What are baluns?

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What are baluns?
"A transmission linetransformer for converting balanced inputto unbalanced output or vice versa. Itmay or may not provide wide frequency range impedance transformation dependingupon the configuration used."
An old typical usage of thebalun was (and still is) with TV antennas. The foldeddipole is part of a yagi antenna which looks something like a pole withrods set across it at right angles. The second last one is folded into anoblong or rectangular shape. A folded dipole exhibits two importantcharacteristics (a) its bandwidth is good for over an octave (e.g. 50 Mhz to100 Mhz or say 120 Mhz to 240 Mhz) AND its characteristic impedance is a moreor less a constant 300 ohms.
In earlier days extensive usewas made of 300 ohm twin lead ribbon cable to feed the signals to the TVreceiver. BTW you can use just a length of 300 ohm ribbon cable to make afolded dipole.
When colour (or color if youprefer) TV was introduced the ribbon cable often created problems which couldbe rectified by the use of co-axial cable. This is not strictly correct becausecoax had earlier uses in TV because of other problems such as ghosting whichbecame intolerable with the introduction of colour.
Now 300 ohm coax can be and ismade. However 50 and 75 ohm cable is preferred for a variety of reasons. Our foldeddipole also exhibits a "balanced" feed characteristic whilstcoaxial cable has an "unbalanced" characteristic. Two problems. Eachsolved by the use of

the balun. file:///C:/DOCUME%7E1/GY/LOCALS%7E1/Temp/msohtml1/01/clip_image002.gif

Fig 1.- Schematic of a balun transformer

Perhaps we should have someclarification about this balanced versus unbalanced jazz. Mentally visualise itthis way. If we have a plus 12V D.C. supply with ground return. That could beregarded for our illustrative purposes as the unbalanced 75 ohm input. On theother hand we could a symmetrical power supply referenced to ground whichprovides a + 12V D.C. AND a  - 12V D.C. voltage.  The only differenceis we are dealing with RF which of course is very high A.C.
A practical 300 / 75 ohm baluncould consist of four pieces of 0.4mmwire wound 2 1/2 turns through a balun core (look a bit like binoculars). In sodoing we have achieved our two goals, (a) the impedance transformation and (b)gone from unbalanced to balanced.

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