September 8, 2016 at 1:43 pm #1138
Ch. Zeshan YousafParticipant
Calculation of Electric Field Intensity:
For estimation of broadcasting efficiency, the electric field intensities created by the Broadcasting Station during day and night hours at different distanced were calculated.
The field intensities were determined for 1000 kW transmitter power and 517 m operating wave. The gain of APPT antenna at the operating wave is equal to 1.75, the efficiency of the antenna – 0.9, the efficiency of the feeder is 0.985. At calculations, the feeder length was assumed equal to 500 m.
The electric field intensity of the ground wave (daylight field) was calculated, using ground wave propagation curves given in Recommendation N 368 CCIR (Oslo, 1966) for ground conductivity equal to 3 mmho/m.
The electric field intensity of the space wave (night hour filed) was calculated according to CCIR charts (Report 264-I, Oslo, 1966).
The threshold minimum value of the field intensity in non-shared channel for the covered areas is assumed equal to 60 db/muV (1 mV/m).
The field intensity of the useful signal more than 50 times (34 db) exceeds the field intensity value of atmospheric disturbances.
In accordance with Report N 322 CCIR (Geneva, 1963), the filed intensity of atmospheric disturbances in various points of the signal of the desired Broadcasting Station amounts to approx. 23 – 24 db/muV in evening in summer, when the level is the highest.
As it is well known, at evening and night hours, the radio wave propagation is realized with a ground beam and an ionosphere-reflected beam as well.
The character of variations of the fields with distance is different. The ground wave filed is gradually decaying as moving away from the Broadcasting Station, but the ionospheric wave field is continuously increasing, reaches the maximum value and then decay again.
Due to different character of variations of the ground and reflected wave filed at some distances from the Broadcasting Station, the intensity of both filed becomes commensurable.
In view of a quick change of the space beam phase, an interference of the ground and space beams may occur at a reception point, which leads to sharp variations of the filed intensity (nearest fading). For designed Broadcasting Station, the nearest fading zone is about 60 km long.
Depending on specific wave propagation conditions, the position of this zone relative to the Broadcasting Station can be modified.
Employment of the anti-fading antenna makes it possible to remove the nearest fading zone away from the Broadcasting Station, however, to move it off beyond the ground wave service area at the given specific conditions (the transmitter power, operating wave, soil conductivity, threshold level of 60 db/muV) has failed.
Therefore in the evening, the radius of the ground wave service area is reduced to about 170 km. Reception of the space wave at a threshold level of 60 db/muV is possible at a distance of the order of 230 to 1400 km from the Broadcasting Station.
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