A conducting circular loop is rotated about its diameter at a constant angular speed of 100rad / s in a magnetic field of 0.5 T perpendicular to the axis of rotation. When the loop is rotated by $30^{\circ}$ from the horizontal position, the induced EMF is 15.4 mV . The radius of the loop is $\_\_\_\_$ mm.
$\left(\right.$ Take $\left.\pi=\frac{22}{7}\right)$