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agricultural fields the contrast between emission from bare and vegetated soil decreases as the 
frequency increases. Considering the atmospheric absorption characteristics, a good compromise 
could be the use of a frequency close to 37 GHz (Choudhury 1989). Thus, it appeared interesting 
to further investigate microwave features at 37 GHz of the whole standing vegetation and of the 
single plant constituents. The achieved results, which show some interesting peculiarities of 37 
GHz emission from wheat and alfalfa canopies, are discussed in this paper. 
2 - THE EXPERIMENT 
The field measurements have been carried out using a dual polarized (V and H polarizations) 
microwave radiometer at 36.6 GHz and a thermal IR sensor (8-14 /tm) (Fig. 1). 
The microwave instrument is a portable, battery operated system, which can be easily used in field 
measurements as well as from an aircraft. The instrument is based on a total power, frequently 
calibrated radiometer with two reference loads at different temperatures (continuously monitored). 
It uses two corrugated conical horns, one for each polarization channel, and the signal is switched 
between the two horns at a frequency close to 10 Hz. The detected signal is digitized with a 
voltage to frequency converter and all the functions of the radiometer (integration time, 
calibration/observation sequences, etc.) are programmable with an internal microprocessor. 
Finally, row and calibrated data are recorded by means of a notebook computer. The achieved 
measurement accuracy (repeatability) is in the order of ± 0.5 K, with an integration time of 1 sec. 
The infrared sensor is a commercial type hand-held radiometer with an accuracy (repeatability) 
of ± 0.5 K. 
The experiments have been organized in the following basic steps: 
- Measurements of emission from well 
developed standing vegetation, in 
natural condition and after sequential 
cut of fruits, leaves, and stems. 
- Measurements of emission from single 
plant constituents (fruit, leaves, stems) 
placed on a reflecting plate. 
The microwave and infrared brightness 
temperatures have been measured at an 
incidence angle 0 = 45* and, in the case 
of plant elements, for various azimuth 
angles (#). Total biomass and 
geometrical characteristics of plant 
constituents have been measured with 
standard methods. 
During the measurements wheat was in 
the flowering stage and had a global 
plant water content (PWC) equal to 1.0 
Kg/m 2 , the fraction due to ears, leaves 
and stems was respectively of 0.2, 0.15, 
and 0.65 Kg/m 2 . The leaf area index was 1.2 m 2 /m 2 and the average height 70 cm. The PWC of 
alfalfa, which was well developed (76 cm height), was 1.4 Kg/m 2 . 
Figure 1 - The experimental equipment 
3 - THE RESULTS 
The emissivity of a canopy covered soil can be approximated by the normalized temperature Tn, 
i.e. by the ratio between the brightness temperature measured by the microwave radiometer and