The array factor of an N element linear uniform array is \(\newline\) (\(\Psi = \beta d \cos\theta + \alpha\), \(\beta = \frac{2\pi}{\lambda}\), d = spacing between elements and \(\alpha\) = inter element phase shift)
A closed-loop system has the characteristic equation given by: $ s^3 + k s^2 + (k+2) s + 3 = 0 $.
For the system to be stable, the value of $ k $ is: