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Old August 24th 16, 02:29 PM posted to rec.radio.amateur.antenna
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First recorded activity by RadioBanter: Jun 2006
Posts: 440
Default Loop Antenna Polarization

J.B. Wood clip: " ... Anyone, ham or other, who claims that an antenna in
the far (several wavelengths from the transmitter) field "receives" (or
favors) an E-field or an H-field is demonstrating a lack of
understanding of basic electromagnetic theory. ..."
_____________

For far-field conditions, it is a given that the E field and the H field of an e-m wave are orthogonal to each other. Neither field can exist without the other.

A simple experiment will illustrate that a single antenna can favor one field but not other, even though that other field exists.

AM broadcast stations transmit using vertical polarization (polarization is defined as the physical orientation of the E-field vectors with respect to the horizontal plane). Vertical polarization maximizes their groundwave coverage areas.

A conventional AM broadcast band receiver (other than in an automobile) uses a loopstick antenna consisting of a close-wound loop of wire wound along a ferrite core. It responds to the H field of the arriving e-m wave, and for maximum r-f output it must be oriented in the horizontal plane -- even though that arriving wave is "vertically polarized."

Such a receiver can work very well when the axis of its loopstick lies in the horizontal plane, and normal to the direction of the arriving e-m wave. But when that receiver is vertically rotated 90° around the bearing to the transmit site so that the loopstick axis is vertical, reception is much poorer than before.

So the loopstick does not respond well to the E field, even though the E field is present at the receive site.

My experiment using a Tecsun PL-880 portable receiver had about s 30 dB reduction in the value of the signal strength shown on its front-panel display, when changing its loopstick orientation from horizontal to vertical.

Richard Fry, CPBE
 
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