The terrestrial line-of-sight link was established between the Howard College and the Westville campuses of the University of KwaZulu-Natal, Durban in 2004 by Naicker and Mneney to monitor signal attenuation due to Rain. The system for monitoring the attenuation due to rain was installed at 19.5 GHz terrestrial link between the two campuses of University of KwaZulu-Natal.
A block diagram of the system is provided in Fig. 4-1 below. The transmitter antenna was installed 178 m above sea level, on the roof of the Science building at Westville campus. The receiver antenna was setup on the Electrical Engineering building in Howard College campus, 145 above sea level [Naicker and Mneney, 2006; Naicker 2006].
Rhode & Schwarz FSIQ40 Spectrum Analyzer
Computer
Oregon Scientific WMR928N
Agilent 83018A Power Amplifier
Agilent 83018A Power Amplifier
Meteorological Sensors
Agilent E8251A Signal Generator T ransmitter
Receiver
Fig. 4- 1: Block diagram of the of the monitoring System [Naicker and Mneney, 2004]
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A valuline ® WR42/R220 parabolic antennae of diameter 0.6 m are used at both the receiving and the transmitting stations. The antennae can operate within the 17.7 – 19.7 GHz and 21.2 – 23.6 GHz bands and provide a gain of 38.6 dBi and a 3 dB beamwidth of 1.9O at 19.5 GHz [Naicker and Mneney, 2006; Naicker 2006]. These parabolic antennae are protected by a weatherproof material known as radome4. The radome prevents ice and freezing rain from accumulating directly onto the metal surface of the antenna [11].
The cabling of the terrestrial link consists of FSJ1-50A super-flexible coaxial cable which produces an attenuation of 22 dB per 100 m. An agilent E8251A signal generator is used at the transmitting station to provide the source signal which can operate between 250 KHz – 20 GHz.
This is used with an agilent 83018A microwave system amplifier which can operate from 0.5 GHz – 25 GHz and provide a gain of up to 27 dB [12]. This setup produces unmodulated continuous wave signals operating at a frequency of 19. 5 GHz.
At the receiver, another agilent 83018A power amplifier is used to give additional gain before feeding the signal into the Rhodes & Schwarz FS1Q40 spectrum analyzer. The signal strength is logged every second or minute onto a computer. The calculated expected noise power in the receiver when no signal is transmitted lies between –80.5 to –80.2 dBm. This defines the noise floor. The noise floor is determined from:
i. The noise temperature of the antenna TA of 206oK (with an estimated efficiency of 63.4%, equivalent background temperature of 150oK [Pozar, 1988], and a maximum physical temperature of 303oK);
ii. The transmission line noise temperature of 97.4oK (with an attenuation of 22 dB per 100 m);
iii. 83018A Agilent amplifier (with a gain of 27 dB, and noise figure of about 9.5 dB at 19.5 GHz [12]) with noise temperature of 2398.5oK;
Therefore resulting in total receiver noise temperature of 3470.4oK, or a noise power of –80.2 dBm [Pozar, 1988]. At the lower temperature of 283OK, the noise power is –80.5 dBm. In the measurements, this noise value varied from –79.5 dBm to –82 dBm [Fashuyi and Afullo, 2007].
4 A radome allows a relatively unattenuated electromagnetic signal between an antenna inside the radome and outside the equipment [11]
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The weather conditions were also logged using Oregon Scientific WMR928N wireless professional weather station to record the rain rate, outdoor temperature, relative outdoor humidity, outdoor dew point temperature, outdoor pressure, wind speed and wind direction. The rain rate was monitored every minute and the rainfall depth over each minute of interval for a period of one year was recorded [Naicker and Mneney, 2006; Naicker 2006]. More details on the link setup at the receiver and the transmitter end can be seen in Naicker [2006].
4.2.1 Path Profile
The terrestrial link has a path length of 6.73 km and passes over both hilly and the suburban terrain of Durban. The azimuth angle at the Westville transmitter station is 30.980O and that of the Howard College receiver station is 30.943O. Fig. 4-2 shows the digital elevation map of the propagation path, and Fig. 4-3 is the aerial map of the link showing the distance between the two stations. The terrestrial path is nearly horizontal, with a slight inclination of 0.3O at the receiver.
Fig. 4-4 gives the path clearance of the line-of-sight link
from the first Fresnel ellipsoid. A
k- factor or effective radius factor is used to examine the path clearance of the worst case of ray bending on line-of-sight link.Fig. 4- 2: Digital elevation map for the 6.73 km terrestrial link between the two stations
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Fig. 4- 3: An aerial photograph of the 6.73 km line-of sight terrestrial link
Fresnel ellipsoids can be used to estimate the diffraction and reflections along radio propagation paths. The radius of the first Fresnel ellipsoid is given by:
1 17.3 d d1 2
F fd (4.1)
where F1 denotes the radius of the first Fresnel ellipsoid in m, f is the frequency in GHz, dis the total path length in km and d1and d2are the distances from either stations in km.
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0 40 80 120 160 200
0 1 2 3 4 5 6
Distance in km
Height above sea level in m
Howard College (Receiver) Westville (Transmitter)
LOS Path
Transmitting antenna
Receiving antenna
Fresnel Ellipsoid
Topography Altitude of the
receiver station
Altitude of the transmitting station
Fig. 4- 4: Path profile for the 6.73 km terrestrial line-of-sight link from Howard College campus to Westville campus [Naicker and Mneney, 2006; Fashuyi et al., 2007]