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## [TowerTalk] Question re current dist. on 1/3 w. dipole

 To: [TowerTalk] Question re current dist. on 1/3 w. dipole ccc@space.mit.edu (Chuck Counselman) Thu Aug 7 22:50:52 2003
 ```At 12:38 AM +0000 8/8/03, Rob Atkinson, K5UJ wrote: >One of my antennas is a dipole ~30 feet up that's ...88' long. I >transmitted [in] the 80 cw band and walked around under the dipole >with a cheap field strength meter..... [T]he field was strongest at >the ends of the dipole and weakest near the feed point. I was not >expecting this, mainly because with a 1/2 wave dipole I understand >the current is max at the feedpoint.... You were within the "near field" region, where the antenna's radiation field is dominated by its quasi-static field. Here, the amplitude of the (oscillating) electric field is approximately that of a static charge distribution equal to that on the antenna at the moment of maximum charge (or voltage) and zero current; and the amplitude of the (oscillating) magnetic field is approximately that of the instantaneous current distribution in the antenna at the moment of maximum current and zero charge (or voltage). The max.-charge distribution is a sinusoidal function of distance along the antenna, with zero at the center, whether the antenna is a half-wavelength long, or shorter. The max.-current distribution is a cosinusoidal function of distance along the antenna, with a peak at the center, whether the antenna is a half-wavelength long, or shorter. Your field-strength meter probably sensed the vertical component of the electric field, by means of a short vertical whip. This component is strongest under the ends of the dipole, because the charge density on the wire is greatest there. If you had been sensing magnetic field, e.g., with a small loop, you would have found the strongest magnetic field under the center of the dipole, directed horizontally, perpendicular to the wire. In the near-field region, the electric field and the magnetic field are not related as they are in the "far," or radiation-field. That is, they are not necessarily perpendicular with the ratio |E|/|H| equal to 377 ohms, and their cross-product is not in the direction of radiation. -Chuck, W1HIS ```
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