where E is the electric field within the material. Figure 12.34 shows how the electric field E inside bodies of different shapes is related to the field E0 in the surrounding air.

Frequencies of 27 MHz and 2.45 GHz are allocated for power applications in Europe. Microwave ovens in the 2.45-GHz range are the most common power application. The oven shown in Figure 12.35 is a large cavity resonator within which many resonance modes are excited to realize a field that is as even as possible. The household ovens have a magnetron oscillator and their power is typically 600W to 1,000W. In industry, radiofrequency power is used to dry lumber, bake bread, vulcanize rubber, seam plastic, dry concrete, and so on.

12.11 Medical Applications

Radio waves are used in thermography, diathermy, and hyperthermia. Diathermy and hyperthermia are based on the absorption of radio-frequency

Figure 12.34 Electric field E inside bodies of different shapes. E0 is electric field in the surrounding air.

Figure 12.35 Microwave oven.

358 Radio Engineering for Wireless Communication and Sensor Applications

energy by a tissue, whereas in thermography, the thermal emission of the tissue is measured.

12.11.1 Thermography

According to Planck’s law, the maximum intensity of thermal radiation is at infrared wavelengths if the temperature of a blackbody is 310K, the temperature of a human being. However, a human being can easily be detected with a microwave radiometer also. At microwaves, the human body is a gray object: at 3 GHz the emissivity of skin is about 0.5 and at 30 GHz over 0.9 [20].

The skin depth d s given in (2.69) is a measure of how deep the waves penetrate into the material. At a depth of d s from the surface, the field has attenuated by a factor of e from its original value at the surface. Correspondingly, the skin depth determines from how deep the thermal radiation of the tissue can be detected. The skin depth depends on the frequency and electric properties of the material. In bone and fatty tissue, water content is low and, thus, the skin depth is large. Muscles and skin have much higher water content and smaller skin depth, as shown in Figure 12.36 [21].

Thermography offers a noninvasive technique for early detection of cancer. A tumor having a temperature 1°C to 5°C higher than its surround-

Figure 12.36 Skin depth for different tissues. (After: [21].)