A lot of antenna if half Zhen Zitian line, amount to antenna of Zhen Zitian line, round is balance feed, they have two feed place, they have a characteristic: The signal tension that two feed hint (or electric current) phasic it is each other is in opposition. And advocate feed cable often is to use coaxial cable, coaxial cable is belonged to lopsided (asymmetry) line of make a present of, the conductor inside its is feed dot, and outer conductor is ground wire dot, do not participate in feed. The character that calculates aerial so is hired fight as identical as coaxial cable also cannot join directly, otherwise, can destroy the symmetry of aerial, make aerial the voltaic size on two arms differs, this kind of lopsided sex can change the direction of aerial to pursue, the made way that makes asymmetry pursues, make line of make a present of may receive all sorts of interference thereby wave and make line of make a present of and antenna are broken match. Accordingly, when aerial and coaxial line join, should consider impedance to match not only and have a balance even- - lopsided commutation.
1, λ / 4 balance transformer (the wavelengh that λ is signal frequency)
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λ / 4 balances commutation is shown 1 times like the graph, half brace up child input impedance is 75 EuropeThe balance is laden, the coaxial cable that uses 75 Europe deserves to although impedance is to match,be received to it, but do not match however evenly, must join transformer of a balance.
Half brace up child one arm and advocate outer conductor of wire of make a present of is linked together (graph 1 medium A is nodded) , another arm and λ / of the end on 4 conductor and coaxial cable inside conductor photograph joins (graph 1 medium B is nodded) , λ / the lower end of 4 conductor is passed short receive ferrule and advocate the outer conductor of line of make a present of is conterminous (graph 1 medium C is nodded) . So A- - " the distance between B dot is λ / 2, so, when the signal that B nods sends A the drop, just turn over, turn into the asymmetry of coaxial line so symmetrical. From A, B at 2 o'clock to short receiving ferrule to look is a paragraph of λ / the short circuit line of 4, its impedance is infinity, match to impedance so won't cause an effect.
2, asymmetry U annulus balances commutation
If the graph is shown 2 times, it all is the coaxial line of 75 Europe to form by two paragraphs of characteristic impedance, Among them one paragraph is λ / 4, another paragraph is 3 λ / 4, of two paragraphs of coaxial lines inside conductor parts with half brace up child two arms A, B is linked together, another end and advocate feed cable is linked together to be nodded at C, visible advocate line of make a present of arrives brace up child the wave Cheng of method of two feed dot differs for 3 λ / 4- λ / 4= λ / 2, namely the signal voltage volume that two feed burn is equal, way is opposite. Assured to balance feed consequently.
Impedance matchs: As a result of half brace up child be balance type, every feed nods right impedance to be 75/2=37.5 Europe, feed dot A passes λ / the impedance that the 75 Europe cable of 4 nods to C is: 75 square / 37.5=150 Europe, feed dot B passes 3 λ / 4 (λ / of 4 times odder) the impedance that 75 Europe cable nods to C is: 75 square / 37.5=150 Europe, so the synthetic impedance that C nods is: 150/2=75 Europe. Mix apparently advocate the impedance of feed cable matchs.
3, λ / 2 balance transformer
λ / 2 balances transformer calls U balance transformer again, if the graph is shown 3 times, amount to halfWave Zhen Zitian line (input impedance is 300 Europe) attend the meeting when the coaxial line of 75 Europe joins, both impedance does not match, because this must add transformer of outfit U balance between them. From the graph 3 can see, feed nods the right impedance of A and B to be 300/2=150 Europe, signal from advocate feed cable is passed to A bit minutes to become two, furnish respectively the load that Zhenzi controls both sides. The wave path difference that as a result of A, B two feed nod is λ / 2. Accordingly, the power source volume that two feed of A, B burn is equal, way is opposite, achieved the goal that alternates evenly thereby.
See impedance side again, as a result of A, B the right impedance 2 o'clock all is 150 Europe, so after synthesis is together, the paralell connection that the impedance that A nods should nod for two feed is worth namely 150/2=75 Europe, so impedance also matchs.
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