Sleeve Antenna Calculator

This sleeve (bazooka) antenna calculator sizes the two key dimensions of a coaxial sleeve dipole — the upper radiating element and the sleeve (skirt) that surrounds the coax feedline — from operating frequency and the coax's velocity factor. It's a common build for VHF/UHF base and repeater antennas where a clean, balanced vertical radiator without a separate balun is desired.

Enter Frequency, Velocity Factor exactly in the units shown by this sleeve antenna. Check the operating band, unit prefix, and decimal position before calculating; these are the inputs used by the formula.

• Frequency — use MHz.
• Velocity Factor.

The results describe the calculated sleeve antenna values for the inputs you entered. Check each value against the available space, selected components, feed system, and operating conditions before making a final design decision.

The Sleeve Antenna uses Element Length = (299.792458 / f) × Velocity Factor. Supply Frequency (MHz), Velocity Factor in the displayed units, then use the calculated values as the first engineering target for this specific antenna types design or analysis.

A sleeve antenna solves a specific problem: feeding a vertical dipole-like radiator directly with unbalanced coax, without the pattern distortion and feedline radiation that comes from connecting coax straight to a dipole's balanced feedpoint. The sleeve — a conductive tube or braid section surrounding the coax outer jacket for a quarter-wavelength below the feedpoint — acts as an RF choke, presenting a high impedance to shield currents and preventing the outer conductor of the coax below the sleeve from becoming part of the radiating structure. This gives the sleeve antenna a naturally symmetric, close-to-omnidirectional vertical pattern similar to a standard dipole, without needing a separate 1:1 or 4:1 balun. Both the upper element and the sleeve are sized as electrical quarter-waves, so their physical length depends on the coax or conductor's velocity factor, not just free-space wavelength — this is why the calculator asks for velocity factor as a required input rather than assuming free space.

Velocity factor is the detail most builders get wrong: it applies differently to the upper radiating element (usually built from solid wire or rod, often with a velocity factor close to but slightly under 1.0 depending on diameter) versus the sleeve, which is typically formed from the coax's own braid or an added conductive tube and inherits the coax's velocity factor (commonly 0.66–0.85 for common coax types). If the upper element and sleeve are built from different materials with different velocity factors, this calculator's single shared velocity-factor input is a simplification — for best accuracy, run the calculation twice with the correct velocity factor for each element and use each result independently rather than assuming they match. As with any quarter-wave element, always cut a few percent long and trim to resonance while monitoring SWR, since real-world conductor diameter (which affects the antenna's effective capacitance) shifts resonant length slightly from the idealized formula.