Bazooka Balun (Sleeve Balun)

A Bazooka Balun (more commonly called a sleeve balun) is a type of 1:1 current balun made by placing a conductive tube or braided sleeve over the outside of a coaxial feedline. It is highly effective at choking off common-mode currents, preventing your feedline from radiating and distorting your antenna’s pattern. However, because its design relies on specific physical wavelengths, it has strict practical limitations depending on the band you are operating on.

How It Works: The Quarter-Wave Stub

The Bazooka balun works by exploiting transmission line theory—specifically, the behavior of a quarter-wave shorted stub.

1. The Construction: A metal sleeve (often aluminum tubing or stripped copper braid) is slipped over the main coaxial cable.
2. The Connection: The top of the sleeve (at the antenna feedpoint) is left completely open. The bottom of the sleeve is electrically shorted to the outer shield of the main coax.
3. The Length: The sleeve is cut to exactly one-quarter wavelength (λ/4) of the target operating frequency (adjusted for the velocity factor of the dielectric between the coax jacket and the sleeve).

When common-mode RF current tries to flow back down the outside of the coax shield, it encounters this sleeve. Because the sleeve and the outside of the coax shield act as a secondary coaxial line that is shorted at the λ/4 mark, it presents an infinite impedance (a massive RF choke) at the open end.

This forces the currents on the inside of the main coax to remain perfectly balanced, providing an elegant transition from an unbalanced coax to a balanced antenna (like a dipole or the driven element of a Yagi).

The Catch: Monoband and Physical Size

Because the Bazooka balun relies on a physical λ/4 length to create that infinite impedance, it is strictly a monoband device. Its choking effectiveness drops off rapidly if you move too far from its center design frequency.

More importantly, the physical scale dictates where this balun is actually useful:

• VHF / UHF (Highly Practical): On the 2m band (144 MHz), a quarter-wave sleeve is only about 50 cm (19 inches) long. For a 70cm LEO satellite Yagi, it’s a mere 17 cm. Here, slipping a piece of aluminum tubing over the boom’s feedline is lightweight, cheap, and structurally easy.
• HF / Low Bands (Impractical): If you were building a resonant 80m dipole, a Bazooka balun is generally a non-starter. At 3.6 MHz, a quarter-wavelength is roughly 20 meters. Suspending a 20-meter conductive sleeve vertically from a feedpoint is physically unmanageable and incredibly heavy.

For HF bands like 80m or 160m, broadband ferrite chokes (like coax wrapped around an FT240-31 or FT240-43 toroid) are heavily preferred over sleeve baluns because they provide high choking impedance across multiple bands without the massive physical footprint.

Note: The Bazooka Balun is frequently confused with the Double Bazooka Antenna. While both use coaxial cable in their construction, the Double Bazooka is a broadband half-wave dipole made entirely out of coax, whereas the Bazooka balun is strictly a feedline choke.