Prediction of Positive Refractivity Gradients for Line-of-Sight Microwave Radio Paths

Line-of-sight microwave radio transmission performance is susceptible to atmospheric conditions, in particular to spatial variations in the microwave refractive index. Nearly always during the day and frequently at night, sufficient vertical mixing occurs of the low-level air layers, through which the microwave beam propagates, so that vertical refractive index gradients encountered are small. However, horizontal layers containing strong vertical gradients in air temperature and water vapor pressure, which determine the refractive index, sometimes develop at night. The presence of these layers can cause the received signal to fade (decrease in strength). This paper addresses the meteorology of obstruction fading, which 803 is caused by a large positive gradient in the refractive index, which is, in turn, caused by a large positive water vapor pressure gradient. The ray path between the transmitting and the receiving antennas is lower than normal in the presence of a positive refractive index gradient, and the terrain can block the ray path when the positive gradient is sufficiently large. Although infrequent, the consequences of obstruction fading are severe: transmitted signal is steadily blocked for as long as an hour at a time. Obstruction fading can be avoided by locating microwave antennas sufficiently high above the ground. Prolonged fades can also be caused by strong negative gradients that refract the transmitted microwave beam downward to such an extent that it ceases to illuminate the receiving antenna.