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Physics
Classical Physics
Electromagnetism
Estimation of E-field strength at a distance from dipole antenna
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[QUOTE="tech99, post: 6595852, member: 477979"] The dipole in free space (no ground anywhere) radiates waves which fall in intensity with the inverse square law. By this I mean that the power/sq metre falls off with 1/D^2. The field components of this wave, electric and magnetic, fall inversely with distance, so for instance, E falls off with 1/D. The formula you quote is correct for distances greater than about a sixth of a wavelength. E = (7x sqrt P)/D in Volts, metres and Watts. So the electric field strength falls off inversely with distance. Now some more difficult information. At distances closer than about a sixth of a wavelength, we see some intense additional fields known as induction fields. Further, large antennas also have a region called the Near Radiation Zone where the pattern is distorted and it is possible for hot spots to occur. I think a dipole has a slight Near Radiation Zone and this may account for the slight hump seen around 600 mm. The distant pattern of a dipole resembles a ring doughnut, but is different when measured close up. Then we see a Near Radiation Zone, and the hole in the centre disappears. If we approach the antenna in its equatorial plane, the electric field increases with 1/D until a distance of, say, lambda/2 from the antenna, then remains constant. As we almost touch the antenna we see an intense local field from the voltage across the feedpoint. If we similarly measure the magnetic field starting at D=lambda/2, it does not level off but continues upwards, following 1/D, until we touch the antenna conductor. [/QUOTE]
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Physics
Classical Physics
Electromagnetism
Estimation of E-field strength at a distance from dipole antenna
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