In engineering, some parameters are often used to characterize the characteristics of the antenna used for transmission or reception. They are the pattern, main lobe level, gain, polarization, frequency bandwidth, standing wave ratio, and noise temperature. Only a few of the main parameters are briefly introduced here.
Pattern, main lobe width and side lobe level
Whether the antenna power radiation strike is concentrated can be expressed by the parameter of the main lobe width; the angle between the two vector diameters when the radiated power in the main lobe is half of the maximum value is called the main lobe width. The smaller the width of the main lobe, the sharper the pattern, indicating that the antenna radiation is more concentrated.
The ratio of the maximum value of the side lobe to the maximum value of the main lobe is called the side lobe level, which is generally expressed in decibels and is defined as:
101g side lobe maximum power / main lobe maximum power
If the ratio of the maximum power of the side lobe to the maximum power of the main lobe is 0.01, the side lobe level is -20dB.
If the strength of the anti-antenna's radiation strike in each direction is expressed by the length of the vector from the origin, then the envelope formed by connecting all the vector endpoints is the antenna's pattern. It shows the relative size of the antenna's radiation in different directions. This pattern is called a stereo pattern. The direction of the vectorial diameter represents the direction of radiation, and the length of the vectorial diameter represents the intensity of the radiation strike. The directional pattern contains many lobes, and the lobe containing the direction of maximum radiation is called the main lobe. Others are called first side lobe, second side lobe, etc. in turn.
Antenna gain G
We can also use the gain to indicate the degree of concentrated radiation of the antenna. The gain of the antenna in a certain direction is defined as: the square of the electric field strength (E2) generated by the antenna at a certain position in a certain direction under the same input power and the electric field strength generated by the lossless ideal point source antenna at the same position in the same direction The ratio of the square of (E02), usually expressed in G.
G = E2 / E02 (same input power)
Similarly, the gain can also be determined in this way: under the condition that the same electric field strength is generated in a certain direction to a certain position, the ratio of the input power (Pino) ​​of the lossless ideal point source antenna to the input power (Pin) of the antenna is called Is the gain of the antenna at that point.
G = Pino / Pin (same electric field strength)
Usually the gain of the antenna in the direction of maximum radiation is taken as the gain of this antenna. Gain is usually expressed in decibels. That is: G = 101gPino / Pin antenna gain calculation: G = η4πS / λ2 = η (π / λ) 2D2 In the formula, S-antenna aperture area (m2); λ-operating wavelength (m); D-parabolic aperture (That is, the diameter of the face opening) (m); η-antenna efficiency.
Antenna noise temperature:
The noise entering the antenna mainly comes from the cosmic noise of the Milky Way and the thermal noise from the earth and the atmosphere. Antennas with different apertures, different frequency bands, different elevation angles and different environments have different noises. In the C-band, the cosmic noise is very small, mainly the thermal noise of the earth and the atmosphere. In the Ku band, these noises also increase with frequency. At the same elevation angle, the larger the antenna size, the narrower the beam, so the antenna's noise temperature TA (K) is smaller, but as the elevation angle increases, this difference becomes smaller. For the same antenna size, the larger the antenna elevation angle Φ, the lower the antenna noise temperature TA (K), conversely, the smaller the Φ, the higher the TA. This is because the smaller the elevation angle Φ, the greater the thickness of the signal passing through the atmosphere, and the stronger the meteorological noise and atmospheric noise.
Polarization of electromagnetic waves:
The polarization forms of electromagnetic waves can be divided into linearly polarized waves and circularly polarized waves. Linearly polarized waves can be further divided into horizontally polarized waves and vertically polarized waves. Circularly polarized waves can be divided into left-handed and Right-handed circularly polarized wave. At present, China's satellite signal parties must use linearly polarized waves.
Only when the polarization mode of the receiving antenna is consistent with the received electromagnetic wave polarization form, can the signal be effectively received, otherwise the quality of the received signal will be deteriorated, or even the signal will not be received at all. This phenomenon is called polarization loss Match. When a rectangular waveguide is used as the feed source, its polarization direction is determined by the direction of the waveguide port. The narrow side of the waveguide opening parallel to the ground plane is horizontal polarization, and the wide side parallel to the ground plane is vertical polarization. When a circular waveguide feed is used, the direction of the probe in the waveguide should prevail. It is worth noting that the linearly polarized wave transmitted by the satellite transponder is defined based on the satellite's axis system, so only when the longitude of the local surface station antenna and the satellite are consistent, the polarization direction of the wave on the ground is the same as that of the satellite The same, there should be a slight deviation in other regions. Therefore, during installation, the feed should be rotated slightly to maximize the level indication of the receiver to achieve polarization matching.
Antenna polarization adjustment
1. The adjustment of polarizer when linear polarization and circular polarization coexist
Right-handed circularly polarized waves were launched from the leased international communications satellites, and horizontally polarized waves were launched from our satellites. When changing from a leased star to a satellite in China, you have to rotate the phase-shifting dielectric sheet in the antenna feed to make it perpendicular to the horizontal plane, and adjust the rectangular position so that the narrow side of the waveguide output port is parallel to the horizontal plane, then you can receive Horizontally polarized waves.
2. The adjustment of the polarizer when only linear polarized waves are received
When China no longer leases international communication satellites and uses satellites launched by China itself, the phase shifter should be removed. A phase shifter is a component that receives circular polarization. When receiving a linearly polarized wave, even if the polarizer is perpendicular (or horizontal) to the waveguide, loss of phase shift will occur, increasing antenna noise. Therefore, when only linearly polarized waves are received, the phase-shifting polarized medium can be removed, and the narrow side of the rectangular waveguide is parallel to the horizontal plane, so that the horizontally polarized wave whose electric field vector is parallel to the horizontal plane enters the waveguide.
Due to the influence of the ground station's geographic position and satellite attitude, the polarization mode of the radio wave emitted from the satellite may sometimes deviate slightly from the horizontal polarization. Therefore, during the adjustment and installation, the feed should be rotated slightly left and right until the receiver's level indication is the maximum At this time, the purpose of polarization matching is achieved.
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