RAIN DEPOLARIZATION OF LINEARLY POLARIZED CENTIMETRIC WAVES ON TERRESTRIAL AND SLANT PATHS IN NIGERIA

Abstract

This work presents an investigation of the amount of signal depolarisation experienced by terrestrial and earth-space communication systems during the passage of linearly polarized (horizontal and vertical) radio waves of frequencies in the range of 4 to 50GHz through rainfall in Nigeria. Cross Polarisation Discrimination (XPD) is investigated during the passage of radio waves through tropical widespread. shower and thunderstorm rain types at three elevation angles relevant to Nigeria - 0'. 23' and 55' and at rain rates applicable to each rain type. XPD is also calculated in terms of co-polar Attenuation (CPA) using an empirical scaling relationship for quick practical application. The results obtained in all the estimates show consistently that rainfall in Nigeria will cause XPD levels to fall below the acceptable margins of 20 to 30dB on terrestrial communication operating at around 10GHz and above and at rainfall rates > I2.5mm/h. For example, during a widespread rain event of rain rate 5mm/h. XPD falls to 6.5dB at a frequency of 50GHz for horizontal polarization. It is -8.5dB if the polarization is vertical. XPD becomes poorer in convective rain types (shower and thunderstorm). The study shows levels which are consistently below 20dB. It is better in widespread rain. Poor XPD level is responsible for the near total outage of satellite TV channels operating ain the C band (4/6GHz) and in addition to signal attenuation at the Ku band (14/11 GHz). XPD was also found to drop with increasing CPA and with frequency up to 40GHz. above which it rises slightly. It also becomes poorer with increasing rain rate. the fall being sharper at rain rates 5mm/h. XPD improves consistently with elevation angle. Terrestrial line of sight propagation exhibits the worst levels. Comparison between standard and scaling estimations of XPD show an average percentage difference of ±40% for widespread rain at elevation angle 0" An average variation of about ±5% was observed for all the other cases.