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Transmission line transformers commonly use small ferrite cores in toroidal rings or two-hole, binocular, shapes. The Guanella transmission line transformer (Guanella 1944) is often combined with a balun to act as an impedance matching transformer. Putting balancing aside a transformer of this type consists of a 75 Ω transmission line divided ...
Practical impedance-matching devices will generally provide best results over a specified frequency band. The concept of impedance matching is widespread in electrical engineering, but is relevant in other applications in which a form of energy, not necessarily electrical, is transferred between a source and a load, such as in acoustics or optics.
A circuit that has the specific purpose of allowing interfacing between balanced and unbalanced circuits is called a balun. A balun could be a transformer with one leg earthed on the unbalanced side as described in the transformer balance section above. Other circuits are possible such as autotransformers or active circuits. [4]
The quarter wave transformer is an alternative to a stub; but, whereas a stub is terminated in a short (or open) circuit and the length is chosen so as to produce the required impedance transformation, the λ/4 transformer is in series with the load and its length and characteristic impedance are designed to produce the required impedance ...
The balun transfers power between the single-ended coax and the balanced antenna, sometimes with an additional change in impedance. A balun can be implemented as a transformer which also allows for an impedance transformation. This is usually wound on a ferrite toroidal core.
Transformers, autotransformers, and baluns are sometimes incorporated into the design of narrow band antenna tuners and antenna cabling connections. They will all usually have little effect on the resonant frequency of either the antenna or the narrow band transmitter circuits, but can widen the range of impedances that the antenna tuner can match, and/or convert between balanced and ...
Another application is the design of impedance matching networks. Impedance matching at a single frequency requires only a trivial network—usually one component. Impedance matching over a wide band, however, requires a more complex network, even in the case that the source and load resistances do not vary with frequency.
At radio frequencies and microwave frequencies, a quarter-wave impedance transformer can provide impedance matching between circuits over a limited range of frequencies, using only a section of transmission line no more than a 1 / 4 wave long. The line may be coaxial cable, waveguide, stripline, or microstrip.
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