In the extensive realm of wireless communication, the antenna, as an indispensable critical component, serves as a bridge linking the information world. Its performance directly dictates the quality of communication. The three primary indicators of antenna gain, polarization, and bandwidth are analogous to the cornerstones of a bridge, underpinning the functionality of the antenna. A profound comprehension of these three indicators is pivotal for optimizing wireless communication systems and enhancing the quality of signal transmission and reception. Below, we delve into each of these key indicators sequentially.
I. Antenna Gain: The "Focusing Mechanism" for Signal Concentration
(1) Definition and Connotation of Gain
Antenna gain is a key metric employed to quantitatively characterize the extent to which an antenna concentrates and radiates input power. From a communication standpoint, it reflects the efficacy of the antenna in generating signals within a specific direction. In an ideal scenario, an isotropic radiator with uniform power distribution radiates energy omnidirectionally in space. For such a radiator, the gain is defined as 1, equivalent to 0dB when expressed in decibels. However, practical antennas, through meticulously engineered structures, deviate from this uniform radiation pattern, strategically concentrating energy for radiation in specific directions, thereby achieving a gain superior to that of an ideal point-source antenna.
Mathematically, antenna gain is the ratio of the square of the field intensity generated by the actual antenna to that produced by an ideal radiating element at the same spatial point, given equal input power, i.e., the power ratio. For instance, to generate a signal of a particular intensity at a spatial point, an ideal radiation source might necessitate an input power of 126W. When utilizing an antenna with a gain of 18dBd, calculations reveal that a mere 2W of input power suffices to attain the same outcome. This vividly illustrates the "amplification-like" effect of antenna gain on signals. Importantly, this "amplification" does not involve an actual increase in signal power, as in active circuits, but rather a more efficient directional allocation of energy.
(2) Calculation Methods of Gain
The computation of actual antenna gain is not a straightforward arithmetic process; it is the product of the directivity coefficient and antenna efficiency. The directivity coefficient quantifies the ratio of the radiation intensity of the antenna in its maximum radiation direction to the average radiation intensity of an ideal point-source antenna, intuitively demonstrating the antenna's capacity to focus energy in a specific direction. Antenna efficiency accounts for inevitable energy losses during the conversion of input power to radiated power, such as thermal losses arising from the resistive properties of the antenna material.
Diverse antenna types employ distinct gain calculation methodologies. For the common parabolic antenna, gain can be approximated using the formula G (dBi) = 10Lg {4.5×(D/λ0)²}, where D denotes the diameter of the parabolic reflector, λ0 is the central operating wavelength, and 4.5 is empirical data derived from extensive practical observations. The gain of a vertical omnidirectional antenna can be estimated via G (dBi) = 10Lg {2L/λ0}, with L representing the antenna length. Additionally, gain can be calculated based on the half-power (3dB) beamwidths in the two principal planes (E-plane and H-plane) using the formula G (dBi) = 10Lg {32000/(2θ3dB,E × 2θ3dB,H)}, where 2θ3dB,E and 2θ3dB,H are the beamwidths in the respective principal planes, and 32000 is also empirical data.
(3) Practical Applications of Gain
In long-range communication scenarios, high-gain antennas, akin to precision instruments, play a vital role. Take satellite communication: the substantial distance between satellites and ground stations results in significant signal attenuation during transmission. Here, high-gain antennas can intensely focus signal energy, enabling it to traverse vast spatial distances and accurately reach the target receiver. In microwave relay communication, high-gain antennas ensure that signals maintain sufficient strength along the lengthy transmission path, facilitating stable and reliable communication links.
Conversely, in short-range communication environments such as indoor wireless coverage, the situation differs. The complex indoor setting demands uniform signal distribution across multiple directions to cater to users in various locations. Hence, low-gain, omnidirectional antennas are predominantly utilized. These antennas function like signal spreaders; although the signal strength in any single direction is relatively modest, they can radiate signals omnidirectionally within a defined range, providing relatively uniform signal coverage for indoor users.
II. Antenna Polarization: The "Spatial Orientation" of Electromagnetic Waves
(1) Definition and Essence of Polarization
Polarization is a physical quantity that precisely describes the spatial orientation of the electric field vector of electromagnetic waves, profoundly revealing the temporal variation law of the electric field direction of waves radiated by the antenna. From a microscopic perspective, polarization reflects the rotational mode and orientational characteristics of the electric field vector in space, a property that exerts a profound influence on the antenna's signal transmission and reception capabilities.
(2) Analysis of Polarization Types
Antenna polarization encompasses three fundamental categories: linear polarization, circular polarization, and elliptical polarization. Linear polarization is further subdivided into horizontal and vertical polarization. A vertically polarized wave has its electric field direction perpendicular to the ground, while a horizontally polarized wave has an electric field direction parallel to the ground. Additionally, polarizations at 45° to the ground, such as +45° or -45°, fall under the linear polarization category. Circular polarization is classified into left-hand circular polarization and right-hand circular polarization based on the rotational direction of the electric field vector, whose spatial trajectory is circular. Elliptical polarization is a more general form, combining features of linear and circular polarization, with the electric field vector tracing an elliptical path in space. Both circular and linear polarizations can be regarded as special cases of elliptical polarization under specific conditions.
(3) Application Examples of Polarization in Various Fields
In radio and television broadcasting, vertical polarization is frequently adopted to ensure stable wide-area signal coverage. This is because vertically polarized waves are relatively less susceptible to ground reflection and multipath effects during propagation, enabling stable signal transmission.
Mobile communication base station antennas predominantly utilize horizontal polarization or ±45° cross-polarization. Horizontal polarization offers advantages in mitigating co-channel interference, while ±45° cross-polarization better adapts to the complex and dynamic mobile communication environment, enhancing the ability to receive signals from diverse directions and improving the reliability and capacity of the communication system.
In satellite communication, circularly polarized antennas are preferred. Owing to the continuous attitude variations of satellites in space and the interference from various complex factors during signal propagation, circularly polarized antennas can effectively reduce signal loss caused by polarization mismatch, ensuring unimpeded communication between satellites and ground stations.
In RFID systems, circularly polarized antennas are also crucial. They enable effective identification of tags in different orientations, significantly enhancing the system's recognition efficiency and accuracy, and providing robust support for numerous application scenarios such as logistics management and access control systems.
III. Antenna Bandwidth: The "Frequency Range" for Effective Operation
(1) Definition of Bandwidth
Antenna bandwidth refers to the frequency range within which the antenna can operate effectively. Within this range, the antenna meets predefined performance criteria, including gain, standing wave ratio, and polarization characteristics. It serves as a frequency band where signals of varying frequencies can be transmitted and received, with the antenna ensuring a favorable environment for these processes.
(2) Distinction of Bandwidth Types
Common definitions of antenna bandwidth include absolute bandwidth and relative bandwidth. Absolute bandwidth is the difference between the upper and lower limits of the antenna's operating frequency range, with units such as hertz (Hz), kilohertz (kHz), or megahertz (MHz). For example, an antenna operating from 1GHz to 2GHz has an absolute bandwidth of 1GHz. Relative bandwidth is the ratio of the absolute bandwidth to the center frequency, typically expressed as a percentage. The center frequency can be calculated using the arithmetic mean, fcenter = (fmax + fmin)/2, or the geometric mean, which is more prevalent on a logarithmic scale, fcenter = sqrt(fmax⋅fmin). Relative bandwidth can also be computed as BWrel = 2*(fmax − fmin)/(fmax + fmin) × 100%. Generally, narrowband antennas have a relative bandwidth of less than 5%, wideband antennas range from 5% to 25%, and ultra-wideband antennas exceed 25%.
(3) Application Requirements of Bandwidth in Different Scenarios
Narrowband antennas, due to their strong frequency selectivity, are employed in communication systems requiring high frequency precision. For instance, in radio and television transmission, where operating frequencies are relatively fixed, narrowband antennas provide stable signal transmission at specific frequencies, avoiding interference from other frequency signals. Specialized wireless communication systems, such as those in certain industrial control domains with stringent requirements for frequency stability and interference immunity, also benefit from narrowband antennas.
Wideband antennas are suitable for complex communication scenarios requiring coverage of multiple frequency bands. In mobile communication base stations, the evolving communication technology necessitates support for multi-band signal transmission to meet diverse user and service demands. Wideband antennas maintain satisfactory performance across a broad frequency range, ensuring efficient communication between base stations and various terminal devices. Similarly, wireless local area networks (WLAN) rely on wideband antennas to accommodate wireless devices of different standards and frequency bands, offering users convenient and high-speed network connectivity.
Ultra-wideband antennas play a unique role in radar detection. Their extremely wide bandwidth provides high-resolution target detection capabilities, enabling accurate identification of target position, shape, and motion status. In short-range high-speed communication, such as indoor high-speed data transmission, ultra-wideband antennas leverage their large bandwidth to achieve data rates of several gigabits per second, meeting the demand for high-speed, large-capacity data transmission.
IV. Interrelation and Comprehensive Consideration of the Three Indicators
The three key indicators of antenna gain, polarization, and bandwidth are not isolated; they are interrelated and mutually influential. Antenna design often requires careful trade-offs and optimizations among these indicators.
Increasing antenna gain typically involves narrowing the radiation beamwidth. While this enhances signal strength in a specific direction, it concurrently reduces bandwidth. This is because narrowing the beamwidth alters the antenna's response to signals of different frequencies, shrinking the effective operating frequency range.
Polarization characteristics also impact antenna bandwidth and gain performance. Antennas with different polarization modes exhibit distinct spatial distributions and variation patterns of the electric field vector during signal radiation and reception, leading to differences in their coupling capabilities with signals of varying frequencies. For example, a circularly polarized antenna may exhibit excellent gain performance within certain frequency ranges but experience gain degradation in others due to factors like polarization mismatch, thereby affecting bandwidth performance.
In practical applications, a comprehensive consideration of these three indicators is essential based on specific communication requirements and scenarios when selecting or designing an appropriate antenna. For example, in a mountainous communication project with high demands on signal coverage range, directionality, and strict frequency band constraints, a high-gain, narrowband antenna with a polarization mode suitable for mountainous terrain may be required to ensure signals traverse complex terrain and accurately cover the target area. In a large shopping mall's indoor communication environment, which needs to support multiple wireless devices and has high requirements for signal bandwidth and coverage uniformity, a wideband, low-gain antenna with a polarization mode adaptable to complex indoor reflections is more suitable, providing stable and high-speed wireless network services for customers and staff.
In summary, a thorough understanding of the three key indicators of antenna gain, polarization, and bandwidth, along with their interrelationships, forms the foundation for achieving efficient and reliable wireless communication. Only through rational optimization and configuration of these indicators based on specific needs in practical applications can antennas deliver optimal performance, providing a solid foundation for the advancement of wireless communication.
