General Forums > Antennas
Double Bazooka antenna plans
K3DAV:
I have built several Bazooka antennas in my past, and I have to honestly say that every one I built out performed the basic wire dipole it replaced. A lot of how they perform depends on the materials used in the construction. ie type of coax, type of wire used for the tails, and even down to the kind of solder used on the connections. And one big mistake that many guys make when building a Bazooka is the exact measurements on each side of the feedpoint.
Too many times, guys just do a rough measurement and one side turns out to be a few inches longer than the other. They do the same with the tails when trimming for SWR. This makes it impossible to get a good low SWR across the band. You have to make the measurements exactly the same on each side of the main element and on the tail trimmings. It is how you get the best impedance balance over the entire length. If these aspects are paid close attention to, a Bazooka will outperform a dipole every time. And yes it is possible to get a 1.1 SWR in the band center and as low as 1.4 SWR at the band edges. I have proven it time after time.
Another mistake is the type of coax used to make the antenna. Using any kind of coax with a double shield or a foil lining around the center conductor, actually lowers the performance. A Bazooka is designed to work best with leaky type coax. The only 2 coax types that should ever be used to make a Bazooka are RG-8X and RG-8U. Basic RG-8X (Mini) works very well on almost any HF band. But the basic standard RG-8U coax works better. The main reason is due to the larger diameter of the RG-8U shield wire. The larger diameter wire provides a wider bandwidth for better SWR at the band edges.
I built a Bazooka with RG-8X for a friend to use on 60 meters. I cut it to the center frequency of the 5 channel spectrum. The SWR does not go above 1.2 at the edges and a 1.1 in the middle. He has had great contacts around the country and into Europe and Asia with it. Build the the right way and they will work the right way.
I have an article on my website that explains the details a lot better than I could do here. Anyone wishing to read my article can CLICK HERE
W5LZ:
Here are some observations I've noticed about bazooka type antennas.
It's a 1/2 wave length antenna and will behave/radiate as any 1/2 wave length antenna will. It will be
have the same radiation pattern and be affected by all the things any such antenna is affected by,
height above ground, whats around it etc. The biggest difference is in how a bazooka antenna is fed.
It's fed through a tuned circuit, a lot like a 'beta' or 'delta' match. That feeding method is why it has
such a 'wide' usable SWR range.
What type of coax cable used in the construction can make a huge difference in performance. Coax with
a 'foam' type insulation will not handle the higher voltages present as well as a cable that uses a 'solid'
type of insulation. So, RG-8X isn't a great type of cable to use for a bazooka antenna. (A voltage increase
is always a product of impedance transformations.) The old RG-8 type cable with solid inner insulation
tends to work better.
The old 'classic' bazooka with the open wire feed line type 'ends' uses those larger 'conductors' because of
end-effect, high voltage at the ends of a 1/2 wave antenna. Makes for less arcing, corona discharge.
Considering the intent of the original bazooka antenna, radar, which involves high power 'pulses', corona
discharge was a real problem. With typical amateur use those large conductor 'ends' aren't really needed.
You can still use them, won't harm a thing if you do.
The type of coax used in it's construction will also determine the different lengths of various parts of a
bazooka antenna. The overall length will stay pretty much a 1/2 wave length, but the lengths of those
center sections, the tuned circuits, will be affected by the velocity factor of the coax cable used. Those
center section lengths (electrical 1/4 wave lengths) are affected by the velocity factor of the cable, making
them 'shorter' than the full length of 1/2 of that antenna. That's why they are shorted toward the ends of
the antenna instead of just continuing to the ends, 'tunes' those center sections, right?
So why don't the 'numbers' work out exactly? Because there are a lot of variations in coax cable, none of
it is ever exactly the same on the same spool, much less the same manufacturing 'batch'. See how that
affects things?
Performance or efficiency is determined by what you are measuring with. A bazooka antenna is not very
efficient away from it's design or center frequency. A typical SWR meter is a terrible means of measuring
performance or efficiency. At best, an SWR meter can only indicate how well impedance matching is done
which has very little to do with how well an antenna 'works', how it performs. A dummy load is a very good
example of that, great SWR, lousy antenna. If your main criteria is SWR then a bazooka antenna is a very
nice antenna to use. If it serves your purpose well, then use one! There still ain't no such thing as a free
lunch, but if the price isn't too steep, why not?
- Paul
K3DAV:
I agree with most of what you say Paul. The Bazooka is subject to the same conditions as a dipole for height above ground and the surroundings.
But one of the main points I was making is how guys build these antennas and do not pay close attention to the measurements. If one side of the main element or the tails are off even 1 or 2 inches from the opposite side, the antenna will not operate with the best efficiency it should give. That is a mistake I learned the hard way the first few times I built one.
But I have to say that the Bazookas I have built did a better job than their late dipoles they replaced. Now there has to be something to that. For example, I helped a friend build a basic dipole for 40 meters several years ago. He runs only the 100 watts from the radio. We were extra careful of the measurements, and used my MFJ-269 analyzer to tune it before it went into operation. It worked very well for my friend always with great reports.
Then I built him a 40 meter Bazooka. Tuned it again with my MFJ analyzer, then put it to the test on the air. The Bazooka was strung in the exact same space the dipole used to occupy, so height above ground and the surroundings were the same. The first thing we noticed was the background static was lower which made us think the antenna was not quite as good. But the locals noticed his signal was higher, and the receive signals were higher also. And then the DX test. My friend here in PA talks on a 40 meter net every afternoon. The net controller is up in CT which is about 250 miles east with 100 watts in the daytime on 40 meters. The net controller noticed the increase in signal instantly, and the same over the next few weeks. The same improvement was noticed by most of the operators on the net also.
For what it's worth, the analyzer said the impedance was 43 ohms at the lowest band edge and 59 ohms at the highest band edge, but it was 51 ohms in the center where we tuned it to a 1.15 SWR. I have never found a basic dipole that could do that well. Many can get a good 49 to 51 ohms at the band center, but drop to 30 or lower at the low band edge and to 75 at the high band edge. The tuner was never needed on any frequency within 40 meters.
The lower noise level is probably due to the DC grounding effect by the continuous loop type connection of the center wire. This usually helps reduce typical QRM and QRN on any DC ground antenna. The SWR was lower on each band edge which at least helps the radio run a bit more efficiently. But the fact is, the design of the Bazooka does create a slightly larger signal pattern than the average 1/2 wave dipole. This actually produces a tiny amount of gain. It's not much, but even a 0.2 dB gain is more than 0 dB gain. It may not be a big bad increase in the ERP, but it does make the signal pattern larger which directs a bit more of the signal towards the horizon. Again nothing to write home about, but it is noticeable. Any increase no matter how big or small is still an increase over what it was. This only proves that not all 1/2 wave antennas radiate the same exact way. If you notice the actual length of the Bazooka actually measures less than a 1/2 wave. But the design gives it a larger radiated pattern than a 1/2 wave dipole.
All I can say is that I must have made more than 50 Bazookas in my time, and those that replaced basic dipoles outperformed the dipole. Some were made with RG-8X and some with RG-8U. The RG-8X should be able to handle the same power it is rated to carry. If that is the case, then RG-8X should handle full legal limit on any HF band. The power rating drops big time for VHF and UHF so I would not use the 8X on those bands.
I am only reporting the results that I have gotten with the many Bazookas I have built. But I know guys who have seen the same observations you have with their own built Bazookas. I still believe from good experience that those who get less than expected results, have built the antenna with measurement errors and did not notice them or even think to check them. As I said I found out the hard way how crucial those exact measurements are between both sides from the feedpoint. But now I know better and I pay very close attention to the measurements of each side. If the Bazooka is built correctly with the right measurements and materials, it will indeed outperform any basic dipole.
KE7TRP:
The bad press on the bazooka comes from a foreign ham who ran tests on dipole vs his homemade bazooka. His tests show a major loss in performance and in short, Slammed the antenna. Ham read this and just parrot what he wrote. I try to run my own tests here and will as soon as I get my antenna up in the air. I plan a back to back test with two antennas up at the same time.
Recently two stations I know switched from wire to the bazooka and both have stronger signals here and they don't have to use the tuner anymore. If this antenna does not work as well as my wire, I will report it and I will find the drawback.
C
K3DAV:
Ahhhh That explains a lot. Have you ever read reviews for anything on Eham.net? There are usually a majority who loves or hates the product. But nine times out of ten, those who are in the minority who hates something, usually did not construct or install the product properly. This is more so with antenna reviews. When a vast majority love the antenna, and a few hate it, it's usually because the haters built it or installed it wrong. And you know they would never come back and say, "Ooops I made a big mistake and now I love the antenna."
Well Clark, I eagerly await your test results, so get out in that Arizona hell heat and get to work. LOL
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