Baluns ( BALanced to UNbalanced ) are still a mystery for hams; the only way to understand this is to learn 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 most type of antennas) we have on both terminal the same voltage referred to common ground (if it's not then is 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), While applying RF we originate 2 currents flows on the coax cable:
The two currents, equal and opposite null themselvesf so no radiation takes place from coaxial cable. The two currents go on the dipole to be irradiated, part of it cames back, the one on the shield gets back trough the external side of the shield. 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 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 below left); As consequence the radiation pattern will be distorced (see right). Mostly this new wire is often near TV and telephon cables irrading directly on those: so producing a source of TVI and RFI. This new wire, when its impedance is low, change the dipole impedance so we can have high S.W.R, that is the reason why (without balun) varying the coaxial cable length the S.W.R. changes. See the difference using a balundevice to void this current (I3), we need an high impedance for RF on the external side of coaxial cable, so having high impedance casually for some lenght of coax cable (odd multiple of 1/4 lambda) or using a balun. The simpler balun device is a coil made with some lenght of coaxial cable placed 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).