Baluns are still a mystery to some radio amateurs and the only way to understand them is to learn what they are and how to use them.
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The word balun means balanced-unbalanced. It's used to adapt a balanced device to an unbalanced one and vice versa.
In a balanced device (as most type of dipoles)
we have the same voltage on both terminals in relation to common ground and if it's not the same, then it is an unbalanced device.
A dipole with direct feed is balanced, whereas 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 fed direct (i.e. RG213). While applying RF we originate 2 current flows on the coax cable:
I1, which flows through the central wire of coaxial cable, from the transmitter up to the dipole;
I2, which flows (for the skin effect) on the inside part of the copper shield, from the dipole back to the transmitter
The two currents, equal and opposite, null themselves so no radiation takes place from the coaxial cable.
The two currents on the dipole are irradiated. Should part of it return, this will then come back on the external side of the shield.
We now have 2 currents on the shield, I2 and I3). The value of this current (which we'll call I3)depends on the impedance value of the external side of coaxial cable with respect to
ground or in other words, it will find high or low resistance. If the impedance is high, I3 will find a high resistance and its current
will be low. If the impedance is low, likewise resistance will also be low and I3 current will be high.
In this way I3 will radiate RF on the external side of the coaxial cable, which will radiate as a third wire to the dipole.
It is as though we have a dipole with 3 wires, and as a consequence to this, the radiation pattern will be distorted as below:
Mostly, this 3rd radiating current often causes EMC problems, producing a source of TVI and RFI and in many cases RF in the Shack.
This new radiating 3rd wire, when its impedance is low, (current high) changes the dipole impedance resulting in a high S.W.R.
To avoid this current within this 3rd wire (I3), we need to use a balun device in order to achieve a high impedance to RF on the external side of coaxial cable.
It will also be possible to achieve high impedance with some lengths of coax cable feeder (odd multiples of 1/4 wavelength) or simply use a balun.
Hence, without balun, varying the coaxial cable length changes the S.W.R.
Many years ago my father installed VHF aerials and VHF TV (rental) sets.
The business was called 'Radio Rentaset Ltd' (later merged with Radio Rentals) and located on Steep Hill in the city of Lincoln.
He remembers, when on site, they had problems receiving good pictures.
What he would do to solve this was to run another piece of coax (75" length) in parallel with the main coax feeder.
He would then cut this down until the picture became good and secondary imaging (ghosting) had gone.
He could have cut the main coax feeder down in stages until it produced the same desired effect.
I guess the 1/4 wave tuning stub was easier or should we call it a balancing device?
The simple balun device is a coil made with some length of coaxial cable placed just below the antenna feed point.
This inductance will make the impedance higher on the external side of the coax.
This effect will result in the 3rd RF current finding a high resistance and its value will be very low and hence will not disturb I2 and I1, which flow inside the coaxial cable.