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.