الملخص الإنجليزي
In the design of the terrestrial microwave links, it is important to know the atmospheric
radio refractivity. Radio refractivity depends on temperature, pressure and vapor
pressure that varies daily and seasonally by region. International Telecommunication
Union (ITU) produced contour maps for radio refractivity using global metrological
data for use in the design of microwave. However, ITU recommends using local data
to get more accurate contour maps of radio refractivity. So, this study aimed to produce
more precise contour maps which show the distributions of radio refractivity over
Oman for all months of the year by using local data for Oman. Meteorological data
from 39 locations in Oman covering ten years were used to compute radio refractivity
using ITU formula to generate contours maps for refractivity. The results show that
refractivity is changing geographically and seasonally. The highest values of the radio
refractivity have been observed in coastal regions. Also, the values of radio refractivity
during summer are higher than in winter. The maximum refractivity during the study
period was about 400 N-unit recorded in Al-Hallanyyat during May, and the minimum
was about 240 N-unit recorded in Saiq during December. These will contribute in
helping local telecommunications companies in the design of terrestrial microwave
links.
Some terrestrial microwave links in Oman face fading problem more than others. This
work also aimed to investigate the main causes which contribute to this problem by
analyzing five links: Zakher-Shaleem, Ayn ALmasharfah-Bufshaighah (AlAshkharah), Kawther (Adam), Sahma-Zolyia (Hima) and Kuff-Yibal (Ibri). These
links are located in different areas of Oman )not included in the previous 39 stations).
This was achieved by correlating hourly and monthly variations of meteorological
parameters (temperature, pressure and humidity) with refractivity computed from
relevant daily and hourly meteorological data. This data was available only for the
three years 2017-2019, their link profile and corresponding transmitted and received
signal levels. Monthly rainfall was also used to investigate the fading problem. The
results showed that fading occurs in these five links mostly during the summer season
and from sunset till sunrise time because of high humidity, low temperature and
fluctuations in refractivity. The duration of the fading lasted for about 11 hours. Rain
is also one of the causes of fading. However, rainfall varied from link to link depending
on the region. Its impact was showed mostly in Zakher-Shaleem link in Dhofar during
the rainy season from May till August
