Thermal radiation is the process of transmission of energy from a source either by motion of material particles or by waves as in sound, light, and x-rays. Here, the rays are the curved lines along which the energy is sent from the source. One can experience radiation in daily life; when he holds his hand in front of a hot object and radiant heat is felt. We can feel radiant heat as we stay out in the sun. Moreover we need light to see the surroundings. All these are the examples of electromagnetic radiation. According to the wavelength or frequency, this radiation is grouped or classified and named as x-rays, ultraviolet rays, solar rays, infrared, and radio waves. But, we will restrict ourselves only with radio waves including microwaves and millimeter waves. Table 1.1 is presented for a better understanding of grouping or classifications or naming of electromagnetic waves. The movement or oscillation of electrical charges in a conductive material or in antenna produces the electromagnetic waves. These waves are radiated away from the transmitting antenna and are then intercepted by a receiving antenna such as a TV antenna or hand-held device (e.g., cellular phone). Electromagnetic waves travel through space at the speed of light. The wavelength and frequency of an electromagnetic wave are inversely related as λ = c f https://s3-euw1-ap-pe-df-pch-content-public-p.s3.eu-west-1.amazonaws.com/9781315217420/8a83eea1-db1b-46eb-92da-df1890125e35/content/unequ1_1.tif"/>