AbstractSolar radio zebras are used in the determination of the plasma density and magnetic field in solar flare plasmas. Analyzing observed zebra stripes and assuming their generation by the double-plasma resonance (DPR) instability, high values of the gyro-harmonic number are found. In some cases they exceed one hundred, in disagreement with the DPR growth rates computed up to now, which decrease with increasing gyro-harmonic number. We address the question of how zebras with high values of the gyro-harmonic numbers $s$ s are generated. For this purpose, we compute the growth rates of the DPR instability in a very broad range of $s$ s , considering a loss-cone $κ$ κ -distribution of superthermal electrons and varying the loss-cone angle, electron energies, and background plasma temperature. We have numerically calculated the dispersion relations and the growth rates of the upper-hybrid waves and found that the growth rates increase with increasing gyro-harmonic numbers if the loss-cone angles are $∼80^{∘}$ ∼ 80 ∘ . The highest growth rates for these loss-cone angles are obtained for velocity $v_{κ}= 0.15\,c$ v κ = 0.15 c . The growth rates as a function of the gyro-harmonic number still show well distinct peaks, which correspond to zebra-stripe frequencies. The contrast between peak growth rates and surrounding growth rate levels increases as the $κ$ κ index increases and the background temperature decreases. Zebras with high values of $s$ s can be generated in regions where loss-cone distributions of superthermal electrons with large loss-cone angles ($∼80^{∘}$ ∼ 80 ∘ ) are present. Furthermore, owing to the high values of $s$ s , the magnetic field is relatively weak and has a small spatial gradient in such regions.