Antenna system definition, performance parameters, antenna type and feeder system
An antenna system is a set of components 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 operation, converting spatial electromagnetic waves back into guided wave signals. These two components are often interchangeable in many applications, as their fundamental characteristics remain consistent due to the principle of reciprocity. This principle highlights that the performance of an antenna remains the same whether it is used for transmission or reception.
Another critical function of an antenna is its ability to concentrate electromagnetic energy in specific directions. When acting as a transmitter, the antenna directs energy toward the desired direction while minimizing radiation in other areas. As a receiver, it captures more signal energy from the intended direction and reduces interference from unwanted directions through phase cancellation. This directional behavior is known as the antenna's directivity. The gain of an antenna refers to how much it amplifies the energy in the desired direction compared to an isotropic (non-directional) antenna. Additionally, the suppression of energy in non-communication directions is referred to as side lobe reduction, which helps minimize interference in both transmitting and receiving scenarios.
The mobile communication antenna system is a key component in wireless networks, responsible for converting electrical signals into radiated electromagnetic waves. This system typically includes base station antennas, feeder cables, RF devices, and installation services. It plays a vital role in ensuring efficient signal transmission and reception across different coverage areas.
**Performance Parameters of Base Station Antennas**
1. **Working Frequency Band**: The range of frequencies over which the antenna operates. For example, omnidirectional antennas usually have a 3–5% bandwidth relative to the center frequency, while directional antennas can reach up to 5–10%.
2. **Input Impedance**: Typically 50 Ω for most mobile communication antennas. A well-matched impedance ensures minimal signal loss and optimal power transfer.
3. **Voltage Standing Wave Ratio (VSWR)**: Measures the efficiency of power transfer between the antenna and the transmission line. A VSWR below 1.5 is generally acceptable in mobile systems.
4. **Isolation**: Refers to the degree of signal separation between different ports in a dual-polarized antenna.
5. **Third-Order Intermodulation**: Caused by nonlinearities in the system, leading to unwanted frequency components that degrade signal quality.
6. **Power Capacity**: The maximum continuous power the antenna can handle without performance degradation.
7. **Gain**: A measure of how effectively the antenna directs or receives energy in a particular direction. Higher gain means better directivity and narrower beamwidth.
8. **Half-Power Beamwidth**: The angular width of the main lobe where the power drops to half its maximum value, measured in horizontal and vertical planes.
9. **Electrical Downtilt**: Adjusts the direction of the main lobe downward, improving coverage in lower areas. It can be fixed or electronically adjustable.
10. **Front-to-Back Ratio**: The ratio of the power radiated in the forward direction to that in the backward direction, indicating the antenna’s directional efficiency.
11. **Sidelobe Suppression and Null Fill**: Reduces unwanted side lobes and fills in low signal areas to improve coverage and reduce interference.
12. **Cross Polarization Ratio**: Measures the difference in signal strength between co-polarized and cross-polarized reception.
13. **Circularity**: Evaluates the uniformity of the horizontal radiation pattern.
14. **Polarization**: Defines the orientation of the electric field. Common types include vertical, horizontal, and ±45° polarization.
**Types of Mobile Communication Base Station Antennas**
Base station antennas come in various forms, including indoor distributed antennas, outdoor base station antennas, and aesthetic antennas designed for visual integration.
- **Indoor Antennas**: Such as ceiling, wall-mounted, Yagi, log-periodic, and parabolic antennas, used for localized coverage.
- **Outdoor Antennas**: Including omnidirectional, directional, and smart antennas, offering broader coverage and advanced beamforming capabilities.
- **Active Antennas**: Combine passive elements with active components for enhanced performance.
- **Aesthetic Antennas**: Designed to blend into urban environments, such as street lamps, signs, and simulated trees.
**Feeder System and Passive Components**
The feeder system connects the transceiver to the antenna, carrying high-frequency signals. Key components include RF cables, combiners, filters, and POI (Point of Interface) units. These elements ensure efficient signal distribution, isolation, and interference management, playing a crucial role in maintaining communication quality and system reliability.
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