Balun
Balun comes from the words BAlanced to UNbalanced, and is a device to connect balanced and unbalanced parts of a circuit together. Hams frequently use baluns with antennas.
A balun has two purposes:
- to isolate a transmission line
- to provide balanced output current
What is involved in effective balun design?
Effective balun design will include:
- high winding inductance
- low stray capacitance
- very short internal transmission lines, less than 1/4 wavelength
- high power components that can withstand high power and mismatches
- large wire guage to reduce power losses
- large cores to prevent saturation and help provide high winding inductance
- effective weatherproofing
- rust proof hardware
- robust case that will withstand physical stress
What is saturation?
If the current through the balun is too high, the core is said to saturate. This means an increase in current does not result in a corresponding increase in magnetic flux. This is a bad thing, because the inductance of the coil decreases - resulting in ever increasing current.
When a ferrite core balun saturates, SWR will increase - possibly to excessive levels before the balun fails. Core saturation can be caused by too great a mismatch at the load (antenna) or by running two much power or a combination of both.
Examples of balun designs
Choke balun
A choke balun can be built by simply looping 8 to 10 times a coax line on itself. In fact, a "choke balun" is a bit of a misnomer because it doesn't actually balance the line, but does provides isolation for the transmission line. The proper name for such a device is an RF choke.
4:1 UHF baluns
The fairly common connectors to connect ladder-line wires to coaxial lines in television setups are in fact 4:1 baluns for the UHF band.
See also
External links
- Center-Fed Half-Wave Dipole (3-30MHz) by Martin E. Meserve - good examples on how to build baluns
- A 4:1 Air-wound Balun
- Common mode chokes - efficiency analysis
- GM3SEK's amateur radio technical notebook, particularly this article: Cost-effective chokes and baluns
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