Antenna system is composed of transmitting antenna and receiving antenna. The former is a transmission mode converter that converts the radio frequency current or electromagnetic wave of guided wave mode into the space electromagnetic wave of diffuse wave mode, and the latter is a transmission mode converter of inverse transformation.

As the transmitting antenna used in the mode conversion of guided traveling wave to diffusion wave, the receiving antenna I used in the mode conversion of diffusion wave to guided traveling wave is called reciprocity theorem, except that the transmitting antenna's power bearing capacity and voltage bearing capacity are far greater than the receiving antenna's, both of them can be exchanged and the basic characteristic parameters of the antenna remain unchanged. Another important function of the antenna is to concentrate the energy of the electromagnetic wave, that is, to concentrate the energy in the transmitting direction while reducing the energy in other directions when it is used as the transmitting antenna; when it is used as the receiving antenna, more energy can be intercepted from the incoming waves in the receiving direction, while the incoming waves in other directions can be reduced by phase cancellation. This is the directivity of the antenna. Compared with the non directional antenna, the increase of the energy concentration is called the gain of the antenna. The extended meaning of antenna directivity is negative gain (attenuation) in non-communication direction, which can be used to describe another related performance index of antenna, i.e. sidelobe (interference) radiation rejection of transmitting antenna or incoming wave interference rejection of receiving antenna in non-communication direction.

1. Definition of mobile communication antenna system

Definition and scope of antenna system

In the mobile communication system, the communication antenna is the converter of the communication equipment circuit signal and the space radiation electromagnetic wave. This paper mainly analyzes the antenna and feeder system in mobile communication system, including base station / room sub-antenna, related feeder cable and other radio frequency devices and related installation services.

Two. Performance parameter description of base station antenna

Ge index

1, working frequency band (Frequency Range)

Working frequency band: whether antenna or other communication products, always in a certain frequency range (bandwidth), depending on the requirements of the indicators. Under normal circumstances, the frequency range that meets the requirements can be the working frequency of the antenna.

The working bandwidth is called working bandwidth. Generally, the working bandwidth of omnidirectional antenna can reach 3-5% of the central frequency, and the working bandwidth of directional antenna can reach 5-10% of the central frequency.

2. Input impedance (Input Impedance)

Input impedance: the ratio of the signal voltage to the signal current of the antenna input is called the input impedance of the antenna. The input impedance of the general mobile communication antenna is 50 ohms.

The input impedance is related to the structure, size and working wavelength of the antenna. In the required operating frequency range, the imaginary part of the input impedance is very small and the real part is quite close to 50_. This is necessary for the antenna to be in good impedance matching with the feeder.

3. Voltage Bobbi (VSWR)

Voltage Standing Wave Ratio (VSWR): The VSWR of an antenna is the ratio of the maximum value to the minimum value in the VSWR pattern generated along the transmission line when the antenna is used as the load of a lossless transmission line.

The standing wave ratio (SWR) is generated by the superposition of reflected waves generated by the incident wave energy transmitted to the input of the antenna and not completely absorbed (radiated). The bigger the VSWR, the bigger the reflection and the poorer the match. In mobile communication systems, Bobbi is generally less than 1.5.

4, isolation degree (Isolation)

Isolation represents the ratio of signals fed to one port (one polarization) of a bipolar antenna to signals fed to another port (the other polarization).

5 and three order intermodulation (Third Order Inter modulation)

Third-order intermodulation signal: refers to two signals in a linear system, due to the existence of non-linear factors so that the second harmonic of one signal and the fundamental wave of another signal to produce beat (mixing) after the parasitic signal.

Intermodulation is the phenomenon of system performance degradation caused by two or more carrier frequencies outside the band mixing and falling into the new frequency components in the band.

6. Power capacity (Power Capacity)

Power capacity: The power capacity of an antenna is the maximum continuous RF power that can be continuously added to the antenna within a specified period of time without degrading its performance.

Space radiation index

7, gain (Gain)

The ratio of the radiated power flux density of the antenna in a given direction to the maximum radiated power flux density of the reference antenna (usually using an ideal point source) at the same input power.

Antenna gain is used to measure the antenna's ability to transmit and receive signals in a specific direction. It is one of the important parameters in selecting base station antenna. The higher the antenna gain, the better the directivity. The more concentrated the energy, the narrower the lobe.

8, H/V-Plane Half Power Beam Width

In the main lobe of the power pattern, the angle between the beam widths of the two points which reduce the relative maximum radiation power to half or less than the maximum 3dB is called the half power lobe width.

The horizontal half-power beam width is called the horizontal beam width; the vertical half-power beam width is called the vertical beam width.

9. Electrical Down Tilt.

The dip angle refers to the angle between the maximum radiation direction on the vertical radiation surface of the communication antenna and the antenna normal.

The communication antenna is divided into fixed dip antenna and tuning antenna according to whether or not it supports electric dip adjustment: fixed dip antenna refers to the fixed dip antenna generated by configuring the amplitude and phase of the antenna radiation element array according to the wireless coverage requirements; and tuning antenna refers to the change of different radiation elements in the array through the phase-shifting element. Phase difference results in different radiation main lobe dip states. Generally, the dip state of an electrically tuned antenna is only in a certain range of adjustable angles.

10, front to back ratio (Front-to-Back Ratio)

The antenna front-to-back ratio is defined as the ratio of the maximum radiation direction of the main lobe (specified as 0 degrees) to the maximum power flux density near the opposite direction (specified as 180 degrees (+ 30 degrees) F/B = 10 log (front-to-back power / backward power).

11. Sidelobe suppression and zeros filling (Elevation Upper Side lobes & Null Fill)

Side lobe suppression: The side lobe of the main lobe in the vertical direction (i.e. the direction toward the zenith angle) is called the superior side lobe. In order to cover the effect of base station antenna, the antenna will be mechanically declined in network planning. This may lead to the antenna's first (or a certain angle range) upper sidelobe may be in a horizontal position or even lower than the horizontal position, it is easy to cause adjacent interference, so it is necessary to suppress it, that is, the upper sidelobe suppression.

The upper sidelobe not only wastes the energy radiated by the antenna, but also interferes with the adjacent cells, especially the high-rise buildings in the adjacent cells. Therefore, the upper sidelobe should be restrained as far as possible, especially the first upper sidelobe with larger energy.

Zero filling: refers to filling the first zero of the lower sidelobe in the vertical plane of the antenna with beam-shaped design to improve the coverage of the base station near the dead zone and reduce the near zone coverage and blind spots.

12, cross polarization ratio (Cross Polarization Ratio)

The difference between the power level of the antenna co-polarization receiving (the maximum receiving level) and the power level of the heteropolarization receiving (the minimum receiving level) within the beam width of Direction 3 dB

13. Direction of roundness (Circularity)

The pattern roundness of omnidirectional antenna refers to the deviation between the maximum or minimum level and the average value in the horizontal pattern.

Average refers to the arithmetic average of the level dB value at a maximum interval not exceeding 5 degrees in a horizontal pattern.

14. Polarization mode (Polarization)

The electric field direction of electromagnetic wave radiated by antenna is the polarization direction of antenna. If the electric field direction of a wave is perpendicular to the ground, we call it a vertical polarized wave; if the electric field direction of the wave is parallel to the ground, it is called a horizontal polarized wave; if the electric field direction of the wave is at an angle of 45 degrees with the ground, it is called +45 degrees or -45 degrees polarization.

Three. Types of mobile communication base station antennas

There are many kinds of mobile communication antenna products with different models. According to the different application scenarios, they can be divided into indoor distributed antenna products, outdoor base station antenna products, beautifying antenna products.

I. indoor distribution and cell coverage antenna products.

1. ceiling antenna

The roof-absorbing antenna is generally used for indoor wireless coverage. According to its different radiation forms, it can be divided into directional roof-absorbing antenna and omni-directional roof-absorbing antenna. The omni-directional roof-absorbing antenna can be divided into two types: unipolar roof-absorbing antenna and bipolar roof-absorbing antenna.

2. wall mounted antenna

Indoor wall hanging antenna is a typical small plate antenna product, mainly used for indoor wireless coverage scene, according to the different polarization mode, can be divided into single polarization wall hanging and dual polarization wall hanging.

3. Yagi antenna

Yagi antenna is mainly used for link transmission and repeater, the cost is relatively low, in the two-dimensional plane before and after the reflectance is better.

4. log periodic antenna

Logarithmic periodic antenna, similar to Yagi antenna, is a multi-element bidirectional antenna with broadband coverage, mainly used for link relay.

5. parabolic antenna

Parabolic antenna is a high gain bidirectional antenna consisting of a parabolic reflector and a central feed antenna.

II. Outdoor base station antenna products

Definition, performance parameters, antenna type and feeder system of antenna system

1. omnidirectional base station

The omnidirectional base station antenna is mainly used for 360 degree wide coverage, which is mainly used to cover sparse rural wireless scenes.

2. directional base station antenna

Directional base station antenna is the most widely used fully enclosed base station antenna, which can be divided into several types, including vertical polarization antenna, vertical and horizontal polarization antenna, +45 degree dual polarization antenna, multi-band antenna and so on. It can be divided into fixed tilt antenna, electrically tuned antenna and three-sector cluster antenna.

3. electrically modulated base station antenna

Electronically tuned antenna is a kind of antenna that changes the phase difference of different radiation elements in the array by phase shifting element, and thus produces different radiation main lobe dip state. Generally, the dip state of electronically tuned antenna is only in a certain range of adjustable angles. The electric dip adjustment has manual adjustment and RCU electric regulation.

4. smart antenna

A dual polarized radiation unit is used to form an array of directional or omnidirectional antennas, which can scan the beam in 360 degrees or a specific direction; a smart antenna can determine the spatial information of the signal (such as the direction of propagation) and an intelligent algorithm for tracking and locating the signal source, and can filter the sky in the spatial domain according to this information. Line array.

5. multimode antenna

Multimode base station antenna is different from common base station antenna in that it integrates more than two different frequency bands in a limited space. Therefore, the emphasis of this product is to eliminate the interaction between different frequency bands (decoupling effect, isolation degree, near-field interference).

6. multi beam antenna

Multi beam antennas can produce multiple sharp beams. These sharp beams (called elementary beams) can be combined into one or more shaping beams to cover a particular airspace. Multi beam antennas have three basic forms: lens, reflector and phased array.

III. active antenna

The passive antenna is combined with the active device to form an integrated receiving antenna.

4. Beautification antenna

1. indoor beautification antenna

The beautification of different indoor distributed antenna products not only solves the problem of indoor signal coverage, but also does not destroy the layout of finishing and decoration. It is.

According to the shape of products, indoor covering beautification antenna can be roughly divided into: ceiling lamp beautification antenna, mural beautification antenna and exhaust fan and so on.

2. outdoor cover beautification antenna

The outdoor covering and beautifying antenna mainly aims at the antenna application products such as residential area and base station. It camouflages and modifies the appearance of the antenna through the application of various materials, structures and patterns without increasing the propagation loss. It not only beautifies the city visual environment, but also reduces the public's fear and resistance to the radio electromagnetic environment. Emotion can also prolong the life of the antenna and ensure the quality of communication.

According to the shape of the product, the outdoor covering beautifying antenna can be roughly divided into: streetlight beautifying antenna, logo beautifying antenna, monitoring spherical beautifying antenna, air conditioning beautifying antenna, rockery beautifying antenna, speaker beautifying antenna, simulated tree beautifying antenna, square column beautifying antenna, chameleon beautifying antenna, etc. Water tower beautification antenna, fence type beautification antenna, exhaust pipe type beautification antenna and so on.

Four, mobile communication feeder passive devices and others

The feeder system is connected between the transmitter, the receiver and the antenna. The feeder system is mainly used to transmit the high frequency power of the transmitter to the antenna and transmit the reflected signal received by the antenna to the receiver.

The mobile communication system includes not only base station/compartment antenna, but also feeder cable, passive devices (such as combiner, filter, POI, etc.) and other radio frequency devices. These are essential components of communication system.

1. RF feeder cable

RF feeder cable can be divided into semi-flexible coaxial cable and semi-rigid coaxial cable; according to the different types, it can be divided into 1/4, 3/8, 1/2, 5/8, 7/8, 1-1/4, 1-5/8, and other different sizes, which are mainly used for indoor and outdoor radio frequency signal transmission.

The RF cable inside the mobile communication antenna is also RF feeding cable, which is mainly used for feeding jumper connectors, power distribution network feeding, network impedance matching and so on.

2. way divider

The combiner is mainly used to combine multiple system signals to a set of indoor distribution systems. In engineering application, a set of indoor distributed system can work in different communication frequency bands simultaneously by using a combiner. 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 the signal which needs part of the frequency to pass smoothly, while the signal of the other part of the frequency is greatly suppressed. The filter is generally divided into active filter and passive filter. Cavity filters in passive filters are commonly used in mobile communication systems. Their main characteristics are wide frequency coverage, high reliability, good stability, input and output impedance matching, easy cascade use, flat in-band amplitude-frequency characteristics, low insertion loss, and high out-of-band suppression.

4.POI

Point Of Interface, multi system road platform. It is mainly used for indoor coverage of large buildings such as subway, convention and Exhibition Center, exhibition hall and airport. The system uses frequency combiner and Bridge combiner to introduce antenna-fed distribution system after combining multiple operators and multiple mobile signals, so as to make full use of resources and save investment.

To avoid interference, POI is divided into two platforms, upstream and downstream, respectively, the uplink and downlink signals are transmitted separately. POI is a bridge between the donor signal and the distributed coverage signal (leaky cable and antenna array) in wireless communication. Its main function is to combine and channel the upstream and downstream RF signals of each operator, and filter out the interference components between each band. The main function of the upstream part of POI is to collect signals from different mobile phones through antennas and transmit them to the upstream POI through feeders. The signals of different frequency bands are detected by POI and sent to the base stations of different operators. The main function of the downlink part of POI is to synthesize carrier signals of various operators and different frequency bands and send them to the sky-feed distribution system of the coverage area. (MWRF junior editor)