Balun

Realization

A balun is generally produced using rolled up coaxial cable or of a small section of two-wire line wound on a Tore in Ferrite or on a chuck without core (balun in the air). Such a balun can function on broad a Waveband S (2 to 4 octave S). The transmissible power depends primarily on the size of the torus and the quality of ferrite used; to in no case the temperature of the torus cannot exceed the point of Curie of material. One can also manufacture a balun using a loop of coaxial cable an electric length equal to the half-length of wave. The balun is then mono-frequency, makes some it functions correctly on a narrow waveband, of some pourcent. The balun can possibly fulfill a function of adaptation of impedance, with for example a ratio from 1 to 4 (and reciprocally from 4 to 1), thus making it possible to connect a two-wire line 300 Ohm S to a coaxial cable 75 ohms.

Example of application

A Doublet Demi-onde is a theoretically symmetrical antenna. Its radiation passes by a maximum in its symmetry plane perpendicular to its axis. If one feeds it directly in his center with an asymmetrical coaxial cable, its diagram of radiation will not be perfectly any more regular and the least loss of adaptability of impedance on the level of the antenna will cause the formation of a current of sheath which, can be awkward by causing jammings. A solution consists in placing a balun of report/ratio 1/1 between the cable and the antenna; it then fulfills the role of a Self of stop in common mode. It is however necessary to keep in mind that the balun introduces losses, this one should not exceed in importance the disadvantages which the balun is supposed to correct.

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