What is a Double Bazooka Antenna for Amateur Radio?
A Double Bazooka antenna is a specialized type of broadband dipole antenna designed specifically for amateur radio. It is constructed using a combination of coaxial cable and standard antenna wire, making it a popular choice for operators looking for a resonant antenna with a wider bandwidth than a traditional wire dipole.
How It Works
The antenna functions as a half-wave dipole, but the “magic” lies in its construction. The center region of the antenna is made from coaxial cable (like RG-58 or RG-8), while the outer ends are made from 300 ohm twin lead according to K5IJB. Coax length is 325/f(MHz) in feet. Total length of two tails is 460/f(MHz) minus length of coax (divided by 2 to get measurement for one side).
- The Feed Point: The coax from your transmitter connects to the center of the antenna. At the other end, the inner conductor and the outer shield of the antenna’s coaxial elements are bridged. The center conductor of your feedline connects to the shield of the right coaxial leg. The shield of your feedline connects to the shield of the left coaxial leg. Inner conductor is continuous in the center and is not connected to the feedline.
- The Resonant Stub: The coaxial sections act as a shorted stub. This creates a high-impedance parallel resonant circuit.
- Bandwidth Expansion: As you move away from the center frequency, the reactance of the stubs changes in a way that partially cancels out the reactance of the antenna itself. This effectively “flattens” the SWR curve, allowing you to use more of the band (like the 80m or 40m bands) without a tuner.
Advantages and Disadvantages
| Pros | Cons |
| Broadband: Offers a lower SWR across a wider frequency range compared to a standard dipole. | Weight: The use of coaxial cable makes the antenna significantly heavier, requiring stronger supports. |
| Quiet Reception: Because it is a “closed” DC loop (the shield and center are connected), it often has lower static noise (QRN) than a standard wire antenna. | Wind Load: The thicker profile of the coax catches more wind and can accumulate more ice in cold climates. |
| No Balun Required: It is naturally balanced and typically fed directly with 50-ohm coax. | Complexity: It is more difficult to build and tune than a simple wire dipole. |
Construction Basics
To build a Double Bazooka for a specific frequency:
- Total Length: Calculated using the standard dipole formula: 468/f(MHz) in feet [460/f(MHz) was used by K5IJB].
- Coax Section: This usually makes up the middle 64-66% of the total length (based on the velocity factor of the coax, typically 0.66 for solid polyethylene). 325/f(MHz) in feet in K5IJB design.
- Tails: The remaining length at each end is made of twin lead shorted at both ends to reach the full half-wavelength as per K5IJB. Some have suggested single conductor copper wire instead.