Shortwave communication is to point to wavelengh 100 - 10 meters (frequency is 3 - 30 MH ｚ ) the radio that electromagnetic wave undertakes corresponds. Shortwave communication transmission channel is had change ginseng is characteristic, ionosphere suffers an environment easily to affect, be in constant change in the center, accordingly, its communication quality, as other communication means is like satellite, microwave, fiber-optic good. The effect stand or fall of shortwave communication system, basically depend on the bandwidth of the stand or fall of a function of use broadcasting station and antenna, gain, standing wave comparing, directional wait for an element. In recent years shortwave broadcasting station raises development as new technology very fast, realized digitlization, solid state to change, miniaturization, but the development of aerial technology however relatively lag. Because shortwave compares the wavelengh of ultrashort, satellite, microwave to grow, so, shortwave antenna volume is larger. In shortwave communication, it is very important to improving communication result to choose aerial. Below antenna of simple introduction shortwave how type selecting and a few kinds of commonly used aerial function.
, measure aerial property factor
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Aerial is wireless communication system the mainest part, decided the character of communication system. Different antenna has different radiation type, polarity, gain and impedance.
１ . Radiation type: Decided the allocation of radiation energy, it is the aerial is the mainest factor in all character, it includes complete to model with direction model.
2. Polarity: Polarity defined aerial the direction of electric vector of of direction of the biggest radiate. Antenna of perpendicular or odd polarity (whip antenna) have perpendicular polarity, horizontal antenna has horizontal polarity.
3. Gain: The gain of aerial is the major attribute of aerial, can judge the actor of low quality of aerial. Gain is the intensity of the biggest radiate that appoints direction to go up and antenna the ratio of intensity of the biggest radiate, use half ambipolar antenna to serve as referenced antenna normally, other type antenna is the easiest can undertake comparative with referenced antenna to the radiation intensity that go up, reach aerial gain. The bandwidth of antenna of average tall gain is narrower.
4. Impedance and standing wave compare (VSWR) : The input impedance of aerial system affects aerial to launch efficiency directly. Compare when standing wave (VSWR) １ : There is reflection wave when １ , voltage reflectance is １ . When VSWR is more than １ , reflection power also increases subsequently. The standing wave ratio that emissive antenna gives out is the biggest allow a value. For example: VSWR is 2: Mean when １ , reflection power uses up the 11 ％ of total emissive power, signal loss 0. 5 ｄ B. VSWR is １ . 5: When １ , loss 4 ％ power, signal is reduced 0. 18 ｄ B.
2, a few kinds of commonly used shortwave aerial
１ . 8 wood antenna (ａ of ｎ of ｎ of ｅ of ｔ of ｎ of A of ｉ of Y ａ ｇ ) 8 wood antenna is in shortwave communication is used at be more than frequency of above of 6 MH ｚ normally paragraph, 8 wood antenna can achieve 19 ｄ B in the gain below good case, 8 wood antenna applies at tape and communication of tall gain shortwave, but erect installation is had in the on iron tower very strong directional. Go up in an iron tower but at the same time erect antenna of a few 8 wood, the main good point of 8 wood antenna is low-cost.
2. Logarithm cycle antenna (ａ of ｎ of ｎ of ｅ of ｔ of ｎ of A of ｃ of ｉ of ｄ of ｏ of ｉ of ｒ of ｅ of P of L ｏ ｇ ) price of logarithm cycle antenna is high, but can use go up in a variety of frequency and angle of elevation. Logarithm cycle antenna agrees with medium, shortwave corresponds, use day wave signal, efficiency is tall, adjacent expect to be worth at blasting off. With other photograph of tall gain antenna is compared, logarithm cycle antenna is directional stronger, bigger to the attenuation of useless direction signal.
3. Long term aerial (ｇ of L ｏ ｎ - ｓ of ａ of ｎ of ｎ of ｅ of ｔ of ｎ of A of ｅ of W ｉ ｒ ) long term aerial advantage is a structure simple, the price is low, gain moderate. With 8 wood the line mixes the day to be compared to extremely periodic antenna, long term aerial length is directional with gain low. But its advantage depends on, as a result of its gain and line length are concerned, the user can be found optimal the length that receives a line and angle. Through comparing signal wavelengh, computation gives the length of the line, special agree with telecommunication. Compare gain with ambipolar antenna when the line grows 4 times wavelengh to be 25 degrees in angle of elevation tall 3 ｄ B, grow when the line 8 times when Yu Bo is long, gain is tall 6 ｄ B, angle of elevation drops 18 degrees, graph １ is long term aerial gain shows a picture.
4. The car carries mobile antenna (ｓ of ａ of ｎ of ｎ of ｅ of ｔ of ｎ of A of ｅ of ｌ of ｉ of M ｏ ｂ ) mobile antenna works commonly in 2. 25 MH of 0 ～ ｚ frequency paragraph go up, for perpendicular polarity antenna, function and physical characteristic are concerned, aerial length is shorter, when low angle of elevation works, emissive efficiency moderate. Connecting general reason condition to fall, the car carries aerial angle of elevation to should be more than 45 degrees, because aerial length is shorter, it is small effect antenna. On the car, physical characteristic limitted the choice of aerial, but aerial can be placed to pour " L " model, raised the perpendicular radiate scale of aerial so, can improve emissive efficiency, pour antenna of " L " to be used at intermediate waves communication aptly.
3, commonly used shortwave aerial function
Directional antenna, simple ambipolar antenna applies to stone's throw to correspond, but shortwave telecommunication signal is faint, when be being flooded by all sorts of noise even, aerial chooses taller than ambipolar antenna gain antenna with respect to need. Ideal directional antenna has very tall gain on working way and the gain on useless direction is 0.
4, different environment issues aerial type selecting
１ . Between fixed station because communication of far ／ close quarters secures direction of the communication between the station it is changeless, use tall gain commonly so, directional strong shortwave antenna. Communication distance is in 1000 - 3000 kilometer, can use tall gain, antenna of cycle of logarithm of low angle of elevation (LP) , but aerial price is high. In practice 100 W shortwave gets used to broadcasting station to match this kind of antenna oneself, can realize Beijing basically to reach Kunming, urumqi even Lhasa is round-the-clock communication. If communication quality demand is not too high,the antenna with also can use the price relatively cheap is like 8 wood antenna, long term aerial, but long term aerial needs to use day of air. The distance uses a level when less than of 600 K ｍ ambipolar antenna can obtain better result, but ambipolar antenna covers an area of the level bigger, central station broadcasting station does not suit cloth antenna more relatively blast.
2. The communication between fixed station and mobile station stands in motion as a result of shift, communication direction not
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