Idealized Radiates power equally in all directions Omnidirectional Dipole antennas Half-wave dipole antenna Hertz antenna Quarter-wave vertical antenna Marconi antenna Parabolic Reflective Antennaġ5 Propagation Modes Ground-wave propagation Sky-wave propagationįollows contour of the earth Can propagate considerable distances Frequencies up to 2 MHz Example AM radioġ7 Sky Wave Propagation Signal reflected from ionized layer of atmosphere back down to earth Signal can travel a number of hops, back and forth between ionosphere and earth’s surface Reflection effect caused by refraction Examples Amateur radio CB radio Power output, in a particular direction, compared to that produced in any direction by an isotropic antenna Effective area Related to physical size and shape of the antennaĨ Antenna Gain (cont.) Relationship between antenna gain and effective area G antenna gain Ae effective area f carrier frequency c speed of light (» 3 x 108 m/s) carrier wavelengthĩ Antenna Gain (cont.) An antenna with a G = 3dB improves over the isotropic antenna in that direction by 3dB or a factor of 2ġ0 Polarization Defined as the orientation of the electric field (E-plane) of an electromagnetic wave Types of polarization Linear Horizontal Vertical Circularġ1 Polarization Vertically Polarized AntennaĮlectric field is perpendicular to the Earth’s surface e.g., Broadcast tower for AM radio, “whip” antenna on an automobile Horizontally Polarized Antenna Electric field is parallel to the Earth’s surface e.g., Television transmission (U.S.) Circular Polarized Antenna Wave radiates energy in both the horizontal and vertical planes and all planes in betweenġ3 Types of Antennas Isotropic antenna Dipole antennas Graphical representation of radiation properties of an antenna Depicted as a two-dimensional cross section Reception pattern Receiving antenna’s equivalent to radiation patternīeam width (or half-power beam width) Measure of directivity of antennaħ Antenna Gain Antenna gain Effective area
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Reciprocity Radiation Patterns Isotropic Radiator Gain PolarizationĤ Reciprocity In general, the various properties of an antenna apply equally regardless of whether it is used for transmitting or receiving Transmission/reception efficiency Gain Current and voltage distribution Impedanceĥ Radiation Patterns Radiation pattern Reception pattern IE 419/519 Wireless Networks Lecture Notes #5 Antennas and PropagationĢ Introduction An antenna is a transducer that converts radio frequency electric current to electromagnetic waves that are radiated into space In two-way communication, the same antenna can be used for transmission and reception
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1 Lecture Notes #5 Antennas and Propagation