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VTU eNotes On Antennas and Propagation (Electronics and Communication)

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Product Specifications

Publisher VTU eLearning
Author: Panel Of Experts
Number of Pages 139
Available Available in all digital devices
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About this eBook

Antenna Basics Introduction, basic Antenna parameters, patterns, beam area, radiation intensity, beam efficiency, directivity and gain, antenna apertures, effective height, bandwidth, radiation efficiency, antenna temperature and antenna filed zones. Introduction It is a source or radiator of EM waves, or a sensor of EM waves. It is a transition device or transducer between a guided wave and a free space wave or vice versa. It is an electrical conductor or system of conductors that radiates EM energy into or collects EM energy from free space. is an impedance matching device, coupling EM waves between Transmission line and free space or vice versa. Some Antenna Types Wire Antennas- dipoles, loops and Helical Aperture Antennas-Horns and reflectors Array Antennas-Yagi, Log periodic Patch Antennas- Microstrips, PIFAs Principle- Under time varying conditions , Maxwell s equations predict the radiation of EM energy from current source or accelerated charge . This happens at all frequencies , but is insignificant as long as the size of the source region is not comparable to the wavelength. While transmission.lines are designed to minimize this radiation loss, radiation into free space becomes main purpose in case of Antennas . For steady state harmonic variation, usually we focus on time changing current For transients or pulses ,we focus on accelerated charge The radiation is perpendicular to the acceleration. The radiated power is proportional to the square of . I L or Q V Where I Time changing current in Amps sec L Length of the current element in meters Q Charge in Coulombs
V Time changing velocity Transmission line opened out in a Tapered fashion as Antenna a As Transmitting Antenna Here the Transmission Line is connected to source or generator at one end. Along the uniform part of the line energy is guided as Plane TEM wave with little loss. Spacing between line is a small fraction of . As the line is opened out and the separation b n the two lines becomes comparable to , it acts like an antenna and launches a free space wave since currents on the transmission Line flow out on the antenna but fields associated with them keep on going. From the circuit point of view the antennas appear to the tr. lines As a resistance Rr, called Radiation resistance
b As Receiving Antenna Active radiation by other Antenna or Passive radiation from distant objects raises the apparent temperature of Rr .This has nothing to do with the physical temperature of the antenna itself but is
related to the temperature of distant objects that the antenna is looking at. Rr may be thought of as virtual resistance that does not exist physically but is a quantity coupling the antenna to distant regions of space via a virtual transmission .line