A Double Bazooka Antenna is a broad-banded half-wave Antenna. This antenna can operate efficiently across an entire band without needing an antenna tuner. In the 1940es, the MIT staff designed and developed this antenna as a radar antenna for the military.
Like a dipole, the Double Bazooka is a balanced antenna. It is made of coax instead of a single wire. Each branch of the antenna has 2 parts. The first part consists of a quarter-wave shorted stub made of coax. The second part is a wire at the end of each branch. The coax shield and the wire tips operate as a half-wave dipole. The stubs present a high impedance to the feed point at resonance. Off resonance, the stub cancels the antenna reactance and increases the antenna's bandwidth. With this antenna, you can expect a good matching at 50Ω impedance and an SWR below 2:1 over the entire band.
According to the literature, the Double Bazooka antenna offers a 14% wider bandwidth than a dipole. The bandwidth will depend on the diameter of the coax. A noise reduction on receive should be expected over a traditional wire dipole.
The antenna can be mounted horizontally or in an inverted "V" configuration without performance losses. This antenna will perform best on HF bands if installed at 1/4λ to 1/2λ height.
To summarize, the Double Bazooka is a mono-band antenna that provides a larger bandwidth and offers a lower SWR than a dipole. Its typical impedance at the feed point is 50Ω, making it possible to feed directly with a 50Ω coax1.
The Double Bazooka antenna is popular among amateur radio operators, especially on the 80/75 meter band, because of the larger bandwidth compared to a dipole.
The Coax
The best coax for that antenna is a cheap coax. Avoid any coax that has a double shield with a foil wrapping. The foil wrapping will make the construction of the antenna harder. For example, the foil on LMR2 cables is fused with the dielectric, making it nearly impossible to remove.
Popular coaxial cables for building this antenna are RG-8, RG-8x, RG-58, or RG-213. The RG-8 coax has a larger diameter, making an antenna with a larger bandwidth. An antenna made of RG-8 will also be heavier, especially if you are building an 80m antenna. Because of its weight, the connection between the tip and the coax might be a weak point. A heavy antenna might also be more challenging to install.
This antenna can also be built using 75Ω TV coax, but most TV coax found at hardware stores uses an aluminum shield instead of copper. Aluminum should be avoided because it is impossible to solder. The RG-59 Belden 8212 or RG-11 Belden 8261 are good options.
Because the coax parts of the antenna are matching stubs, they should be cut at 1/4λ. The length of these stubs will depend on the velocity factor of the coax. You can use your favorite search engine to download the datasheet of the coax you are using. The datasheet should indicate the velocity factor. For an "unknown" coax, you can use a modern Antenna Analyzer, a NanoVNA, or an Oscilloscope with a pulse generator to find the velocity factor.
Bazooka Antenna Calculator
The following calculator will help you calculate the length of all your antenna elements. Enter the center frequency of the band you want to use and the velocity factor of the coax.
Construction
Use the calculator above to calculate all the dimensions. Make sure you have the exact velocity factor for the type of coax you are using. Depending on how you want to mount your antenna, you might want to leave a certain length of wire at both ends. It will help you install an insulator. Having some margin to play with is also helpful to fine-tune the antenna to the exact frequency you want.
Remove 40mm (1.5inch) of the outer jacket at the center of the antenna. Cut the shielding in the center and separate it into two braids. One of the two braids will receive the center wire of your feed line, and the second braid the shield. Do not cut or damage the center dielectric of your coax.
At the desired length from the center of the antenna, remove the outer jacket and the shield, exposing the center dielectric of your coax. Leave a small quantity of braid exposed. It will be connected to the center conductor.
Carefully remove a small slice of the dielectric to expose the center conductor. Do not damage the center conductor. Damaging this conductor will structurally weaken your antenna. Solder the shield and the center conductor together. see the schematic below.
Finish
Seal all surfaces exposed to rain or moisture. Water getting inside the coax will ruin your antenna. You can use any product that blocks moisture from entering the coax. I use marine-grade heat-shrink tubing. These tubings contain glue that melts with heat. They will waterproof the connections, and they will also mechanically re-enforce that weak section of the antenna.
The results
I quickly put together an antenna for the 6-meter band using an RG-58. I chose to center the antenna at 50.200 MHz because I mainly use the SSB part of that band. The antenna is installed on the rafters in the attic of my garage. It follows the roofline as an inverted "V" at more than 120º. My RigExpert3 gave me the following measurements. The SWR is below 2:1 across the entire 6-meter band.
The antenna can cover the entire 6-meter band with a thin coaxial cable such as the RG-58. Building a Double Bazooka Antenna is as easy as making a dipole. The most challenging part of building this is removing the dielectric without damaging the center conductor.
The following PSK Reporter map shows the contacts made during a test session with FT8.
The antenna is installed in the attic of my garage, but I still used marine-grade heat shrink with glue to re-enforce the antenna where the center conductor is connected to the shield.
I hope to talk to you soon on 6 meters.