The word Balun is derived from two words Balanced and Unbalanced. Thus, Balun is a device that transforms balanced signal to unbalanced signal or vice versa. Balanced signals are double ended, ie, both the terminals of a balanced port are hot with voltage whereas in an unbalanced port, only one terminal carries the signal and other is ground. A balun is a device that connects a balanced impedance to an unbalanced impedance. It usually also provides impedance transformation, and is hence named as balun transformer.
Microwave baluns can be coaxial or planar. Planar baluns are commonly implemented as microstrip structures. The two most popular microstrip baluns are the Marchand balun and the tapered line balun, both of which are capable of decade bandwidths if designed properly.
Tapered microstrip baluns typically consist of two broadside-coupled tapered lines. The two lines are of equal width at the balanced end, while at the other end, one line is at least three times wider than the other so as to form a conventional microstrip line.
Tapered microstrip baluns can be classified according to the type of taper used. Four widely used tapers are:
- Linear
- Exponential
- Triangular
- Klopfenstein
There are mathematical functions that define the tapering of the above tapers. The equations are complex but can be implemented in MATLAB and thus the dimensions of the device can be found according to the impedances required at each end. These equations can be found in [1].
Recommended Readings:
[1] David M. Pozar, “Microwave Engineering”, Addison-Wesley, 1993.
[2] Inanc Yildiz, “Design and Construction of Reduced Size Planar Spiral Antenna in the 0.5-18 GHz Frequency Range”, MS Thesis, Middle East Technical University.
[3] Franco di Paolo, “Networks and Devices using Planar Transmission Lines”, CRC Press, 2000
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