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Abstract: This article details the knowledge related to high-power FM coverage calculation, including influencing factors, common calculation methods, and key points in practical applications. By elaborating on these contents, it aims to help relevant technicians better understand and master FM coverage calculation to achieve more efficient and high-quality FM broadcasting coverage.
Frequency Modulation (FM) broadcasting is widely used in the broadcasting industry due to its superior sound quality and strong anti-interference capabilities. For high-power FM transmission systems, accurately calculating their coverage area is crucial. This not only determines whether broadcast signals can effectively reach audiences but also relates to rational resource utilization and cost control.
Transmitter: The output power of the transmitter directly determines the signal coverage area—the higher the power, the wider the coverage range. Additionally, performance indicators of the transmitter, such as the signal-to-noise ratio, frequency response, distortion, and left-right channel isolation of the exciter, affect signal quality and propagation. Faults or poor performance in the transmitter may cause signal distortion or attenuation, thereby reducing coverage.
Antenna: Characteristics of the antenna, such as gain, radiation pattern, and polarization, significantly impact FM signal coverage. High-gain antennas can concentrate signals in specific directions, enhancing signal strength and expanding coverage. The antenna’s radiation pattern determines the signal’s radiation intensity in different directions; reasonable selection of the radiation pattern ensures optimal coverage of the target area. Moreover, mismatched polarization between the transmitting and receiving antennas reduces signal reception efficiency, affecting coverage.
Topography: Different terrains (mountains, hills, plains, etc.) affect FM signal propagation differently. In mountainous and hilly areas, signals are easily blocked by mountains, forming shadow zones and reducing coverage. In plain areas, signals propagate more smoothly but may still be scattered or absorbed by obstacles such as buildings and trees.
Urban Environment: With rapid urbanization, high-rise buildings in cities can reflect, refract, and scatter FM signals, creating multipath effects that cause signal distortion and fading. Meanwhile, the complex electromagnetic environment in cities, with interference from various electronic devices, may also affect signal propagation and reception.
Frequency Interference: Strong signal sources within the FM band may interfere with the target FM signal, reducing the signal-to-noise ratio, degrading reception quality, and narrowing the effective coverage area.
Meteorological Conditions: Although FM signals are relatively less affected by weather, extreme conditions (heavy rain, fog, strong winds, etc.) may slightly influence signal propagation, causing minor changes in coverage.
This is a common method for FM coverage engineering calculations. The field strength E (in dB(μV/m)) at a distance d (in km) from the transmitter can be calculated using the formula:E=Pe+E1+A
where:
o Pe is the effective radiated power (in dB(kW));
o E1 is the field strength (in dB(μV/m)) formed by a 1 kW effective radiated power on a smooth spherical surface at the observation point (values can be obtained from relevant charts);
o A is the attenuation correction coefficient (in dB) related to the terrain ruggedness Δh (values can be obtained from corresponding charts).
The effective radiated power Pe is calculated as:Pe=P+G−L
where:
o P is the transmitter’s rated output power (in dB(kW));
o G is the power gain of the transmitting antenna relative to a half-wave dipole (in dBd);
o L is the feeder loss (in dB).
This method is relatively simple and macroscopically satisfies engineering calculation needs. However, it ignores the specific terrain at the receiving point, making it less accurate for calculating field strength at specific receiving points in complex terrains (especially when there are obstructions in the near-front area).
This method provides more accurate field strength calculations for specific receiving points in complex terrains. It analyzes the topographic profile between the transmitter and receiver, considering factors such as obstacle diffraction and reflection to calculate signal propagation loss.
However, it requires detailed topographic data and professional calculation software, making it suitable for scenarios with high precision requirements.
When constructing high-power FM transmitting stations, coverage calculations help reasonably select station locations and parameters such as transmitting antenna height and gain to achieve effective coverage of the target area. Additionally, they evaluate the coverage effectiveness and costs of different schemes to select the optimal construction plan.
By calculating the coverage area and field strength distribution of FM signals, potential frequency interference can be analyzed. For areas prone to interference, measures such as adjusting transmission frequencies or adding filtering equipment can be taken to improve signal quality and stability.
Regular field strength testing in existing FM coverage areas, combined with comparative analysis of test results and calculation data, can identify discrepancies between actual and expected coverage. If inconsistencies are found (e.g., equipment faults, environmental changes), optimization measures such as adjusting antenna parameters or increasing transmission power can be implemented to enhance coverage quality.
High-power FM coverage calculation is a critical link in FM broadcasting engineering, involving multiple factors and calculation methods. Accurate coverage calculations enable rational resource allocation, improve broadcast signal quality and efficiency, and provide a better listening experience for audiences. In practical applications, selecting appropriate calculation methods based on specific scenarios, combined with on-site testing and optimization, is essential to continuously refine FM coverage systems and adapt to evolving broadcasting needs and environmental conditions.
