Answer
$\Phi = A~B~cos~\theta$, where $\theta$ is the angle between the area of the antenna and the direction of the magnetic field. If the angle $\theta$ is non-zero, the induced emf in the antenna is reduced from its maximum possible value by a factor of $cos~\theta$.
Work Step by Step
According to Faraday's law, the induced emf is equal to $-N~\frac{\Delta \Phi}{\Delta t}$, where $\Phi$ is the magnetic flux.
Note that $\Phi = A~B~cos~\theta$, where $\theta$ is the angle between the area of the antenna and the direction of the magnetic field. This value will be a maximum when $\theta = 0$. If the angle $\theta$ is non-zero, the induced emf in the antenna is reduced from its maximum possible value by a factor of $cos~\theta$.
