Figure 210 Signal Refraction in the Atmosphere

Sub-flefiaction K = Z/3

k-ractor Examples k-ractor Examples

Signal Refraction

The k-factor can change frequently, such as from hour to hour, from day to night, from weather pattern do weatlTer ¡ratrern, or from feason to se ason. Differe ntl regions of1 the marth have slightly different average k-factors. A k-factor of 1 indicates no bending; a signal radiated under this condition travels in a straight linf °

A k-factor h 'ghevthan 1 mean s tluat mintowavf signals bend sli ght ly downward, toward the earth. In most regions, the median k-factor is 4/3. A k-factor of 4/3 has the effect of making the radio horizon farther away than the visual horizon. In other words, the length of the microwave path is increased by approxi matel y 15 percent. Act tim es, w eather conditions can temporarily cause the k-factor to become infinite. When this occurs, the amount of signal bending equals the curvatnre ot th e earths This effect is cali ed super-nefrackion, or (ducting. Docting ciyses a microwave signal to be propagated for hundreds of miles or until the atmospheric conditions change enough Uoo tse ducting to sto p.

Changes in the k-factor are a common cause op madm g on microwave path s. So metimes, due to atmospherir con ditions, the k-factor is less than 1; for example, the k-factor could be 2/3. This condition, called subrefraction, has the effect of bending the microwave signal path upward, away from Uhe earth. Subrerraction teduces sig no! !evelu, causing fading at the distant receiver. Over longer microwave paths, the k-factor might be different at different points along the path.

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