关于巴伦使用

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巴伦的作用是什么，在有些接收电路中用巴伦，有些没有用，到底在哪种情况需要，哪种情况不要用呢？

巴仑通常用来实现不平衡到平衡变换和阻抗匹配用，天线设计中经也常用到，接收机器件级连中可以将单端变平衡从而抑制共模干扰，天线设计常用来做馈线。

高手能解释一下什么是巴伦吗？没有听说过！

ABOUT BALUN
May be baluns are still a mystery for hams; the only way to understand it is learning what it is and how to use it.
The word balun means balanced-unbalanced: it's used to adapt a balanced device to an unbalanced one; in a balanced device (as larger type of antennas) we have on both terminal the same voltage respect to the ground (if not so it's an unbalanced device);
a dipole with direct feed is balanced, a coaxial cable is unbalanced.
So, when we connect a balanced device to an unbalanced one the following occurs:
we have a dipole with coaxial cable direct fed (i.e. RG213); normally, transmitting, there are 2 currents on the cable:
I1, which flows trough the central wire of coaxial cable and from transmitter goes up to the dipole;
I2, which flows (for the skin effect) on the inside part of the copper shield;
The two currents, equal and opposite, humble itself and we have no radiation from coaxial cable.
The two currents cames on the dipole to be irradiated; part of it cames back; the one on the shield cames back trough the external side of the shield (no more the inside: so we have 2 current on the shield, I2 and I3); the value of this current (which we'll call I3) depends to the impedance value of the external side of coaxial cable respect to the ground (in a word if it will find high or low resistance);
if impedance will be high, I3 will find high resistance and its value will be low; if impedance will be low, resistance will be low and I3 value will be high; in this way I3 will radiate RF and the external side of the coaxial cable will radiate as a third wire of dipole: it's as we have a dipole with 3 wire (see picture 1); as consequence the radiation pattern will be distorced (see picture 2);
the big problem is that this new wire is often near TV and telephon cables irrading directly on those: so we have more probabilities to cause TVI.
This new wire, when its impedance is low, change the dipole impedance so we can have high S.W.R; this is the reason why (without balun) varying the coaxial cable length the S.W.R. changes.
Let's see now why use a balun; to delete this current I3, we need an high impedance for RF on the external side of coaxial cable; so we have high impedance casually for some cable length (odd multiple of 1/4 lamda) or using a balun: so the first reason to use a balun is TO AVOID THAT ON THE EXTERNAL SIDE OF COAXIAL CABLE FLOWS A CURRENT I3.
The simpler balun is a coil with some coaxial cable turns just belowe the antenna feed point: this inductance make the cable impedance (we mean the external side of cable shield) higher so that the RF current will find an high resistance and its value will be very low (that will not disturb I2 and I1 which flow inside the coaxial cable).
TECNICHAL DATA ABOUT BALUN
We can build a balun in various ways:
choke balun (see picture 1) with ratio 1:1 : as we said, the easier way is a coil with 6-8 turns (diameter 20-30 cm for 14-30 mhz) of coaxial cable just below the dipole (or other balanced antenna) feed point; for low band add more turns or use a ferrite (like Amidon) wich has no inflence on I1 and I2.
collins balun (see picture 2) with ratio 1:1: the bandwidth is very wide; we have also a similar one but with ratio 4:1 (see picture 3);
transmission line balun: a transmission line transformer is composed of two (or more) transmission lines of X impedance connected between in series at one end and in parallel at the other one; turning this line as a coil we obtain high impedance for RF;
the balun on picture 4 has a ratio of 1:1;
the balun on picture 5 has a ratio of 4:1;
the naluns on picture 6 have a ratio of 6,25:1 and 9:1;
the line length should be 1/4 lambda at the lower frequency.
We can use a ferrite (as Amidon) to make the size smaller.
bazooka balun (see picture 7) with ratio 1:1: it's a simple conductor pipe of 1/4 lambda length placed around the coaxial cable and short-cut and the low-end; it has a small bandwitdh and usually it's used on VHF;
in VHF it's made with a metallic pipe, in HF it's made with metallic sheet wrapped around the coaxial cable (es.:carta stagnola) and short-cut with a pin at the low end (short-cut only the metallic sheet with the cable shield).

VHF balun (see picture 8) with ratio 4:1: it's very common in VHF. The length L should be exactly 1/2 lambda.
All above baluns can be made using ferrite to make the size smaller (see picture 9).