Maxwell’s equations predict the radiation of electromagnetic energy from current sources under time varying conditions. Althrough such phenomenon occurs at all frequencies, its relative magnitude is insignificant until the size of the source region is comparable to wavelength. Our main interest is in the radiation of this electromagnetic wave or loss of energy from the system into free space. While transmission lines or waveguides are generally made so as to minimize radiation, antennas are designed to radiate energy as effectively as possible.
The transmitting antennas are devices used in terminating a transmission line or waveguide with the intent of efficiently launching electromagnetic waves into free space and hence they can be considered as source of such waves in space. The radiation of electromagnetic waves from a transmitting antenna to a receiving antenna is of considerable interest. Thus, we are concerned with the analysis of the radiation fields achieved from typical antenna source depicted.
When RF energy is fed into a mismatched transmission line, standing waves take place, Energy is lost or radiated into the space surrounding the line. This process is considered undesired in the transfer of energy to the radiation device. If we examine this process and expand upon it, by separating the ends of the transmission line, that more surface area of the wire is exposed to the atmosphere and enhances the radiation process.
Radiation efficiency of this system is improved even more when the two wires are bent at 90* (right angles) to each other. Now, the electric and magnetic fields are fully coupled to the surrounding space instead of being confiend between the two wire, and maximum radiation results. This type of radiator is known as a dipole. If the total length of the two wires is a falf wavelength, then antenna is called a half-wave dipole.
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