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2 years ago in Electrical Engineering By Anuj Patel
I’m tuning a split-ring resonator’s capacitive gap using an interdigital capacitor (IDC). How does the number of interdigital fingers primarily affect the total capacitance and the Q-factor of the metamaterial unit cell?
In my left-handed metamaterial design at 5 GHz, I've replaced the standard SRR gap with a 5-finger IDC to increase capacitance and lower the resonant frequency. I can add more fingers within the same footprint, but I'm unsure of the trade-off. Does each additional finger linearly increase capacitance, or does mutual coupling between fingers cause diminishing returns? More importantly, does increasing the finger count (and thus metal length) significantly increase ohmic loss and lower the Q of the resonance, potentially weakening the negative permeability effect?
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By Email1@gmail.com Answered 1 year ago
Adding fingers increases capacitance, but not linearly due to mutual coupling and fringing field saturation. The first few fingers give the biggest jump; beyond 5-7 fingers in a fixed width, you get diminishing returns as the electric fields from inner fingers become strongly coupled. More critically, each finger adds series resistance (narrow current paths) and dielectric loss (more fringing into the substrate). This directly lowers the Q of your resonant circuit. For a strong, low-loss magnetic resonance, I've found an optimal sweet spot at 3-5 fingers. If you need more C, it's often better to increase finger length or reduce the gap rather than pack in more fingers. Always simulate the full unit cell's S-parameters and extract the complex effective permeability to see the true impact on your negative index bandwidth and loss.
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