The antenna system is a system composed of a transmitting antenna and a receiving antenna. The former is a transmission mode converter that transforms the radio frequency current or electromagnetic wave in the guided wave mode into a space electromagnetic wave in the diffuse wave mode; the latter is the transmission mode converter for its inverse conversion.
As a transmitting antenna for mode conversion of guided traveling wave to diffuse wave, and a receiving antenna for mode conversion of diffused wave to guided wave mode I, except that the power carrying capacity and voltage withstanding capacity of the transmitting antenna are much greater than that of the receiving antenna, both are It can be used interchangeably, and the basic characteristic parameters of the antenna remain unchanged, which is called the reciprocity theorem. Another important function of the antenna is the concentration of electromagnetic wave energy, that is, when used as a transmitting antenna, the energy is concentrated in the transmitting direction while reducing energy in other directions; when used as a receiving antenna, more energy can be intercepted from incoming waves in the receiving direction. For incoming waves in other directions, the input energy is reduced by phase cancellation. This is the directivity of the antenna. Compared with non-directional antennas, the increase in energy concentration is called antenna gain. The extended meaning of antenna directivity is the negative gain (attenuation) in the non-communication direction, which can be used to describe another related performance index of the antenna, that is, the side lobe (interference) radiation suppression of the transmitting antenna or the incoming wave interference in the non-communication direction of the receiving antenna Inhibition.
Definition and scope of antenna system
In the mobile communication system, the communication antenna is the converter of the circuit signal of the communication device and the electromagnetic wave radiated from space. This article mainly analyzes the part of the communication antenna and feeder system in the mobile communication system, which mainly includes the base station/indoor antenna, related feeder cables and other radio frequency devices and related installation services.
2. Description of the performance parameters of the base station antenna
General Electric Index
1. Frequency Range (Frequency Range)
Working frequency band: No matter the antenna or other communication products, it always works within a certain frequency range (bandwidth), which depends on the requirements of the index. Under normal circumstances, the frequency range that meets the index requirements can be the operating frequency of the antenna.
The width of the working frequency band is called the working bandwidth. Generally, the working bandwidth of an omnidirectional antenna can reach 3-5% of the center frequency, and the working bandwidth of a directional antenna can reach 5-10% of the center frequency.
2. Input Impedance
Input impedance: The ratio of the signal voltage to the signal current at the input of the antenna is called the input impedance of the antenna. Generally, the input impedance of a mobile communication antenna is 50Ω.
The input impedance is related to the structure, size, and operating wavelength of the antenna. Within the required operating frequency range, the imaginary part of the input impedance is small and the real part is quite close to 50Ω, which is necessary for the antenna to be in good impedance matching with the feeder.
3. Voltage standing wave ratio (VSWR)
Voltage standing wave ratio: The voltage standing wave ratio of the antenna is the ratio of the maximum value to the minimum value of the voltage standing wave pattern generated along the transmission line when the antenna is used as the load of a lossless transmission line.
The standing wave ratio is caused by the superposition of reflected waves generated by the incident wave energy transmitted to the input end of the antenna and not fully absorbed (radiated). The greater the VSWR, the greater the reflection and the worse the match. In mobile communication systems, the standing wave ratio is generally required to be less than 1.5.
4. Isolation
Isolation represents the proportion of a signal fed to one port (one polarization) of a dual-polarized antenna that appears in the other port (another polarization).
5. Third Order Inter modulation (Third Order Inter modulation)
Third-order intermodulation signal: refers to the parasitic signal after two signals are in a linear system, due to the existence of nonlinear factors, the second harmonic of one signal and the fundamental wave of another signal are beat (mixed).
Intermodulation is a phenomenon in which two or more carrier frequencies outside the frequency band are mixed and then fall into the frequency band, resulting in a decrease in system performance.
6. Power Capacity
Power capacity: The power capacity of an antenna refers to the maximum continuous RF power that can be continuously added to the antenna within a specified period of time under specified conditions without reducing its performance.
Space radiation index
7. Gain
The ratio of the radiated power flux density of the antenna in a specified direction to the maximum radiated power flux density of the reference antenna (usually an ideal point source) at the same input power;
Antenna gain is used to measure the ability of an antenna to send and receive signals in a specific direction. It is one of the important parameters for selecting a base station antenna. The higher the antenna gain, the better the directivity, the more concentrated the energy, and the narrower the lobe.
8. Horizontal/Vertical Half Power Beam Width (H/V-Plane Half Power Beam Width)
In the main lobe of the power pattern, the beam width angle between two points where the relative maximum radiation direction power drops to half or less than the maximum 3dB is called the half-power lobe width.
The half-power beam width in the horizontal plane is called the horizontal beam width; the half-power beam width in the vertical plane is called the vertical beam width.
9. Electrical Down Tilt (Electrical Down Tilt)
Electrical downtilt refers to the angle between the maximum radiation direction of the vertical radiation surface of the communication antenna and the antenna normal.
Communication antennas are classified into fixed downtilt antennas and electrically adjustable antennas according to whether they support electric downtilt adjustment: fixed downtilt antennas refer to fixed downtilt antennas produced by shaping the antenna radiating element array in amplitude and phase according to wireless coverage requirements; and Electrically adjustable antenna means that the phase difference of different radiating elements in the array is changed by a phase shifting unit to produce different radiation main lobe downtilt states. Generally, the downtilt state of an electrically adjustable antenna is only within a certain adjustable angle range.
10. Front-to-Back Ratio
The front-to-back ratio of the antenna refers to the ratio of the power flux density in the maximum radiation direction of the main lobe (specified as 0°) to the maximum power flux density near the opposite direction (specified as within the range of 180°±30°) F/B= 10log (front and rear power / backward power).
11. Side lobe suppression and zero fill (Elevation Upper Side lobes & Null Fill)
Side lobe suppression: The side lobe of the main lobe in the vertical direction (that is, the positive direction of the zenith angle) is called the upper side lobe. For the coverage of the base station antenna, a certain mechanical downtilt is usually adopted for the antenna in the network planning. This may cause the first upper side lobe of the antenna (or within a certain angle range) to be in a horizontal position or even lower than the horizontal position, which may easily cause neighboring interference. Therefore, it needs to be suppressed, that is, upper side lobe suppression.
The upper sidelobe not only wastes the energy radiated by the antenna, but also interferes with adjacent cells, especially the high-rise buildings of adjacent cells. Therefore, the upper sidelobe should be suppressed as much as possible, especially the first upper sidelobe with larger energy.
Zero point filling: It means that the first zero point of the lower side lobe is filled by beamforming design in the vertical plane of the antenna to improve the coverage of the near area of the base station and reduce the dead zone and blind spots of the near area coverage.
12. Cross Polarization Ratio (Cross Polarization Ratio)
The difference between the power level of the antenna with the same polarization reception (the maximum receiving level) and the power level of the different polarization reception (the minimum receiving level) within the 3dB beam width of the pattern
13. Circularity of the direction map (Circularity)
The circularity of the pattern of an omnidirectional antenna refers to the deviation of the maximum or minimum level value from the average value in the horizontal plane pattern.
The average value refers to the arithmetic average of the dB value of the level in the horizontal plane pattern with the maximum interval not exceeding 5°.
14. Polarization (Polarization)
The electric field direction of the electromagnetic wave radiated by the antenna is the polarization direction of the antenna. If the electric field direction of the electric wave is perpendicular to the ground, we call it a vertically polarized wave; if the electric field direction of the electric wave is parallel to the ground, it is called a horizontally polarized wave; if the electric field direction of the electric wave is at an angle of 45° with the ground, then It is called +45° or -45° polarization.
3. Types of mobile communication base station antennas
There are many types and models of mobile communication antennas. According to their application scenarios, they can be roughly divided into indoor distributed antenna products, outdoor base station antenna products, and beautification antenna products.
Ⅰ. Indoor distributed and cell coverage antenna products
1. Ceiling antenna
Ceiling antennas are generally used in indoor wireless coverage scenarios. According to their different radiation patterns, they can be divided into directional ceiling antennas and omnidirectional ceiling antennas. Omni-directional ceiling antennas can be divided into single-polarized ceiling-mounted and dual-polarized Ceiling two kinds.
2. Wall-mounted antenna
Indoor wall-mounted antennas are typical small panel antenna products, mainly used in indoor wireless coverage scenarios. According to different polarization methods, they can be divided into single-polarized wall-mounted and dual-polarized wall-mounted.
3. Yagi antenna
Yagi antennas are mainly used for link transmission and repeaters, with relatively low cost, and better front and back reflection ratio in a two-dimensional plane.
4. Log periodic antenna
Log-periodic antennas are similar to Yagi antennas. They are multi-element bidirectional antennas with broadband coverage capabilities and are mainly used for link relay.
5. Parabolic antenna
The parabolic antenna is a high-gain bidirectional antenna consisting of a parabolic reflector and a center-fed antenna.
Ⅱ. Outdoor base station antenna products
1. Omni-directional base station
The omni-directional base station antenna is mainly used for 360-degree wide coverage, and is mainly used for rural wireless scenes with sparse coverage.
2. Directional base station antenna
Directional base station antennas are currently the most widely used fully enclosed base station antennas. They are divided into multiple types, including: vertical polarization antennas, vertical and horizontal polarization antennas, ±45° dual polarization antennas, multi-band antennas, etc. According to the different ways of tilt angle electric adjustment, it can be divided into fixed tilt angle antenna, electric adjustment antenna, and also includes three-sector cluster antenna.
3. ESC base station antenna
Electrically adjustable antenna means that the phase difference of different radiating elements in the array is changed by a phase shifting unit to produce different radiation main lobe downtilt states. Generally, the downtilt state of an electrically adjustable antenna is only within a certain adjustable angle range. There are manual adjustments and RCU electric adjustments for ESC down tilt adjustment.
4. Smart antenna
Using dual-polarized radiation units to form a directional or omnidirectional array, an antenna array that can scan the beam in 360 degrees or a specific direction; smart antennas can determine the spatial information of the signal (such as the direction of propagation) and track and locate the signal source Intelligent algorithm, and based on this information, the antenna array for spatial filtering.
5. Multimode antenna
The main difference between multi-mode base station antenna products and ordinary base station antennas is that they integrate more than two antennas of different frequency bands in a limited space. Therefore, the focus of this product is to eliminate the mutual influence between different frequency bands (decoupling effect, isolation Degree, near-field interference)
6. Multi-beam antenna
A multi-beam antenna can produce multiple sharp beam antennas. These sharp beams (called element beams) can be combined into one or several shaped beams to cover specific airspace. Multi-beam antennas have three basic forms: lens type, reflective surface type and phased array type.
Ⅲ. Active antenna
Passive antennas and active devices are combined to form an integrated receiving antenna.
Ⅳ. Beautify the antenna
1. Indoor coverage beautification antenna
The beautification processing of different indoor distributed antenna products not only solves the problem of indoor signal coverage, but also does not destroy the layout of finishing decoration; general indoor coverage and beautification antennas are beautiful and small in appearance and have good invisible effects. They are suitable for various high-end residential, Shopping malls, hotels, hotels, office buildings, hospitals and other public places.
Indoor coverage and beautification antennas can be roughly divided into ceiling lamp type beautification antennas, mural type beautification antennas, exhaust fan type and so on.
2. Outdoor coverage beautification antenna
Outdoor coverage beautification antennas are mainly aimed at antenna application products such as cells and base stations. Without increasing the propagation loss, the appearance of the antenna is camouflaged and modified through the application of various materials, structures and patterns, which not only beautifies the city’s vision The environment also reduces the public’s fear and resistance to the wireless electromagnetic environment, while prolonging the service life of the antenna and ensuring the quality of communication.
Outdoor coverage beautification antennas can be roughly divided into: streetlight beautification antennas, signage beautification antennas, surveillance ball beautification antennas, air conditioning beautification antennas, rockery beautification antennas, speaker beautification antennas, artificial tree beautification antennas Antennas, square column beautifying antennas, chameleon beautifying antennas, water tower beautifying antennas, fence beautifying antennas, exhaust pipe beautifying antennas, etc.
4. Mobile communication feeder passive components and others
The feeder system is connected between the transmitter, receiver and antenna. The feeder system is mainly used to transmit the high frequency power of the transmitter to the antenna, and transmit the target reflection signal received by the antenna to the receiver.
In addition to the base station/room antenna, the mobile communication system also contains feeder cables, passive devices (including such as combiners, filters, POIs, etc.) and other radio frequency devices. These are all essential components of the communication system.
1. RF feeder cable
RF feeder cables can be divided into semi-flexible coaxial cables and semi-rigid coaxial cables; according to their different models, they can be divided into 1/4", 3/8", 1/2", 5/8", 7/8 ", 1-1/4", 1-5/8" and other different size models, these are mainly used for indoor and outdoor radio frequency signal transmission.
The radio frequency cable inside the mobile communication antenna is also an RF feed cable, which is mainly used for jumper connector feed, power division network feed, and network impedance matching.
2. Combiner and splitter
The combiner is mainly used to combine signals from multiple systems into an indoor distribution system. In engineering applications, the use of a combiner can make a set of indoor distributed systems work in different communication frequency bands at the same time. The combiners used in mobile communication systems generally include two-way combiners, three-way combiners, four-way combiners, and so on.
3. Filter
The function of the filter is to allow signals that require some frequencies to pass smoothly, while signals of other frequencies are greatly suppressed. The filters are generally divided into active filters and passive filters. The cavity filter used in the mobile communication system is generally the cavity filter in the passive filter. Its main characteristics are: wide frequency coverage, high reliability, good stability, input and output impedance matching, easy to cascade use, in-band amplitude Flat frequency characteristics, low insertion loss, high out-of-band suppression, etc.
4.POI
Point Of Interface, a multi-system integration platform. Mainly used for indoor coverage of large buildings such as subways, convention and exhibition centers, exhibition halls, and airports. The system uses a frequency combiner and a bridge combiner to combine the mobile signals of multiple operators and multiple formats and introduce an antenna feeder distribution system to achieve the purpose of making full use of resources and saving investment.
In order to avoid interference, POI is divided into two platforms, uplink and downlink, and the uplink and downlink signals are transmitted separately. POI serves as a bridge connecting wireless communication donor signals and distributed coverage signals (leaky cables and antenna arrays, etc.). Its main function is to combine and split the uplink and downlink RF signals of each operator, and filter out the frequency bands. The interference component. The main function of the uplink part of the POI is to collect the signals from mobile phones of different formats and transmit them to the uplink POI through the antenna collection and feeder. After the POI detects the signals of different frequency bands, they are sent to the base stations of different operators. The main function of the POI downlink part is to synthesize the carrier signals of various operators and different frequency bands and send them to the antenna distribution system of the coverage area.
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