Cross polarization at 18 and 30 GHz due to rain

Analysis indicates that the canting angles of freely falling raindrops are of great importance in determining the magnitude of the cross polarization in higher frequency (18-30 GHz) radio relay systems. Since knowledge of these canting angles is sparse, the images of 463 raindrops obtained with a raindrop camera by personnel at the Illinois State Water Survey have been measured to obtain the distribution of canting angles during two rainstorms. The distributions for both storms are similar, with 40 percent of the drops having positive canting angles exceeding
15\deg
with respect to the horizontal, and 25 percent having negative angles of less than
-15\deg
. Calculations based on the measured distributions and representing upper bounds indicate that a relay system at both 18 and 30 GHz and with a repeater spacing of 3.5 km will experience cross polarization less than -20 dB at rain rates of 150 mm/hr.