1The Mystery of Wave Propagation and Radiation from an Antenna


An antenna is a structure that is made of material bodies that may consist of either conducting or dielectric materials or may be a combination of both. Such a structure should be matched to the source of the electromagnetic energy so that it can radiate or receive the electromagnetic field in an efficient manner. The interesting phenomenon is that an antenna displays selectivity properties not only in the frequency domain but also in the space domain. In the frequency domain an antenna is capable of displaying an external resonance phenomenon where at a particular frequency the current density induced on it can be sufficiently significant to cause radiation of electromagnetic fields from that structure. An antenna also possesses a spatial impulse response that is a function of both the azimuth and elevation angles. Thus, an antenna displays spatial selectivity as it generates a radiation pattern that can selectively transmit or receive electromagnetic energy along certain spatial directions in the far field as in the near field even a highly directive antenna has essentially an omnidirectional pattern with no selectivity. That is the reason researchers have been talking about space division multiple access (SDMA) where one directs a beam along the direction of the desired user but places a null along the direction of the undesired user. This has not materialized as we shall see in the next chapter as most ...

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