Metamaterial transmission lines based on broad-side coupled spiral resonators

Microstrip lines periodically loaded with broad-side coupled spiral resonators (BC-SRs) are designed, fabricated and characterised for the first time. These structures inhibit signal propagation in the vicinity of the resonance frequency of the spirals, f_o. This stopband behaviour is due to magnetic coupling between the line and the BC-SRs at resonance. As long as the electrical size of the BC-SRs is small, the structures can also be considered as metamaterial transmission lines with negative effective permeability, mu_eff . This negative value of mu_eff takes place in a narrow band above f_o, as occurs in split ring resonator (SRR) loaded lines, or in transmission lines coupled to spiral resonators etched in a single metal level (edge-coupled spiral resonators (EC-SRs). However, because each loop of the BC-SR is etched in a different metal level (face-to-face loops), the equivalent capacitance of the resonant tank can be substantially enhanced. Thus, by using narrow microwave substrates, the BC-SRs loaded lines can be made much smaller than SRR or EC-SR loaded lines