Difference between revisions of "Refractivity"
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Latest revision as of 14:32, 21 February 2020
The refractive index (n) of air is defined as:
n = c/v
where:
- c is the velocity of light in a vacuum.
- v is the velocity of light in air.
A typical value of n is 1.000301. For convenience the refractive index is expressed as refractivity (N)
N = (n-1) x 10^6
which results in a typical value of 301 Refractivity is defined by the equation:
N = 77.6 x (P/T) + 3.732 x 10^5 x (e/T^2)
where:
- P is the atmospheric pressure.
- T is the absolute temperature.
- e is the water vapor pressure.
- N varies with elevation. In a well mixed troposphere, the variation is exponential; however, in the range from 1 to 2 kilometers above sea level, the variation is approximately linear. This is referred to as a constant refractivity gradient. The refractivity gradient determines the microwave beam-bending.
Handling the variations of refractivity with elevation is the major challenge in the design of high reliability microwave links
