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A balun / ˈ b æ l ʌ n / (from "balanced to unbalanced", originally, but now derived from "balancing unit") [1] is an electrical device that allows balanced and unbalanced lines to be interfaced without disturbing the impedance arrangement of either line. [2] A balun can take many forms and may include devices that also transform impedances ...
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]
A reference designator unambiguously identifies the location of a component within an electrical schematic or on a printed circuit board.The reference designator usually consists of one or two letters followed by a number, e.g. C3, D1, R4, U15.
A coax balun is a cost-effective method of eliminating feeder radiation but is limited to a narrow set of operating frequencies. One easy way to make a balun is to use a length of coaxial cable equal to half a wavelength. The inner core of the cable is linked at each end to one of the balanced connections for a feeder or dipole.
The goal is to prevent certain current flows (common mode current in the coax shield) while allowing others (opposing currents in the shield vs. center conductor). I should post a good schematic picture. Each type of balun does its job slightly differently. The choke balun prevents the outside current while the coax allows the inside current.
A 10 dB 1.7–2.2 GHz directional coupler. From left to right: input, coupled, isolated (terminated with a load), and transmitted port. A 3 dB 2.0–4.2 GHz power divider/combiner.
Balanced and unbalanced circuits can be interfaced by the use of a balun, often through a DI unit (also called a "DI box" or "direct box"). As a last resort a balanced audio line can be fed into an unbalanced input and vice versa as long as the electronic design used for the output stage is known.
Transmitters feeding a balanced transmission line will need a balun. This transforms the single ended output of the transmitter to a higher impedance balanced output. High power short wave transmission systems typically use 300 ohm balanced lines between the transmitter and antenna. Amateurs often use 300–450 ohm balanced antenna feeders.