Building the Bazooka Coaxial Dipole Antenna VHF/UHF : A Complete Guide with Multi-Band Calculations.
The Bazooka Coaxial Dipole antenna, also known as a coaxial dipole, represents one of the most elegant solutions in amateur radio antenna design. Originally developed during World War II, this antenna achieves a perfect standing wave ratio (S.W.R.) across the 2-meter ham band (144-148 MHz) and can be adapted for UHF, AIS marine band, and 1.25-meter amateur frequencies. The design uses sections of coaxial cable as radiating elements, creating a broadband dipole with excellent impedance matching characteristics.
What makes the Bazooka antenna particularly attractive is its simplicity and effectiveness. Unlike traditional dipoles that require insulators and precise element construction, the Bazooka uses readily available coaxial cable for both the radiating elements and the outer sleeve, which acts as a quarter-wave transformer. This design naturally provides a 50-ohm impedance match to standard coaxial feedlines, eliminating the need for matching networks or baluns in many installations.
The antenna shown in the image is optimized for the 144–148 MHzVHF amateur band, yet the same design can be accurately scaled for other bands such as AIS (162 MHz), 1.25 m (222 MHz), and UHF (70 cm). What makes this antenna special is the use of coaxial transmission line sections as radiating elements, which improves symmetry, suppresses common-mode currents, and eliminates the need for external baluns.
The original design provided by YY5RM focuses on the 2-meter ham band, but the beauty of this antenna lies in its scalability.
How the Bazooka Coaxial Dipole Antenna Works
The operating principle of the Bazooka antenna revolves around the clever use of coaxial cable geometry. The center section (L1) consists of coaxial cable with the outer braid exposed, forming a quarter-wave sleeve balun. This sleeve prevents current from flowing on the outside of the feedline while simultaneously acting as part of the radiating element. The two end sections (L2) use standard #12 AWG wire to complete the half-wave dipole configuration.
The beauty of this design lies in its velocity factor compensation. Since the center coaxial section has a velocity factor (VP) that depends on the dielectric material between the inner conductor and outer shield, we must account for this when calculating dimensions. Common coaxial cables use polyethylene (VP=0.66), solid Teflon (VP=0.70), or Teflon foam (VP=0.82) as the central insulating material. This velocity factor affects how fast electromagnetic waves propagate through the cable compared to free space, requiring us to adjust the physical length of the coaxial section.
The antenna geometry is defined using three main parameters:
- L – Total electrical length of the antenna
- L1 – Length of the coaxial radiating section (velocity-factor dependent)
- L2 – End section compensation length
Velocity factor (VF) depends on the coax dielectric:
- 0.66 → Polyethylene
- 0.70 → Solid Teflon
- 0.82 → Foam Teflon
2-Meter VHF Ham Band Calculation (146 MHz)
For the reference frequency of 146 MHz using VF = 0.66:
- L = 142.5 / 146 = 97.6 cm
- L1 = (150 / 146) × 0.66 ≈ 67 cm
- L2 = (97.6 − 67) / 2 ≈ 15.3 cm
AIS Band Bazooka Antenna Calculation (162 MHz)
AIS (Automatic Identification System) operates around 161.975–162.025 MHz, commonly used by marine receivers and shore stations.
For 162 MHz, VF = 0.66:
- L = 142.5 / 162 ≈ 88.0 cm
- L1 = (150 / 162) × 0.66 ≈ 61.1 cm
- L2 = (88.0 − 61.1) / 2 ≈ 13.45 cm
This configuration works well for fixed AIS reception antennas mounted vertically.
1.25-Meter Ham Band Bazooka (222 MHz)
The 1.25 m amateur band is popular for FM repeaters and weak-signal work.
For 222 MHz, VP = 0.66:
- L = 142.5 / 222 ≈ 64.2 cm
- L1 = (150 / 222) × 0.66 ≈ 44.6 cm
- L2 = (64.2 − 44.6) / 2 ≈ 9.8 cm
Due to the shorter physical size, mechanical accuracy becomes more critical at this band.
UHF Ham Band Bazooka (70 cm – 435 MHz)
For 435 MHz, commonly used in UHF amateur satellites and repeaters:
- L = 142.5 / 435 ≈ 32.8 cm
- L1 = (150 / 435) × 0.66 ≈ 22.8 cm
- L2 = (32.8 − 22.8) / 2 ≈ 5.0 cm
At UHF, construction tolerances must be tight, and low-loss coax with a stable dielectric is strongly recommended.
Once constructed, the Double Bazooka antenna is notably forgiving in real-world installations. The coaxial radiating section (L1) forms a DC-continuous loop, which naturally helps dissipate accumulated electrostatic charge—an advantage over conventional vertical whips that are often prone to noise buildup. During installation, the center feed point must be properly insulated, and the end “tails” (L2) should be kept clear of nearby metallic objects. This spacing is important to preserve the intended current distribution and maintain a clean, symmetrical radiation pattern.