In the antenna community, there has always been a fundamental question: how can we control antenna behavior, e.g., resonant frequency, bandwidth, and mutual coupling in an array, solely by changing materials surrounding the radiating current? To answer this, we analyze the role of a magneto-dielectric substrate/superstrate in controlling the stored energy, impedance, and radiation properties, showing that the bandwidth of the antennas can be substantially increased by reducing the substrate permittivity or permeability, respectively, in magnetic- and electric-current-driven antennas. These theoretical findings are validated through numerical simulations. The same conceptual framework is applied to investigate the problem of mutual coupling between closely spaced elements. The results show that loading magnetic-current antennas with a magnetic superstrate effectively suppresses surface waves and thereby reduces interelement coupling, whereas for electric-current antennas, a dielectric superstrate provides the most efficient coupling reduction.