Is there any way to solve this?

[yungman]

This is part of the antenna field pattern calculation, I want to find out is there any way to find the \;\theta\; of this:

\frac 1 N \left[\frac {\sin(N\theta)}{sin\theta}\right]\;\hbox { = 0.7071 where N is an integer bigger than 1}

[appelberry]

you can use the identity
\sin N\theta = \sum_{k=0}^N \binom{N}{k} \cos^k \theta\,\sin^{N-k} \theta\,\sin\left(\frac{1}{2}(N-k)\pi\right)
to obtain a polynomial in \sin \theta which can then be solved for \theta . However, you will probably need to solve it numerically for N>3.