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High Definition Television (HDTV) ___________________________________ Common TV/FM Antenna Types The dipole is the fundamental unit of antenna design. The impedance of a dipole at its center frequency is about 70 ohms. Impedance is the ratio of voltage to Amperage combined with the "springyness" of a circuit. in the mechanical sense it is like having a number for torque plus RPM combined with a measure for how much the metal twists in a vehicle power train. In the electrical world an impedance transformer (or any transformer) is exactly like a gear box in the mechanical world. By convention transmitters and commercial receivers are designed to operate with 50 ohm systems, while consumer receivers are designed.... more at kyes.com
Keep in mind that antennas are reciprocal, i. e. transmitting and receiving characteristics are identical. Also, they scale with wave length. If the wave is ten times larger, an antenna 10 times larger will be identical in all respects except effective area. However, in real life, with real materials, we must change things depending upon use and wavelength....more at kyes.com [scroll down]
Recently on the SETI email discussion list there has appeared some good dialog regarding possible detection of Earth TV broadcasts at ET locations. [These discussions were stimulated by the question: could beings on worlds 40 light years distant really be viewing I Love Lucy? -- Ed.] This is not a novel idea. An older book titled Communication With Extraterrestrial Intelligence (editor Sagan, 1973, MIT Press, ISBN 0-262-69037-3) mentions the possibility of eavesdropping by ETI in the chapter Techniques of Contact ....more at setileague.org
When planning a communications system the usual practice is to look at radiation patterns. However, patterns developed by antenna manufactures will not necessarily mirror what can be achieved in real conditions, especially in the case of mobile networks using high gain antennas for extended range. The reason for this lies in the differences in propagation of the direct wave....more at moonraker.com.au
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