Antenna system definition, performance parameters, antenna type and feeder system
An antenna system is a setup that includes both a transmitting and a receiving antenna. The transmitting antenna functions as a mode converter, transforming guided wave signals—such as radio frequency currents or electromagnetic waves—into free-space electromagnetic waves. Conversely, the receiving antenna performs the reverse function, converting these spatial waves back into guided signals. These two components are often interchangeable in terms of functionality, as they share the same fundamental characteristics, which is known as the reciprocity theorem.
One of the key roles of an antenna is to concentrate the energy of electromagnetic waves. When used as a transmitter, it focuses energy in a specific direction while minimizing it in others. As a receiver, it captures more energy from the desired direction and reduces interference from other directions through phase cancellation. This directional property is referred to as directivity, and the degree of concentration is measured by the antenna's gain compared to an isotropic (non-directional) antenna. The opposite effect, where energy is reduced in non-communication directions, is called side lobe suppression and is crucial for reducing interference.
In mobile communication systems, the antenna serves as a bridge between the circuit and the wireless medium. It converts electrical signals into electromagnetic waves for transmission and vice versa. The antenna system typically includes the base station antenna, feed cables, RF devices, and installation services. Understanding the performance parameters of these antennas is essential for optimizing network coverage and signal quality.
The main performance indicators of a base station antenna include:
1. **Working Frequency Band**: The range of frequencies over which the antenna operates effectively.
2. **Input Impedance**: The ratio of voltage to current at the antenna input, usually 50 Ω for mobile communications.
3. **Voltage Standing Wave Ratio (VSWR)**: A measure of how well the antenna is matched to the transmission line.
4. **Isolation**: The ability of a dual-polarized antenna to prevent signal leakage between ports.
5. **Third-Order Intermodulation**: Unwanted signals generated due to nonlinearities in the system.
6. **Power Capacity**: The maximum power the antenna can handle without degradation.
7. **Gain**: A measure of the antenna’s directional strength.
8. **Beamwidth**: The angular width of the main lobe, indicating the directionality of the antenna.
9. **Electrical Downtilt**: The angle at which the main beam is tilted downward.
10. **Front-to-Back Ratio**: The ratio of signal strength in the forward direction versus the backward direction.
11. **Side Lobe Suppression**: Reducing unwanted radiation in non-intended directions.
12. **Cross Polarization Ratio**: A measure of polarization purity.
13. **Circularity**: The uniformity of the horizontal pattern.
14. **Polarization**: The orientation of the electric field in the transmitted wave.
Mobile communication antennas come in various types, including indoor distributed antennas, outdoor base station antennas, and aesthetically designed antennas. Each type serves different coverage needs and environments, from small indoor spaces to large outdoor areas.
Feeder systems play a vital role in connecting the antenna to the transceiver. They consist of coaxial cables, combiners, filters, and POI (Point of Interface) devices, all of which ensure efficient signal transmission and reduce interference. These components are essential for maintaining high-quality communication in modern networks.
Overall, the design and selection of antenna systems are critical for ensuring reliable and efficient wireless communication across diverse applications.
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