Vertical Bazooka

[thomast77]

I am looking to build a Scanner antenna. The frequencies I monitor most are in the 155mhz/453-460mhz Range. I am looking for a cheap but effective design. I have been doing alot of research and was looking for plans to build the sputnik from Radio Shack also known as the Antennacraft ST-3. I hear pretty good reviews. But I cannot find plans. It appears that the Sputnik is pretty much a 1/4 wave ground plane at vhf hi so why not just build a 1/4 wave cut at 155mhz. However I have come across another design called the Vertical Bazooka. 2 Meter Vertical Bazooka - Build A Vertical Bazooka from Coaxial Cable by VE3VDC
I like this design it is compact because it does not require a ground plane. Cheap because I will not have to buy an so-239. And I am assuming that since the antenna is essentially part of the coax there is no loss from using connectors. Has anyone here used the Vertical Bazooka? If so I would be interested in how well it performed for you? Any input as to how well this antenna compares to the 1/4 wave ground plane or the sputnik would be greatly appreciated. Thank you for your time.
Thomas

 

[prcguy]

Its a coaxial dipole and they are cheap and easy to make but it needs some things not mentioned in the article. First you really want to decouple the antenna from the feedline with a string of #43 ferrite beads or coil several turns of the feedline near the base and move this up or down the feedline for lowest SWR.

The outer covering of RG-8 coax is usually PVC or similar and a lousy dielectric at VHF. When you fold the braid right over the outer covering it needs to be a good dielectric and since its not there will a little loss. If you were to use Teflon coax or use a conductive pipe much larger than the coax and center the coax inside the pipe with a few styrofoam spacers you will get back the losses.

After all this you will have a 1/2 wave dipole for VHF and that's all it will be good for, its a single band antenna.
prcguy

 

[thomast77]

Thanks for your input prcguy. So what your saying is that the 1/4wave ground plane cut for 155mhz would be better than the bazooka? I am trying to avoid cost here so what if I build the ground plane into the antenna instead of using an so-239. Anything I should know about that type of setup?

 

[prcguy]

A properly designed bazooka (coaxial dipole) should have a very slight advantage over a 1/4 wave groundplane but the groundplane is easier to build correctly, so I think its a better choice for you.

An SO-239 is very inexpensive and a good foundation to build a groundplane on. Or you can use an N type chassis connector. Either way you can use #10 or #12 solid copper house wire for the groundplane elements or brazing rods makes a nicer looking antenna. Goop up the top of the SO-239 at the center junction with some RTV when your done and have a party.
prcguy

Thanks for your input prcguy. So what your saying is that the 1/4wave ground plane cut for 155mhz would be better than the bazooka? I am trying to avoid cost here so what if I build the ground plane into the antenna instead of using an so-239. Anything I should know about that type of setup?

 

[thomast77]

Thanks again for the input prcguy. I do have some #12 solid copper wire. Maybe I'll just build a ground plane.

 

[thomast77]

I wanted to update everyone. I decided to build a Vertical Bazooka because it is cheap and easy to build. I decided to beef it up some. I used rg8x coax I had laying around. I also had some 1/4" aluminum rods threaded on one end and some aluminum tubing from an old antenna. I calculated the measurements for 155 mhz:
7500/155mhz=48.387cm converted to inches I get 19.05" for the top section.
7500/155mhz x .78 velocity factor for rg8x coax=37.741cm converted to inches I get 14.859" for the lower section.

I bought some 3/4" pvc and two end caps. I drilled a 1/4" hole in both caps. Slide one cap onto the coax before you start making the antenna. I stripped about 16" of the outer jacket and slid the shield carefully back over the coax and trimmed it to 14.859". I found some aluminum tubing lying around that fit very snugly around the shield. You probably don't need it but I just felt like doing it. It will ensure that the shield remains the correct length and stays in place. You could use shrink wrap or electrical tape I suppose. I then cut the 1/4" rod to 19.05". The 1/4" rod was threaded about an inch on one end. (For those of you who may want to try this antenna. You will probably not be able to find a 1/4" rod only threaded an inch on one end. I don't see why you could not just use 1/4" or 3/8" all thread rod from the hardware store). I bought some brass nuts and soldered the center conductor to the nut. I also used a nut on the outside of the cap so I could tighten it down good. Then just slide the caps on both ends of the pipe. I used a 5' piece of pvc pipe. So basically the top section is sticking out of the end cap secured with one nut inside the cap and one nut outside of the cap. And the lower section of the antenna is inside of the pipe.

I mounted it on the eve outside my bedroom window. I must say this antenna is working very well for me. My main interest is VHF 154-155 and UHF 453-460. And this antenna has exceeded my expectations. I have decided to eventually build a 1/4 wave ground plane and compare the two and keep the one that works best for me. I just wanted to share what I have done because it is working better than the Radio Shack Discone I used to have mounted at about 20'. If anyone is interested I will take a picture of the antenna and post it here to give you a better idea of what it looks like when done. I like this design because it is compact. It requires no ground plane. Also don't forget to leave a mounting gap if you are mounting it to a metal pole. the measurement would be 3750/155mhz=24.193cm or 9.524" from the bottom of the shield to the mounting bracket or pole. Here is a link to the site where I got the idea for this antenna. 2 Meter Vertical Bazooka - Build A Vertical Bazooka from Coaxial Cable by VE3VDC
Tom