This paper investigates the mutual coupling effects between axial-mode helical antenna arrays. Single-negative (SNG) magnetic metamaterials are developed and used in this work in order to reduce mutual coupling between axial-mode helical antennas used in multiple-input multiple-output systems. The inclusions considered here are composed of broadside coupled split-ring resonators. The magnetic permeability of those SNG inclusions have complex effective magnetic permeability response over a frequency band with high negative real part above the resonance and positive real part below the resonance. The SNG resonators are inserted between closely-spaced axial-mode helical antenna elements. Effective responses of the constitutive parameters of the developed magnetic inclusions are incorporated within the numerical models. It is shown that mutual coupling between the antenna elements can be reduced significantly by incorporating such magnetic inclusions. The SNG magnetic resonators work as antenna decoupler, and thus can be applied in a variety of antenna applications.