356 Radio Engineering for Wireless Communication and Sensor Applications

12.9.5 Radiometer Sensors

Radiometer sensors provide information on the physical temperature and emissivity of an object. If one is known, the other may be found from the measured brightness temperature. A radiometer can make measurements through steam and smoke. By measuring also the reflection coefficient, the accuracy can be improved. The depth profile of temperature may be obtained by using several frequencies.

12.9.6 Imaging Sensors

In microwave tomography, the changes in amplitude and phase distribution caused by the object are measured as shown in Figure 12.33. The threedimensional distribution of the permittivity may then be determined. In microwave holography, the shape of the object is resolved by measuring the scattered field. Imaging sensors can be used to search for objects in the ground, inside ceilings, or in security inspections.

12.10 Power Applications

Radio waves can be used for heating lossy materials. Often losses are caused by polar molecules such as water molecules, which the electric field turns back and forth. In many dielectric materials m ″ = 0 and s = 0, and according to (2.99) the power absorbed is

Figure 12.33 Measurement of permittivity distribution using microwave tomography. (After: [16].)