December 31, 2023 at 3:53 pm #41115
This is imported straight from my QRZ page, so if errors turn up, just call me on the 2 meter or email me and I’ll fix it.
(Please note this design was purely my own and any failures should be attributed to me, not the original antenna design or designers. )
Dan’s Double Bazooka
This is a cost saving center connector for your double bazooka antenna project.
It is not a full set of directions to build a complete double bazooka. It is a project to build only one part of a double bazooka antenna system. The measurements of the limbs and stingers are band (or band section on 80 and 160 meters) dependent, so I’ll add the needed math in a post below this. So … here we go.
Why the more complicated double bazooka over a simple dipole? Well … because you can. The double bazooka does have some performance improvements over a dipole, which for the chronically pragmatic, probably aren’t enough to justify the extra effort, but since when are amateur radio operators all that pragmatic? If it was all about labor saving, amateur radio wouldn’t exist. it’s easier to sit on the spud-couch time-slicing between the cartoon channel and thumb-scrolling a smart phone. On the other hand, after using this thing non-stop for five years, I’ve found the durn thing is a tough as a box of rocks. It’s also modular, and therefore easy to work on and quick to replace parts.
Once again, please note this is not an instructional in how to build a full double bazooka. DBs are band specific, so that’s way deeper into the weeds than I want to get here. What I am dealing with here, is a non-band specific double bazooka CENTER CONNECTOR designed to make building your DB easier and more affordable by eliminating the need to purchase a full length of coax, doomed to be destroyed, and having to fiddle around with center snicking, stripping and soldering, thus also avoiding the inevitable physical weakening of the antenna structure. Besides, I just can’t bring myself to ruin a perfectly good hunk of coax. Nor is it easily tolerable to lop off a perfectly functional and well installed PL259. With this design you may be able to simply re-purpose lengths of coax already existing in your radio-junk pile. It also easily allows you to easily re-purpose the coax used in your DB if you decide to change antenna designs.
Parenthetically, regarding the limbs of your DB, you’re welcome to use whatever suites your fancy, but full diameter RG8 will always give you the best performance and broadest band tuning. It’s also a lot more durable.
I’m going to presume here that you have a moderately well supplied and equipped shop or garage to work in. You can do without, but life will be much easier if you have and use a drill press, bench vise, bench grinder, chop saw with a clean sharp blade, a soldering iron or gun, and a battery operated drill/screwdriver with variable torque settings.
I am also going to presume you know how to use all of the above gear in a safe and efficient manner. Remember safety always comes first, and fingers are irreplaceable, and any screw-ups and injuries you encounter are your own fault and strictly your responsibility. As I have absolutely no control over your skills and quality control, I am also not responsible for the outcome of your project nor any consequences thereof. In short, I ain’t liable, so y’all be careful now, ya hear!
As always, any good antenna project requires spending some quality time — and money — at your local hardware store. Then you’ll want to stop in at your favorite radio store for a quarter dozen chassis mount SO239s.
You will need (per bazooka).
- (3) chassis mount So239
- (1) 1 1/2 PVC Schedule 40 “Tee”
- (3) 1 1/2 to 1/2 inch PVC Schedule 40 reducers
- 18 inches of 1/2 inch schedule 40 PVC
- 24 inches of solid or stranded insulated wire, suitable for RF connections. Your SO239s will dictate appropriate gauge.
- (1 each) PVC cleaner and glue
- (12) appropriately sized stainless steel screws for attaching SO239s. eg. Home Depot #4 by 5/8 inch Phillips pan head sheet metal screws. They come in packages of 8 units so you’ll have to buy 2 bags — but this is not a problem. They are handy little buggers and you’ll use them up sooner or later.
- (1) 1/8 inch shaft machine thread eyelet.
- (4) inches of left over gray water-heater pipe insulation.
- Liberal quantities of latex sealant/gorilla snot.
Once you’ve collected all the requirements, it’s time to prepare some inserts/spacers. Using a saw capable of providing clean smooth cuts, lop off (3) 3/4 inch long pieces of the 1/2 inch PVC.With these, if you err, do it to the short side. it will save you some time and effort in the next step.
Figure 1. Prepared 3/4 inch length spacers.
Apply glue and press the previously cut spacers flush into the 1 1/2 inch PVC reducers as illustrated. This is most easily accomplished with a bench vise. immediately wipe away any excess glue. If they are a bit long and protrude, they can be ground flush with judicious use of a fine grit wheel on a bench grinder.
Figure 2. Inserts pressed (and glued) into place.
***The next step is critical.*** fit, mark and PRE-DRILL the screw holes for mounting the SO239s. Using the suggested screws, you will need a 3/32 drill bit in a drill press. You can hand drill these, but I wouldn’t recommend it. While you have the parts at the drill press, this would be a good time to drill the feed wire pass-through. This should be gauged according to the size of wire you have chosen to use. In my case, I selected a 7/64 bit. See Figure 4 for the placement of this hole. Drill wire pass-through holes in all three reducers at this time.
Figure 3. Pre-drilled SO239 mounting holes.
Figure 4a. Wire pass-through hole location in relation to pre-drilled SO239 mounting holes.
Figure 4b. The consequences of driving the mounting screws without pre-drilling.
Set these assemblies aside and drill a 1/8 inch hole in the “top” of the PVC Tee, and mount the eyelet, securing it with a hex nut on the inside. Be careful to avoid any unnecessary intrusion into the interior. Place a piece of the pipe insulation inside to cover the securing nut and prevent electrical contact with the jumper running between the limb center conductors.
Figure 5. Inserted hanging eyelet.
Figure 6. Securing eyelet hex nut.
Now that the prep work is done, let’s discuss the electrical realities of the DB center connector. The center conductors of the SO239s mounted on the limbs of the “T” are connected between them, and to nothing else. The center conductor of the SO239 mounted on the base of the “T” (feed point) will be connected to the shield/outer conductor of one limb, and the shield of the feed point will be attached to the shield/outer conductor of the other limb.
Let’s put it all together.
Glue and insert both limb reducers into the “T”, making sure the wire pass-through holes are at the bottom (in relation to the stem of the “T”. You may wish to align the reducers by eye, and then use a bench vise to press them into the “T” for final fit. DO NOT glue or install the base reducer at this time.
Strip and solder a jumper wire into one SO239 center conductor which is long enough to reach 1/2 inch out of the other side of the “T” when properly mounted.
Figure 7. Strip and solder the center jumper.
Mount this SO239 onto one limb using three of four screws. Start the screw closest to the pre-drilled wire pass-through hole but do not tighten. Leave it with 1/4 inch of the screw shaft showing above the SO239 surface. You will be attaching a wire to this screw later in the assembly process.
Moving to the other limb, snip the protruding wire as short as possible, but leaving enough to securely solder it to the center conductor of the next SO239. Perform that soldering operation at this time. Once soldering is complete, mount the SO239 as you did before on the other side, leaving the bottom screw extended 1/4 inch.
Depending on the amount of wire slack left between the center conductors, you may need to reach through the bottom with a pair of needle-nosed pliers, and twist the wire until slack is removed. Pinch the extra length together to eliminate electrical distance, and solder the wad if necessary to ensure minimum electrical distance between the limb center conductors.
Figure 8. Twist, wad and solder any remaining slack.
**** ALTERNATIVE METHOD
Alternatively, you can solder a shorter piece of wire onto both SO239s, bending an “eye” into on, and leave a straight probe on the other. When mounting the SO239s, ensure the probe of one side penetrates the eye on the other. Then reaching through the bottom, pinch the eye tight and securely solder the connection. ****
Place another piece of pipe insulation into the “T” covering the jumper wire.
Now feed the limb wires through the the wire pass-through holes and work them down through the base of the “T”, one will be soldered to the feed point center conductor, the other out through the base feed-through hole.
Glue and securely mount the lower reducer after you have fed the wires through their proper location. Align the wire feed-through hole for minimum distance to it’s related limb. See Figure 9 for details.
Figure 9. limb wires fed through the base reducer.
Solder the lead from the appropriate limb onto the center conductor of the remaining SO239, and mount the SO239 as before, leaving the screw closest to the wire feed-through hole. Then wrap the other wire around the protruding screw and secure it firmly.
Moving to the limb which is to be connected to the feed point center conductor, carefully pull the wire coming through the feed hole, and when the internal slack has been removed, secure the wire to the waiting screw, and secure.
Figure 10. Securing feed wires to SO239 mounting screws.
Moving to the other limb, repeat the process of taking up any slack, and secure to the waiting screw. Snug all wire connecting screws.
For additional strength and security, 3/8 inch self-tapping screws may be used to ensure reducers are safely retained under all conditions.
Construction is now complete. Continuity check for electrical integrity.
Figure 11. Completed build.
Last but not least, you may wish to liberally apply a high quality sealant over all screws, mating surfaces and wire entry ports.
EDIT: If water incursion gets to be a problem, drill a drain hole, and using compressed air, blow all the moisture out of the assembly. After you are sure you’ve gotten the water out … use the same hole to inject enough Gorilla Snot electronic-safe sealant to fill the structure, then set a screw into the hole you drilled.
-73-December 31, 2023 at 4:41 pm #41116
Now the math.
a) Total overall length (both limbs, stingers and center connector) = 460 divided by frequency (in mhz)
b) Total single limb (plus stinger) length = (a) divided by 2
c) total “driven” coax section = 325 divided by frequency (mhz)
d) Single limb “driven” coax length = (c) divided by 2
e) Single stinger length = (b) minus (d)
Uhm … yeah. You’re right. Confusing as hell.
Probably better to just include the following explanations from K3DAV.
Our center connector will take the place of the middle section indicated by the big black arrow.
Figure 2 (3a)
Here the “tail” section is your stinger, but we will be using PL259s and a shorted barrel connector in the place of the indicated soldering.
Now attach a soldered PL259 onto each end of both “driven” elements, and one end of each tail section/stinger.
Smoke/continuity check everything, and then set a short barrel connector into a vice, and drive a self tapping screw into the center of it, to short the internal conductor to the outer shell.
Assemble the parts as seems logical, and then prop it up off the ground as high as you can, and using an analyzer or SWR meter, tune it to the intended frequency of use.
That’s just about it.
Comments, criticisms and/or improvement ideas are absolutely welcome below.
December 31, 2023 at 9:10 pm #41120KI5VGQParticipant
- This reply was modified 1 month, 3 weeks ago by Danial Beard.
I picked up the pvc and some hardware for two. Got glue and cleaner also.January 1, 2024 at 8:05 am #41124
Good deal! I suspect the hard parts to get will be the So239s and the PL259s. BTW, I re-checked my QRZ page and the parts list is still there.
Happy New Year!January 1, 2024 at 6:52 pm #41132
1) Just for the kicks and giggles of it, my next build is going to use N type connectors. You may wish to try this yourself. They’re a little more sturdy. The chassis mounts will install the same way, and male N connectors will be needed on the limb center elements. Out at the stinger connections I recommend staying with the PL259s and UHF barrel connectors for simplicity.
2) Richard (at MTC Radio) has some of those hurky-looking wire antenna end grips for tying off wire antenna ends. They’re big, silver and ugly, but they work. I’ll be switching all my Bazooka ends over to those doohickies. They are dirt simple and generally tougher than a bag of hammers. Be sure to pinch the wire bails snug before you hang ’em.
(If you know what the dang things are actually called, please post it below.)
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