Refractivity

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