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Antenna Introduction and Application

Posted by burrows on about 2 years ago.
Antenna feed system can ensure high enough gain and the lowest possible antenna noise. And it can effectively feedback faint radio wave to low noise amplifier with high-frequency. Moreover, it shall provide correct antenna pointing and polarization mode. Besides, antenna system shall possess high enough capacity of bearing wind pressure.
Antenna type
Antenna is divided into two categories which are rotated polarization and non-rotated polarization. Rotated polarization antenna has two types which are spiral and parabolic. And parabolic antenna has two basic forms. One is prime focus parabolic antenna of feed forward with only one reflector. The other is Cassegrain antenna of feed back, which processes main reflector (paraboloid) as well as sub-reflector (hyperboloid). And for Cassegrain antenna, radio wave converges after reflecting twice. Primary focus antenna is simple but its performance is worse. Low noise preamplifier of Cassegrain antenna can be placed at the back of paraboloid so as to prevent from sunlight. Cassegrain antenna is especially suitable for satellite TV station in the tropics; however, the cost is higher.
  Besides, there are flat panel antenna and ring focus antenna. The former is portable, suitable for family receiving. The latter decreases antenna sidelobe and improves receiving efficiency. To flexibly receive TV program on different satellites, power-driven polar axis antenna is produced. This type of antenna possesses the original antenna, feed; moreover, it also possesses electric motor speed-reduction drive antenna mechanism and antenna controller controlled by microcomputer.
  Some cable television system adopts multi-focus parabolic antenna, which uses fixed antenna to receive more satellite’s television signal simultaneously. This type of antenna has more feeds which receive signal from different satellites to form more beams. And its reflector uses deformed sphere and paraboloid. For example, the reflector consists of rectangle cut out from sphere or two kinds of curves. And its horizontal direction is circular arc while vertical direction is a part of paraboloid.
  The diameter of parabolic antenna of large satellite communication station is 15~30m, the medium one is 4~10m. Ku-band receiving antenna of household live satellite TV can be as small as 0.5m. Now C-band satellite TV receiving station mostly uses 1.5~3m antenna while Ku-band uses 0.75~1.5m antenna. With the improvement of efficiency and performance of antenna, feed, and reduction of low-noise amplifier’s temperature, the antenna diameter can be reduced.
Antenna feed system
  Feed, the heart of antenna, serves as junior radiator of high-gain assembled antenna and provides efficient illumination for parabolic antenna.
(1)It has suitable directional diagram. Feed junior directional diagram cannot be too narrow, or the paraboloid cannot be totally irradiated. But it cannot be too wide so as not to leak too much power. Besides, junior directional diagram should be near to rotational symmetry without sidelobe and tail lobe.
(2)It has ideal wave front. Circle parabolic antenna requires that feed wave front is sphere to ensure that parabolic diameter phase is evenly distributed when phase center coincides with focus. Otherwise, it will give rise to distorted antenna directional diagram, reduced gain and increased sidelobe.
(3)No cross-polarization. That is, there is no cross component of main polarization. It requires that cross-polarization component of feed radiation field should be as little as possible.
(4)Impedance changes stably. It requires that input impedance of feed shouldn’t change dramatically to match with feed line within operational band.
(5)Size should be as small as possible. The whole feed system mainly consists of feed horn, 90°phase shifter and rounded rectangle converter. The feed can be divided into feed of feed forward and feed of feed back according to usage mode. And it can be divided into C-band feed and Ku-band feed according to satellite band. At present, C-band and Ku-band shared feed has been developed. Feed of feed forward is applied in ordinary parabolic antenna and feed of feed back is applied in Cassgrain antenna. Common feed form of parabolic antenna has pyramid horn, conical horn, open-end waveguide and corrugated horn, etc. feed of feed forward uses corrugated slot feed mostly, and also coaxial waveguide feed with choke groove. Feed of feed back horn uses dielectric load horn which is common conical horn with the addition of a length of Polytetrafluoethylene liner. Offset antenna chooses offset feed. Offset feed’s ripple disc is funnel-shaped while primary antenna’s ripple disc is horizontal.
1. Polarized wave
When radio wave propagates in the space, its electric field vector’s instant orientation is called polarization. When radio wave propagates, the electric field vector’s space locus is a straight line and always propagates in a plane, so it is called linear polarized wave. Linear polarized wave can be divided into two categories which are horizontal wave and vertical wave. If the electric field vector’s space locus is a circle, namely, the field vector constantly rotates around axis of propagation direction, so it is called circular polarized wave.
Radio wave polarization performance depends on polarization performance of transmitting antenna feed system. Receiving antenna should possess same polarization and rotation performance with transmitting antenna to realize polarization matching, thus it receives all energy. If matching partially, it only receives the portion of energy. A linear polarization wave can be decomposed into two circular polarization wave rotating in opposite direction while a circular polarization wave can be decomposed into linear polarization wave which is mutually orthogonal. Therefore, antenna receiving linear polarization wave can also receive circular polarization wave. And antenna receiving circular polarization wave can also receive linear polarization wave, but it will lose 3dB energy. Either linear polarization antenna or circular polarization antenna cannot receive their own orthogonal component.
2. Polarizer
Polarizer is the device to control polarization direction of antenna feed system, which is used to choose polarization pattern which is in line with satellite TV signal and to restrain other patterns of polarization wave in order to acquire polarization matching and realize best receiving. In the modern satellite TV transmission, different TV programs can be transmitted by making use of mutual isolation performance between vertical and horizontal polarization, left-hand circular polarization and right-hand circular polarization, so as to improve satellite’s transmission capacity. Circular polarization or linear polarization has pluses and minuses. When circular polarization traveling through rain-mist layer and the ionosphere, its entry loss is low and it doesn’t have the problem of polarization panel rotation of linear polarization wave. And the greatest advantage of linear polarization wave is simple realization.
3. Antenna feed receive linear polarization wave
It’s worth noticing that linear polarization wave transmitted by satellite repeater is defined based on satellite shafting. For example, broadcast satellite’s horizontal wave parallels with the tangent of geosynchronous orbit at the fixed point of satellite. But vertical wave is vertical with the tangent. Take horizontal wave as an example, horizontal wave received by ground station A in the diagram which has same longitude with satellite positioning doesn’t change. But horizontal wave received by ground B of different longitude gets tilted. The farther it is from satellite postioning longitude, the more seriously tilted horizontal wave is. To realize matching, the corresponding polarization compensation, namely, turning an angle P, is made for feed of station B. The station C at the equator is an extreme case that feed is turned 90°when receiving horizontal signal.
Antenna system selection
In project, Antenna performance is characterised by some main parameters of direction diagram, main lobe width, sub-lobe level, gain, polarization, frquency band width, voltage standing wave ratio(VSWR), noise temperature and antenna wind load, etc. Please refer to national standard  GB11442- 95 for the requirement of C-band antenna electric performance while refer to national standard  GB/T 16954-1997 for the requirement of Ku-band antenna electric performance.
Attention should be paid in the following problems when choosing antenna.
1. Choose in strict accordance with the requirement for antenna in Receiving Technology Condition for Aluminum Antenna Satellite TV (standard title).
2. Antenna structure is reasonable or not, including whether antenna support is firm, whether elevation and azimuth adjustment bar pitch and regulation is convenient, etc.
3. Selection of plate antenna and mesh antenna is related to user’s station location. In large, medium cities or industry areas, it is probably best to choose panel antenna because of serious air pollution. If the station is located in windy mountain area, choose aluminum mesh antenna which possesses lower gain compared to plate antenna, is easily influenced by rain and snow, but has strong ability of anti-wind.
4. Selection of feed forward antenna and feed back antenna. Due to the appearance of HEMT device and low noise, it is available to directly connect them at the back of feed, which overcomes the weakness that feed forward line loss is higher than that of feed back line, and meanwhile greatly reduces feed forward line’s noise temperature. Now feed forward antenna’s gain is lower 0.2dB than that of feed back antenna, which has little effect on receiving quality of TV signal of satellite TV receiving station. However, feed back antenna can be used as the antenna of satellite communication earth station in the meantime while feed forward antenna cannot.
5. Selection of antenna tracking, drive mode. When using large-caliber antenna, mostly choose double-shaft tracking. When using small-caliber antenna, choose single-shaft or double-shaft tracking.
Antenna has three drive modes which are manual, power-driven and automatic. The first two modes possess characteristics of artificial positioning, simple function and low cost. The third drive mode generally adopts mono-board microcomputer control when using double-shaft tracking. It possesses the function of automatic selection (the accuracy can be more than 0.3°), one satellite tracking, or several satellites preset (the accuracy can be more than 0.1°). Thus antenna can quickly find any satellite that needs. And antenna can be in the best receiving status by means of signal tracking.  When using single-shaft automatic tracking antenna, bridge balance mode is adopted to memory satellite position automatically. And it can also preset more satellites in a geostationary orbit and find any satellite quickly, but it cannot track satellite by signal.
6. The possibility of TVRO. The most effective way to reduce antenna caliber is to increase satellite EIRP, reduce noise temperature of high-frequency tuner, improve antenna efficiency and reduce receiver’s threshold level.
7. Selection and application of offset antenna. Small antenna with Ku-band receiving has two types which are prime focus and offset. Prime focus Ku-band antenna’s focal length is shorter. Under the condition of same size, offset antenna’s gain is a little higher than that of prime focus. Therefore, small antenna with Ku-band almost adopts offset antenna.

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