Omnidirectional antenna, that is, 360 ° uniform in the horizontal patternradiation, that is to say nothingdirectional, on the vertical patternperformanceIt has a certain widthbeamIn general, the smaller the lobe width,gainThe larger.Omnidirectional antenna atMobile communication systemGenerally used insuburban countyThe station type of regional system covers a large area.[1]
directional antennaOn the horizontal pattern, it is shown as radiation in a certain angle range, that is, directivity.As with omnidirectional antennas, the smaller the lobe width, the greater the gain.Directional antenna is generally used in communication systems where the communication distance is long, the coverage is small, the target density is large, and the frequency utilization is high.
Relationship between the two
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The omnidirectional antenna will transmit signals in all directions, and can receive signals in front, back, left and right. The directional antenna is like covering a bowl shaped reflector behind the antenna, and the signal can only be transmitted to the front. The signal that shoots behind is blocked by the reflector and reflected to the front, strengthening the signal strength in front.The figure below shows the signal radiation of directional antenna.The main radiation range of the directional antenna is like an inverted, incomplete cone.[2]
directional antenna
For omnidirectional antenna, the smaller the lobe width, the greater the gain.Omnidirectional antenna atcommunication system In general, the application distance is close, the coverage is large, and the price is cheap.The gain is generally below 9dB.The following figure shows the signal radiation diagram of omnidirectional antenna.[1]The radiation range of omnidirectional antenna is more like an apple.
Schematic diagram of omnidirectional antenna
Purchasing method
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If multiple stations need to be met and these stations are distributed in different directions of the AP, omnidirectional antennas need to be used;If it is concentrated in one direction, it is recommended to use directional antenna;In addition, it is also necessary to consider whether the antenna connector type is the same asAPWhether the matching and antenna gain size meet your needs;[1]
Installation of antenna
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For outdoor antenna, lightning protection equipment shall be added between antenna and wireless AP;For directional antenna, pay attention to the direction that the front of the antenna faces the remote station;The antenna should be installed as high as possible, and the sight distance between the antenna and the station should be as high as possible (visible to the naked eye, avoiding obstacles in the middle).
Radiation pattern
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Monopole
1.1 Microstrip fed monopoles.The planar monopole antenna has a simple structure and nearly omnidirectional radiation pattern. It is widely used in communication, as shown in the following figure.The biggest difference of this antenna lies in the deformation of the ground. The ground of a conventional antenna is often the size of the entire medium, while the ground of this monopole is only half of the medium, and one side is changed into an ellipse and a triangle. The ratio of the long and short half axes of the ellipse is 1.8. The monopole is also an ellipse, and the ratio of the long and short half axes is 1.2. The entire antenna is fed by microstrip,The ground width W and the length h of the feed slot are the main parameters affecting the antenna performance.[1]
1.2 Coplanar waveguide fed monopole antenna.One of its characteristics is the use of coplanar waveguide feeding,Coplanar waveguideAnd traditionalMicrostrip lineThere are two advantages: low frequencydeviationCharacteristic and wideimpedanceBandwidth.Another feature is that the radiation unit adopts hexagonal patch. Compared with other rectangles, squares and triangles, hexagonal has the inherent advantages of broadband characteristics.The feed part of the whole antenna forms a short circuit between the upper and lower layers by opening holes on the top and bottom of the grounded coplanar waveguide, thus preventing the generation of parasitic modes. The number of holes can also change the input impedance of the antenna.It can also be seen from the figure that the ground under the hexagonal patch is removed, which can promote the radiation of the wide side at the bottom.Good matching can be achieved by adjusting the width of the feeder and the gaps on both sides of the feeder, which also makes it easy for the antenna to get impedance matching.[1]
dipole
Monopole sumdipoleAlthough there is an omnidirectional radiation mode in theoryfeederThe electric field on the vertical plane inclines downward, which affects the radiation characteristics in the horizontal direction, so it is difficult to obtain the omnidirectional pattern.The antenna is composed of dipoles. By reasonably arranging the isolation between the dipole radiation unit and the feeder line, the influence of the feeder line is shielded, so that the horizontal direction can reach omnidirectional radiation. The antenna uses another dipole as the parasitic unit, so that the entire antenna works in twoFrequency band0.9GHZ and 2.0GHZ, and the antenna gain is very high. The two frequency bands reach 10dBi and 12dBi respectively.The whole antenna structure is: a metal tube in the middle, four pieces of media around the metal tube, two dipoles with parasitic units on each medium, the dipole length is 137mm, the operating frequency is 0.9GHZ, the parasitic unit length is 47mm, the operating frequency is 2.0GHZ, the center distance between two dipoles on the same substrate is 150mm, and the whole antenna uses a cylindrical shape with a diameter of 110mmRadomeWrap up.The feed part of the antenna uses a four way power splitter andBurren 。[1]
Common full line antenna
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Array omnidirectional antenna
It is difficult for general element antenna to form omnidirectional radiation, so we can consider forming an array to form omnidirectional pattern of array antenna.Moreover, the gain of a single antenna is generally limited. By forming an array, the gain can be improved to meet the requirements of omnidirectional high gain that we want to design.[1]
Microstrip omnidirectional antenna
microstrip antenna Because of its simple structure, low processing cost, light weight and many other advantages, it has become a key technology in the antenna field. Therefore, microstrip antennas with various performances are being studied, and omnidirectivity has also become a trend in the development of microstrip antennas. For example, the omnidirectional radiation performance of microstrip antennas can be achieved by cross feeding microstrip transmission lines.A multipoint excited linear array antenna, which is composed of multiple λ/2 microstrip segments in series. The floor of the microstrip segment and the guide strip are alternately placed on both sides of the dielectric substrate, and the width of the microstrip floor is variable. Cross connection is used to achieve the purpose of phase inversion.In addition to the transmission mode, there is also the discontinuity of the cross connection pointRadiation modeThe wave is transmitted along the conduction band and the inner surface of the floor. The size of the radiation is controlled by the floor width. In order to obtain good omnidirectional performance, the width is limited to about λ/4.To achieve good radiation efficiency of the antennaimpedance matching , add a rectangular patch on each section of the guide belt by addingReactanceComponent to improve matching.[1]
