In this paper, the theoretical foundations of the equivalence between waveguide propagation below cutoff and artificial plasmas are carefully analyzed through the derivation of the propagation constants of normal modes in waveguides filled with anisotropic plasmas. The equivalence between waveguide and dielectric plasma proposed by Marqués et al., which is valid for evanescent TE modes, has a dual counterpart for magnetic plasmas and evanescent TM modes. This new equivalence states that a negative magnetic permeability medium can be simulated by means of TM modes below their cutoff frequencies. The need of an anisotropic filling of the waveguide for the equivalence between plasmas and evanescent modes is also highlighted. To exemplify the applicability of this new equivalence, a structure that implements a double-negative medium has been proposed. Full-wave simulations of the proposed structure and measurements from an experimental setup are presented, both of which corroborate the new equivalence's validity.Index Terms-Backward waves, electric plasmas, evanescent modes, magnetic plasmas, metamaterials, negative permeability, negative permittivity, periodic structures.
The couplers' measured phase performances are depicted in Figure 6. The meander rat-race coupler has the largest bandwidth, followed by that of the triangular meander, and the multimeander has the smallest bandwidth. This emphasizes the fact that the bandwidth and spurious response are the cost of miniaturization. Table 2 shows a comparison of the circuit areas occupied by conventional coupler designs and those proposed in this paper. The sizes of the proposed rat-race hybrid couplers are between 17% and 30% of the size of a conventional design. This level of miniaturization is expected to be achievable for other center frequencies and substrates.
CONCLUSIONIn this paper, a new family of compact rat-race hybrid couplers has been proposed. The size of the proposed couplers is between 15% and 30% of the conventional designs at the same center frequency.
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