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# Rf Waves Help (page 4)

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By — McGraw-Hill Professional
Updated on Sep 12, 2011

## Duct Effect

The duct effect is a form of tropospheric propagation that takes place at approximately the same frequencies as bending and scattering. Also called ducting , this form of propagation is most common very close to the surface, sometimes at altitudes of less than 300 m.

A duct forms when a layer of cool air becomes sandwiched between two layers of warmer air. This is common along and near weather fronts in the temperate latitudes. It also takes place frequently above water surfaces during the daylight hours and over land surfaces at night. Radio waves can be trapped inside the region of cooler air, in much the same way that light waves are trapped inside an optical fiber. Ducting often allows over-the-horizon communication of exceptional quality over distances of hundreds of kilometers at VHF and UHF.

## Rf Waves Practice Problem

#### Problem

Suppose that you are using a handheld radio transceiver to talk with someone across town. You stand on a hill and can see the house where the other person is located, and the two of you are well within the quoted communications range for the radios. Yet the signal is extremely weak. You move over a few meters, and the signal gets strong. What might cause this?

#### Solution

The direct wave and the reflected wave from the other radio’s antenna happen to arrive out of phase at your antenna, so they almost cancel each other out. Moving a few meters remedies this, and the signal becomes strong.

Practice problems of these concepts can be found at: Forms of Radiation Practice Test

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