[not third] as person of V/U wave band gas increases, abstained 8 wood antenna to become popular topic again. Last centuries 20 time, the 8 wood of Japanese northeast university are beautiful second with eaves two people invented Tian Tai 郞 this kind of antenna, be called " 8 Mu Yutian antenna " , abbreviation " 8 wood antenna " .
8 wood antenna is good really with. It has very good directional, relatively occasionally extremely aerial has tall gain. Will measure with it to, telecommunication effect is particularly good. If deserve to go up again,angle of elevation and azimuth rotate control equipment, OKer follow one's inclinations with include dimensional aircraft inside the broadcasting station contact on each direction, this kind is experienced going up from erect antenna is couldn't get.
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Typical 8 wood antenna should have 3 to be opposite brace up child, whole structure is shown " king " glyph. Weigh active with what line of make a present of is linked together brace up child, or advocate brace up child, reside 3 pairs brace up child in, "King " the intermediate one horizontal stroke of the word. Than active Zhenzi grows the weighing of a bit a bit reflector, it is in active brace up child a side, rising to weaken the electric wave that transmits from this direction or the action that launch the electric wave that go from this antenna; Than active Zhen Zilue weighs director shortly, it is located in active brace up child another side, it can increase to be transmitted from direction of this a side or emit the electric wave that go out to this direction. Director can have a lot of, every length wants those who compare its photograph adjacent to stand by active brace up child that slightly a bit shorter. Director is more, acerber, gain jumps over direction tall, but exceed 4 actually, after 5 director, this kind " advantage " increase not quite apparent, and bulk is big, self-prossessed increase, ask to rise to data strength, cost is increased wait for a problem to be highlighted gradually however. There are a pair of 5 units below the circumstance normally 8 wood (have 3 director namely, a reflector and an active brace up child) used quite.
Every director and reflector are to use marvellous make it of a metal. No matter how many have " unit " , all brace up child, it is to press certain span parallel to secure be in " crossbeam " on. Crossbeam also uses metallic stuff make it. These brace up child does midpoint want to be isolated from with crossbeam? Not. Original, electric wave " walk " be half wavelengh length about in these brace up child when going up, brace up child midpoint as it happens is located in inductive signal voltage at 0 o'clock, received at 0 o'clock " the ground " , also was no problem. And still have a profit, the metal that the electrostatic as it happens that arrives in dimensional induction can contact dot, antenna through these establishs lever to guide again connect a building avoid thunder earth mat goes.
The working principle of 8 wood antenna is such (with 3 unit aerial receives for exemple) : Director slightly short at 1/2 wavelengh, advocate brace up child be equal to 1/2 wavelengh, reflector grows slightly at 1/2 wavelengh, two brace up child span 1/4 wavelengh. Right now, director is shown to inductive signal " look sex " , voltaic lead voltage 90 ° ; The electromagnetic wave of director induction is met to advocate brace up child radiation, radiation signal makes through the distance of 1/4 wavelengh its lag is amounted to at arriving directly from inside sky advocate brace up child signal 90 ° , offsetted fitly in front cause " lead " , both and phasic identical, then signal alternate is added, get strengthening. Reflector grows slightly at 1/2 wavelengh, show sensibility, voltaic lag 90 ° , arrive plus radiation advocate brace up in subroutine lag 90 ° , with add directly from reflector direction advocate brace up child the signal just in time that attend differred 180 ° , had quits effect. A direction is strengthened, a direction is weakened, had strong directional. Yi Ran of process of emissive state action.
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Active brace up child an unit that is a key. Have two kinds of familiar pattern: Amount to brace up child with straight brace up child. Straight Zhenzi is 1/2 wavelengh actually occasionally extremely brace up child, amount to brace up child it is its are out of shape. Active brace up child the place that connects with line of make a present of is indispensible the insulation that keeps good with girder, and Zhen Zizhong nods amount to to still be interlinked with crossbeam.
Be modelled on a pair of antenna, but still always need to undertake proper adjustment. What to move? Why to want accent? This needs us to understand the principle that makes antenna.
A of aerial main feature, that is " input impedance " . In syntonic condition, if aerial is received with a resistor carry in line of make a present of. Impedance of line of commonly used make a present of is 50 Ω , if aerial input impedance also is 50 Ω , that was achieved " match " , the signal that broadcasting station outputs can blast off from aerial entirely go out; If not " match " , the result discharge road that one part power can reflex wire back stage.
1/2 wavelengh occasionally extremely the input impedance of aerial is 67 Ω about, 1/2 wavelengh amount to brace up child input impedance criterion prep above 4 times formerer. After adding director, reflector, impedance relation becomes complex rose. As a whole 8 wood are compared only and basic brace up child impedance wants a lot of lower, and 8 wood unit span is big each impedance is tall, conversely impedance becomes low, at the same time aerial loss of efficiency. Have data introduction, director and advocate brace up child span lowest of the impedance when 0.15 wavelengh, 0.2 - 0.25 when impedance is tall, efficiency rises. At this moment the span of impedance is in about 5 - between 20 Ω .
Classical amount to brace up child the characteristic impedance of 8 wood antenna is 300 Ω about, (brace up child span makes an appointment with 1/4 wavelengh) the TV receving antenna that as usual sees. When the span that amount to Zhen Zishe adds up to is narrow, or of 1/2 wavelengh " long side " the diameter is more than those two about 1/4 wavelengh " short edge " when the diameter, its input impedance is taller.
Our communication equipment output is designed by 50 Ω , match 50 Ω cable to make line of make a present of. How can 8 wood antenna just achieve impedance match with line of make a present of? Have to consider this issue apparently. Had a various recipe ways then. Shortwave wave band 8 wood are commonly used " hairpin type " match, it is to be in feed place to receive a paragraph of U conductor, it is having the effect of an inductor, formed shunt-wound resonance to raise aerial impedance thereby with the capacitance of aerial itself; Still have classical " gamma " match, famous HB9CV antenna is waited a moment. The simplest way is wind the string of make a present of that approachs aerial feed point into about 6, about 15 centimeters coil hangs 7 diameters over, the principle that I think to this and hairpin match should be same.
Still a problem wants to notice: 8 wood antenna is " output evenly " , its two feed place is right " the ground " present identical character, but normally port of antenna of transmitting and receiving equipment is however " lopsided " , core line is hot end, outer conductor ground connection. Although we are OK also,ground of turn a blind eye to receives line of core of line of make a present of what in aerial two feed nod at will
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