Consider an antenna with a (sin x / x) pattern. Let (x=(pi r sin theta) / lambda),

Question:

Consider an antenna with a $\sin x / x$ pattern. Let $x=(\pi r \sin \theta) / \lambda$, where $r$ is the antenna radius, $\lambda$ is the wavelength, and $\theta$ is the off-boresight angle. Derive Eq. (3.92). Assume small $x$, and expand $\sin x / x$ as an infinite series.

Equation (3.92)