  We estimated the brightness temperature of radio zebras (zebra pattern --
ZP), considering that ZPs are generated in loops having an exponential density
profile in their cross-section. We took into account that when in plasma there
is a source emitting in all directions, then in the escape process from the
plasma the emission obtains a directional character nearly perpendicular to the
constant-density profile. Owing to the high directivity of the plasma emission
the region from which the emission escapes can be very small. We estimated the
brightness temperature of three observed ZPs for two values of the density
scale height (1 and 0.21 Mm) and two values of the loop width (1 and 2 arcsec).
In all cases high brightness temperatures were obtained. For the higher value
of the density scale height, the brightness temperature was estimated as 1.1
$\times$ 10$^{15}$ - 1.3 $\times$ 10$^{17}$ K, and for the lower value as 4.7
$\times$ 10$^{13}$ - 5.6 $\times$ 10$^{15}$ K. We also computed the saturation
energy density of the upper-hybrid waves (which according to the double plasma
resonance model are generated in the zebra source) using a 3D particle-in-cell
model with the loss-cone type of distribution of hot electrons. We found that
this saturated energy is proportional to the ratio of hot electron and
background plasma densities. Thus, comparing the growth rate and collisional
damping of the upper-hybrid waves, we estimated minimal densities of hot
electrons as well as the minimal value of the saturation energy density of the
upper-hybrid waves. Finally, we compared the computed energy density of the
upper-hybrid waves with the energy density of the electromagnetic waves in the
zebra source and thus estimated the efficiency of the wave transformation.
