Electronic components, Magnetism, RF circuits, Telecomunications

Other baluns and the unun

This post shows the other types of baluns that wasn’t shown in the previous post about the same subject. Also introduces the unun.

It’s recommended that you read the post about the operation of baluns before continue.

What is balun?Click here

Folded balun

Also called a quarter-wavelength balun, due to the parallel conductor have one quarter wavelength of operation frequency. It’s connected to coaxial cable and one part of dipole antenna, as shown in the figure above. Serves to make connection between an unbalanced coaxial cable and a balanced dipole antenna.

folded balun
An example of folded balun for 432 MHz, this component is also called Pawsey stub. Source: DG7YBN.

Coaxial balun

Are used pieces of coaxial cable to make impedance matching and each terminal on the output is linked to a cable end. This type have two settings: on 1:1 settings, 2 coaxial cable pieces connected to the unbalanced side, one whose length is 1/4 wavelength of applied signal and another that measures 3/4 wavelength.

coaxial baluns 1:1
In 1:1 settings, the impedances on input Zi, in the cables Zc and on output Zo are equal.
another 1:1 balun
Another possible arrangement of 1:1 setting, a piece of 1/4 quarter wavelength, where the central conductor and the mesh are connected by a conductor. Source: One transistor.

Other settings is the 4:1, where the antenna’s output has 4 times input’s impedance of unbalanced coaxial cable. For that, it’s used a cable piece whose length is half of signal’s wavelength. This piece must be connected as shown in the figure below.

In both configurations, the unbalanced cable’s length can be arbitrary. Are used for antennas that operate in VHF and UHF frequencies, whose bands are 30 to 300 MHz and 300 MHz to 3 GHz, respectively.

Bazooka balun

bazooka baluns

A sleeve made of conductor material, must have a quarter wavelength (\lambda /4). This is linked to coaxial cable’s external mesh in the opposite side of antenna, in other words, it’s short-circuited. 

sleeve balun
Must have an air gap between the sleeve and the cable. Source: The BRATS.

What is common mode current?

When high frequency currents pass through coaxial cable, occurs the skin effect, that produces one more current that pass through external wall of metallic mesh, this is the common mode current.

skin effect on coaxial cable
The high frequency currents in a coaxial cable: Ia in center conductor, Ib in the mesh inner wall and Ic, the common mode current in external wall. Source: StackExchange.

When a coaxial cable is connected to a dipole antenna, the current on both sides of dipole aren’t equal and the antenna won’t work properly, because it’s irradiation will be distorted. In addition to the cable radiate radio waves.

dipole antenna without balun
(b) shows the unequal distribution of current when there is no balun.

One side is short-circuited so that the another side has an infinite impedance, blocking the common mode current. Consequently, antenna becomes balanced. The folded balun uses the same principle.

baluns bazooka model
\hat{V}_{S} and \hat{Z}_{S} form a current model that enters on transmission line which goes from 0 to L. \hat{Z}_{IN}(0) is the input impedance and \hat{Z}_{L} is the load impedance. Source: In Compliance.

Mathematical demonstration.

\large \hat{Z}_{IN}=Z_{C}\frac{\hat{Z}_{L}+jZ_{C}tan\beta L}{Z_{C}+j\hat{Z}_{L}tan\beta L}

  • Z_{C} is conductor impedance.
  • L is balun’s length.
  • \beta is phase constant, whose value is \large \frac{2\pi }{\lambda }.

\large \hat{Z}_{IN}=Z_{C}\frac{jZ_{C}tan(\frac{2\pi }{\lambda }) (\frac{\lambda }{4})}{Z_{C}}=jZ_{C}tan(\frac{\pi }{2})=\infty

The unun

The name is the abbreviation of unbalanced-unbalanced, makes the impedance transformation between two unbalanced sides. It’s used to connect an antenna with unbalanced structure to a coaxial cable.

unun 49:1
An example of unun 49:1 with a ferrite core. Impedance on output is 49 times higher than on input. Source: WorldWideDX Radio Forum.

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