The amplitude of the gravitational waves emitted by a binary system of arbitrary eccentricity and orientation is developed in Fourier-Bessel series. Periastron advance is also considered. We discuss the influence of the orbital parameters in the spectrum of the wave amplitude. The dominant harmonic in the spectra gives a good estimation of the eccentricity. Periastron advance modulates the shape of the gravitational radiation field, splitting the basical harmonics. The analytical formulae presented here are useful for studying the contribution of binary systems to the gravitational radiation background.