539 
Fig. 3 shows the brightness temperature 
(measured at e = 45*) of a layer of 
wheat ears as a function of the azimuth 
angle The ears are placed close 
together in order to form an almost 
uniform layer. When the main axis of 
ears is in the incidence plane (*=0*), the 
V component of the emission is much 
higher than the H one, while the reverse 
is obtained for * = 90*. A similar 
behavior is shown by the layer of stems 
(Fig. 4) and leaves (Fig. 5). 
All the diagrams confirm the anisotropic 
emission of these elements, the 
emissivity being much higher when the 
main axis of plant constituents is aligned 
with the electric field of the antenna. It 
should be noted that there is not a full 
symmetry between V and H components 
because the incidence angle is 45*. 
Figure 6 - The brightness temperature (f = 37 GHz, 0 
= 45’) at V and H pol. of three layers of wheat leaves 
(PWC = 0.5 Kg/m 2 ) as a function of azimuth angle #. 
When medium becomes optically thick as 
in three layers of leaves, the anisotropy tend to disappear and the normalized temperature is 
almost independent of azimuth angle, with a vertical polarized component higher than the 
horizontal one according to the Brewster effect (Fig. 6). As a consequence, and due to the almost 
random distribution of leaves in the standing crop, the anisotropic effect is significantly smoothed 
when plants ore observed in field 
conditions. 
The measurements on alfalfa indicated 
different emission characteristics of this 
crop. Indeed the alfalfa plant shape is 
characterized by small, quasi circular 
leaves and a ramified structure. The 
anisotropic effects are much less 
conspicuous in this case. As an example, 
Fig. 7 shows the brightness temperature 
of layered stems with leaves of an alfalfa 
crop, with a total plant water content 
equal to 0.70 Kg/m 2 . We see that the 
difference between V and H 
polarizations is quite small, even in the 
case of one single layer; moreover the 
increase of emission with increasing 
biomass is very fast and attains 
saturation from the second layer. 
Figure 7 - The brightness temperature (f «= 37 GHz, 0 
= 45*) of layered stems with leaves of alfalfa plants. (V 
= vertical pol., H « horizontal pol.)