Wednesday, May 19, 2010

Double Bazooka

I have been looking at trying some different antennae. Field Day is coming up and I am working on some double bazooka antenna builds. This antenna has good bandwidth and is rather simple to build. Some designs claim coverage of the entire voice portion of a band without a tuner.

I have RG-6 to build them from and purchased some PVC conduit T enclosures that have the open panel on one side. I think this will allow me to put the SO-329 connector on them and solder inside the box. I am hoping it will allow me to solder inside and seal the feed point.

I cut down one of the outlets to be flush with the enclosure and mounted the SO-239 on it. I have PVC plugs with holes in them to pass the RG-6 through.

Now the math!

460/f-MHz=overall length of the antenna
325/f-MHz=length of 1/4wave shorted stub in center

What does all that mean? Well, the bazooka can be make in different ways. The one thing that stays constant is the coaxial 1/4 wave stub in the middle. I have seen the ends made of 450 ohm ladder line, 300 line, and good old wire like a dipole. I will be using one continuous piece of RG-6 from one end to the other with the center and outer shield shorted at the 1/4 wave point. Just remember to figure an electrical quarter wave using the velocity factor of the line used. The quarter wave shorted stub is where most of the broad banding of this antenna comes from. The use of balanced lines on the ends may add some broadening by changing the length to diameter ratio. The simple wire would likely not add much if any broadening.

The jacket and shield only are stripped back to allow connection to the feed. The center conductor and shield are shorted at the 1/4 point to create the broadband stub. The center conductor is one piece from one end to the other, do not cut it. The total length is found from the formula previously mentioned.