Two concentric coplanar circular loops of radii $r_1$ and $r_2$ respectively carry currents $i_1$ and $i_2$ in opposite directions (one clockwise and other anticlockwise). The magnetic induction at the centre of the loops is half that due to $i_1$ alone at the centre. If $r_2 = 2r_1$, the value of $\frac{i_2}{i_1}$ is