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Message: <blockquote data-quote="KY4TRK" data-source="post: 12034" data-attributes="member: 2">Archive AUG 2010Tech TalkBy Jim Purvis WA7HRGWhat the heck is a Balun??A "BALUN" is a bi-directional adaptor between a BALanced device and an UNbalanced device. Twin-lead feedlines and dipoles are "balanced" (two signal conductors, with equal-but-opposite current: symmetrical). Coax cables and the typical antenna port of a transceiver or antenna tuner, are "unbalanced" (one signal conductor that is referenced to ground; a.k.a. "single-ended"). Baluns other than those with a 1:1 transformation ratio, also act as impedance transformers.There are two basic types of Balun: current baluns and voltage baluns:Current baluns typically consist of ferrite beads on a section of 2-conductor transmission line such as a coax, or conversely, coax wound multiple times through a ferrite ring, or tight-wound coil of coax windings (air core). Its purpose is to provide high impedance to common-mode current on the transmission line: it "chokes" that current. There is no impedance transformation and is said to be 1:1. When placed between a balanced device such as a symmetrical antenna (ignoring stray capacitances etc. from surrounding objects that may actually make the antenna a-symmetrical), and a coax (unbalanced transmission line), the current balun rejects the common-mode current and passes the differential voltage.Voltage baluns comprise a transformer with at least two sets of windings; they may be interconnected like an auto-transformer. If the input and output section of the transformer has the same number of windings (ratio = 1), than the balun has the same voltage and impedance at its input and output. The impedance transformation ratio is the square of the voltage transformation ratio, which is the ratio of the number of windings. This is said to be a 1:1 balun. When the winding ratio is 4 it is said to be 4:1. The impedance is also 4:1. This allows the transformation of 50 ohms impedance applied to one side (the source) to an impedance of 200 ohms on the other (the load).What Problems are Baluns supposed to Solve?Current baluns usually solve problems caused by an imbalance.An imbalance of what? To answer this question, we need to look at current flow in transmission lines. In a coaxial cable, the currents on the inner conductor and the inside of the shield are equal and opposite. This is because the fields from the two currents are confined to the same space. With the presence of skin effect, a different current flows on the outside of the shield than on the inside. The current on the outside, if significant, causes the feed line to act like an antenna, radiating a field that is proportional to this current.Current baluns placed at the Dipole feed point will stop or ‘choke’ that current from going back down the outside of the feed line and to force it out the antenna leg where it belongs.To understand the functions of a balun, it is essential to be familiar with current paths at the feed point of the dipole. These paths are shown in Fig 1. Because of their symmetrical relationship, the dipole arms couple energy of equal magnitude and opposite phase onto the feed line, thus canceling induced current flow on the outside of the feed line. What is disturbing is the discovery that there are three paths for current flow in a coaxial feed line, instead of only two. How can there be three current paths in only two conductors? At RF, skin effect isolates the currents flowing on the inner and outer surfaces of the coaxial shield. This effect, which does not occur significantly at DC or low-frequency AC, prevents currents that flow on the inner braid surface from interacting with those on the outer surface, and vice versa.As shown in Fig 1, while traveling within the transmission line, current I1 flows on the center conductor and I2 flows only on the inner surface of the outer shield. When antenna current is flowing from left to right as shown, I1 flows out of dipole arm 1, downward onto the center conductor, and returns to the generator. Current I2, being of opposite phase and direction, flows upward along the inside surface of the feed-line shield until it reaches the junction of dipole arm 2. At this junction, I2 divides into two separate paths to form I3 and I4. Current I3 flows back down the outside surface of the feed line, and I4, which equals I2 – I3, flows to the right onto dipole arm 2. The magnitude of I3 depends on the impedance to ground provided by the outside surface of the coaxial shield.If the effective path length to RF ground is an odd multiple of 1/4 wave, the impedance to ground will be very high, making I3 negligible. In this case, I1 and I4 will be nearly equal. On the other hand, if the RF path to ground is a multiple of 1/2 wave, the impedance to ground will be fairly low, and current I3 may be substantial. This results in unequal currents in the dipole arms and radiation from the feed line. In many instances, this RF path to ground includes the transceiver line cord and some house wiring, terminating at the power-line ground! Thus, the amplitude of I3 varies with changes in feed-line length because of changes in the impedance between dipole arm 2 and ground.It is evident that, in coupling an unbalanced line to a balanced load (such as a dipole), the primary function of a balun is to block the external current path between the inside and outside surfaces of the coaxial shield. With a balun in the circuit, I2 will not divide at the end of the feed line to form I3, but instead will flow only onto dipole arm 2. Thus, when I3 is zero, I4 equals I1, and the currents flowing on dipole arms 1 and 2 are equal and therefore balanced.Voltage baluns typically transform impendence.It has been determined analytically that current baluns force equal currents into opposite halves of a dipole, independent of the impedance of either half. On the other hand, voltage baluns provide only equal voltage to the opposite halves, and thus do not provide equal currents in each half if the impedances of the two halves are not identical. In other words, if the two arms are not cut the same length, have the same wire characteristics, or are effected by outside forces such as tree limbs or near by buildings. Frequencies other than antenna resonance significantly decreases the impedance transfer ration also. A trifler wound 4:1 balun will match the input 50 ohms to the output 200 ohms at the antenna and provide equal voltages to both legs. And, assuming no loss or leakage, will produce equal current to both legs providing the antenna is resonant.However, voltage baluns do not yield a true 1:1 or 4:1 impedance transfer ratio between their input and output terminals. This is because of losses, leakage reactance, less than optimum coupling, core material, and a host of other variables. There are thousands of pages of theory on voltage baluns, but the bottom line seems to be that with so many variables, unless everything is just right voltage baluns do not correctly balance the currents in both sides of the antenna.So which is the best type of balun to use? There are literally hundreds of thousands of articles and information on the web on this subject. Trust me, I read through enough to know that it is a very difficult and complicated subject. But in my mind it came down to this fact. Power in an antenna is still a function of voltage and current. If you want the most power radiated from the antenna you don’t want to lose any current spilling off to ground down the outside of the coax.So, generally I would say a current balun. As it ensures that power is only radiated from the part of the antenna system you want it to be from. It prevents common mode current from flowing along the feeder which can cause RF in the shack and the pickup of unwanted noise on receive.There are several design concepts for current baluns. Some being an air wound coaxal choke, a coaxial choke wound on a ferrite core, and ferrite beads that encase a portion of the feed line coax. Variations on these are abundant on the internet. In my research one current balun kept coming up time and again. That is the W2DU ferrite-loaded coaxial RF choke which just happens to yield the same results as a 1:1 current balun. It appears to be the balun of choice for many and is very easy to build. Just slide about a foot of ferrite beads over the coax next to the feed point of the antenna. What could be simpler?If your looking for a very simple and quick project that will yield some pretty significant results I suggest you look into some type of current balun on your antennas.I actually borrowed this idea from a QST article so I’m just going to call it my ‘version’ of an integrated center insulator and current balun for dipole antennas. You can find construction details and drawings on the TCARC Yahoo Groups site under files/construction projects.73’s Jim</blockquote>

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