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How does sleeve balun (Balance to Unbalance) works? In the setup below, the ground and signal of the coaxial cable is connected directly to the antenna which is wrong since coaxial is unbalanced while the antenna is balanced. But I don't understand the reason why adding the quarterlength copper sleeve essentially acts as the Balun for the coaxial and the antenna?

enter image description here

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  • \$\begingroup\$ Interesting, how do you calculate lambda/4? The lines speed factor is ruled by insulation electric constant, in this case the insulator is partly made by the sleeve of the RG213 (unknown plastic, hence unknown er) and partly by the air surrounding. Nice problem isn't it? \$\endgroup\$
    – carloc
    Mar 20, 2022 at 10:11

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The sleeve balun acts as an RF choke, preventing RF current from flowing on the outside surface of the coaxial cable shield.

It helps to think of the sleeve and the outer surface of the coaxial cable shield as the outer and inner conductors of a coaxial line. Because it's shorted at the lower end and is one-quarter wavelength long, it presents a high impedance at the upper end.

It's not nearly as broadband as a true balanced-to-unbalanced transformer constructed with coils wound on a ferrite core, but for single-frequency use, the sleeve is simpler to build, lower in loss, and can handle higher power, than the transformer-based balun.

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Great balun. I've been using them for 40+ years, no noticeable loss. I use rg8 braid for the outer sleve. Easy to put on and wrap it in 4 layers of tape. Lasts for many many years, is flexible and I use it on a rotable 20 meter monobander as well as monoband dipoles. About every fifth year I put on another layer of tape.
wb0snf. Very clear construction details on this article.

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    May 7 at 6:00

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