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Fine structures of radio bursts play an important role in diagnostics of thesolar flare plasma. Among them the zebras, which are prevalently assumed to begenerated by the double plasma resonance instability, belong to the mostimportant. In this paper we compute the growth rate of this instability for twotypes of the electron distribution: a) for the power-law distribution and b)for the kappa distribution, in the both cases with the loss-cone typeanisotropy. We found that the growth rate of the upper-hybrid waves for thepower-law momentum distribution strongly depends on the pitch-angle boundary.The maximum growth rate was found for the pitch-angle $\theta_\mathrm{c}\approx$ 50$^\circ$. For small angles the growth rate profile is very flat andfor high pitch-angles the wave absorption occurs. Furthermore, analyzing thegrowth rate of the upper hybrid waves for the kappa momentum distribution wefound that a decrease of the characteristic momentum $p_\kappa$ shifts themaximum of the growth rate to lower values of the ratio of the electron-plasmaand electron-cyclotron frequencies, and the frequency widths of the growth ratepeaks are very broad. But, if we consider the kappa distribution which isisotropic up to some large momentum $p_m$ and anisotropic with loss-cone abovethis momentum then distinct peaks of the growth rate appear and thus distinctzebra stripes can be generated. It means that the restriction for small momentafor the anisotropic part of distributions is of principal importance for thezebra stripes generation. Finally, for the 1 August 2010 zebra stripes, thegrowth rates in dependence on radio frequency were computed. It was shown thatin this case the growth rate peaks are more distinct than in usually presenteddependencies of growth rates on the ratio of the plasma and cyclotronfrequencies.

Growth Rates of the Upper-hybrid Waves for Power-law and Kappa Distributions with a Loss-cone Anisotropy