ra = 350 П and Rb = 1.8 kfi. In Fig. 4.22b, it can be seen that at the aforementioned resonant frequencies of the resistively loaded AMC, the desired dual frequency absorption has been achieved.