The Emergency Vertical Bazooka
your own dipole from coax
(PROJECT COURTESY OF VE3VDC
- L D BLAKE
(See update at bottom of article
Every radio amateur should build at least one antenna to
prove to themselves they can improvise in an emergency. One of the
easiest and quickest antennas to build is the Vertical Bazooka,
which is made entirely from RG-8U coaxial
The plan for a
Vertical Bazooka is in the drawing above. Copy the image to your own
hard disk and you are all set. The design and image are public
domain so feel free to copy them and give them out as you
Beyond being extremely easy to build, these
antennas have several nice features to recommend them:
- Total cost under
- No coils needed.
- No ground radials.
- SWR under 2:1 across
- They are surprisingly
- Can be built as part of
- Can be coiled up and put
in your car trunk.
- Performance like regular
half wave antennas.
itself is an off-center fed vertical dipole made by flipping a
quarter wavelength of braid back over the outside of the coax. The
center lead of the coax forms one half of the dipole and the braid
forms the other. The resulting antenna has a low radiation pattern
and an impedance of 55 - 60 ohms.
One of the more
interesting features is the braid itself. By folding an electrical
quarter wavelength of braid back over the insulated coax we are
forming both a dipole and a "bazooka" (sleeve) balun, a coaxial 1:1
balun that greatly reduces feedline radiation. This makes the
Vertical Bazooka a good choice for use on apartment
The braid side
of the dipole ends up considerably shorter than the top element
because of the interaction between the braid and the coax. The outer
braid couples with the inner braid to form the balun and is thus
affected by the velocity factor of the coax. Note on the plans that
I've compensated for the length difference. Also note the formulas
given will start you off long so you have to trim for the best
You can use the
formulae in the diagram above to create Vertical Bazooka dipoles for
any frequency. However; practical considerations of mounting and
support tend to limit it to the span from 10meters (28mhz) to 70cm
(440mhz). Below about 25mhz the housing tube becomes very long and
can be difficult to support. Above about 450mhz you are pushing the
frequency limits of the coax which are affected by the lower braid
section of the dipole.
with good quality coax should be 100+ watts. However; if you plan to
run more than 50 watts on a regular basis, as a precautionary
measure, you should add a couple of layers of electrical tape or
heat shrinkable tubing between the braid and the outer insulation of
Bazooka is a fun and easy antenna that can be built in an
project was to build a Vertical Bazooka for 2 metres. I was easily
able to build the antenna and get it set up on my balcony in a day,
including several coffee breaks as I went. I had more trouble
getting some weight for the base I used than I did building the
step I would strongly recommend if you are working from an enclosed
location such as an attic or a balcony, is to spend some time with a
handy talkie and find out where you are going to get the best
results. Position the finished Vertical Bazooka in your personal
"hot spot". On my balcony this was just a bit West of center.
On the left you
see all the parts needed to build a Vertical Bazooka antenna. The
larger circle of cable is RG-8U coax which will become the antenna.
The inner coil is my feedline made of 4 wavelengths of RG-8X coax.
At the back of the picture you see the grey plastic tube, a piece of
electrical conduit that will hold the finished antenna.
NOTE: IN THIS PROTOTYPE, GRAY PVC WAS
USED. IT IS HIGHLY RECOMMENDED THAT YOU USE WHITE PVC. GRAY HAS MUCH
MORE ATTENUATION TO RF THAN WHITE!
The first step
is to decide how you want to mount the antenna. It is important to
have enough of a gap between the bottom of the coax shield and the
mounting structure. If this distance isn't at least 1/8 of a
wavelength it is very likely the SWR will be affected when you mount
If it's going
on top of a tower, you can simply make the plastic housing tube as
long as you want, leaving plenty of extra for the mounting gap. If
you are doing a balcony mount a little figuring will have to be done
to ensure the antenna will fit between your balcony and the one
above it, with some clearance above.
For my balcony mount I
decided on 160cm. Mounting this on a 75cm piece of pipe on a round
base gives me lots of mounting gap (certainly more than the minimum)
but doesn't make the antenna so tall that it would not work between
my balcony and the one above. I actually worked it out so the bottom
of the braid would be a couple of centimetres above the steel top
rail of the balcony, which seems to have been a workable
Having decided the lengths of things I then
cut the tube and temporarily assembled it to the base. Next I put a
PL259 connector on the the RG8-U coax and cut it to fit in the tube.
Obviously you want the antenna to be the full length of your tube so
be sure to allow a couple of extra inches when cutting. At this
point I had a piece of RG-8U, the right length, with one connector
installed, as you can see in the picture. This will become my
The next step
was to measure down 1/4 wavelength from the open end of the antenna
so I could cut away the outer insulation and expose the braid.
This distance is determined by the formula:
7500 / frequency = Length (in centimetres)
I wanted to cut
for center band, so, on 2 metres this is 146.000mhz.
Substituting into the formula:
7500 / 146 = 51.37
cm. (20.22 inches)
Being extremely careful not to damage the
braid, I made a cut around the outer insulation of the coax at this
distance and cut along it's length. This allowed me to remove the
outer insulation, leaving me with the braid exposed as shown in the
picture to the left.
Now comes the
fun part. Without unraveling it, I had to push the braid back over
the still-insulated part of the coax, reversing it on the cable. In
other words the braid came up inside the outer insulation and then
went back down on the outside.
this isn't all that hard to do. The braid on coax works like
"Chinese Handcuffs" in that when you push back on it it will expand
in diameter. I worked from the open end back, getting it all loose
and a little bit expanded, then I just grabbed it at the bottom
(where the insulation ends) and rolled it back over itself. This
took about 10 seconds, once I got the hang of it.
The larger picture to
the left shows the antenna with the braid flipped back. At the right
is a close-up of the feedpoint (where the center and braid go
opposite directions) showing how the braid is flipped back over the
Now I hit a
little snag. When you loosen the braid as I did it gets springy and
very difficult to keep in place. I finally ended up wrapping tape
around the bottom to hold it in place while I trimmed. I then found
I could could trim it very easily with cutter pliers.
For your first
cut on the braid use the formula
(7500 / frequency) X
velocity factor = length (in centimetres)
It is very
important that you know the velocity factor of the RG-8U coax you
are using for the antenna. If you have the .80 type, cutting for .66
will make it too short. If you have the .66 cutting for .80 could
cost you a lot of hours trimming it down. So, be sure first.
coax was the .66 velocity factor type so substituting into our
formula we get:
(7500/146) X .66 =
33.91 cm (13.35 inches)
This should get the length of the braid very
close to the correct length and very little trimming should be
On the right is a
picture of the antenna taped to the side of it's housing just before
I took it outside for tuning. Note the tape at the bottom of the
braid, to keep it from springing back.
the top section for best SWR is easy. The final length of the upper
section (center lead) will vary according to the dielectric effect
of the rather thick insulation on it. So I cut very slowly taking no
more than a centimetre, often just a couple of millimetres at a
After each cut I
would check the SWR at 144.000mhz and 148.000mhz. Getting them equal
means the antenna is the right length. If the low frequency is
better the antenna is too long. If the high frequency is better the
antenna is too short. So you know when to stop cutting!
Trimming the braided
shield is a bit tricky. Each cut has a large effect and tends to
affect the SWR equally all across the band. If you find you can't
get the SWR under 1.5 to 1, you can try a small cut on the braid, no
more than a couple of millimetres, and see what happens. If the SWR
improves try it again... if not, stop where you are.
You should note that you
won't likely get a 1:1 SWR out of this antenna. It's feedpoint
impedance is closer to 60 ohms, so anything under 1.5:1 is
I discovered through experimentation that I actually
wanted the antenna just a tiny bit short. Inserting it into it's
tube had an effect like lengthening it slightly, probably due to the
dielectric effect of the plastic. So the first cut you make that's
too short, stop and test it in the tube... it's likely to be the
exact length you need.
To the left is a shot of the antenna all
ready to be assembled and put to work. Note that I have put heat
shrinkable tubing over the braid part of the antenna. This is to
prevent it from moving after assembly, vinyl tape should work as
well. If the braid moves inside the housing, the SWR will be
There were now only a
couple of minor details left:
The first task was keeping the antenna from
sliding out of the plastic tube that was to be it's home. This was
easily solved by making a small hole in the tube and using a wire
tie to clamp the antenna into place.
detail was some waterproofing which I handled by putting a chair leg
cap over the top of the tube. You can get these at most hardware
stores in packs of 4. The nylon ones are really tough and work
And finally once it was all together I put it
on an old fan base I had sitting around and added a little weight to
keep it all in place. It would have been better to clamp it to the
railing, but at the time I didn't have my landlord's permission. The
final assembly, sitting on my balcony is shown on the left.
antenna is almost invisible from the ground. It looks like a mop
handle or broomstick, and not at all like an antenna.
The SWR is under 1.5 across
the entire 2 meter band. It measured 1.3 at each end and 1.1 in the
middle during my tests; not perfect but totally usable.
posting this article I've received a few emails with suggestions for
improving the design. Here are a couple of the better ones...
USE WHITE PVC INSTEAD OF GRAY. MUCH LESS RF
Use copper tube for the
Instead of using the
braid itself, a section of copper tubing could be used to make the
lower section. In this case cut the braid leaving only a short stub
that would be soldered to the copper tube. The length is the same as
for the braid version, but the tube is more dimensionally stable.
You would trim it with a pipe cutter. (I've not tested this version,
so additional feedback would be appreciated)
Use several pieces of
wire to replace the lower section.
- In this
modification you replace the shield section with 3 or 4 runs of
insulated 16ga lamp wire. To do this cut the braid and twist it to
form a short stub for soldering. Make a ring of wire to fit around
the coax, attach the wires to the ring and solder to the shield
braid. The wires are then secured to the sides of the coax with
heat shrink tubing. To adjust this new bottom section, clip bits
off the ends of the wires. This modification has the advantage
that you can still roll the antenna up, if used for an emergency
More mods and
improvements will be posted as they come in. To offer a suggestion
click the "Feedback" link at the bottom of this page.
Putting It To
In checking with a
number of people around my area, up to 30 miles away, signal reports
have been fair, not braggable. Most reports are about the same as I
would expect for a quarter wave, all were far better than for my
handy talkie. The performance is adequate, so the overall project is
what I'd call a success.
Reports from a couple of
other Amateurs who have put Vertical Bazookas on mast pipes are
better than those from my balcony. This I think is to be expected
since the operating environment on my balcony, surrounded by sheet
metal, is far from ideal.
Making an emergency
version of this antenna is simple. Just don't put it in the tube.
You can then coil it up and put it in your emergency communications
kit. Add a little coax and some nylon cord on the top end and you
can string it over a tree branch or hang it from a clothesline while
you need it.
UPDATE! 5-2011 courtesy of
I built this, and mounted it to
one of those fiberglass poles they sell for bike flags. Used half
inch shrink to hold it on. Does tend to flex in the wind, but
seems happy at 70+ mph. I built a mount out of aluminum angle
and a couple of the big Harbor Freight magnets. Looks a little
hillbilly when you get close. The best match I could get was
1.3:1, but it's very broadbanded and stayed under 2:1 for +-5Mhz.
The whole point of the exercise was to find something better
than a mag mount that didn't depend on the car body for a ground
I tried building it with
lmr 400 (corrected for VF .85) Couldn't get it to
Tried again with rg 8x. Finally got close enough to
happening. Using a MFJ259 to sweep, I was
able to identify that this type
of antenna has horrible feedline
coupling to anything metallic in a five
mile radius (perhaps
slightly overstated), making it unsuitable without
cars with metal bodies. It was only after I put 2 passes
the coax through a good sized snap on toroid (just below the
braid), that it
would repeatably tune. My experience in
tuning it was to trim the radiator
portion to get the dip in
the middle of the band, then tweak the shield
length for minimum
Really need to remind people that a
choke is not optional. Even
hanging this from a tree, you need to decouple that
feedline if you expect a
Good luck with
your Vertical Bazooka dipole.