The 2 Meter Vertical Bazooka
Antenna
Courtesy LD Blake, VE3VDC
1-07
Build your own vertical dipole from
coax
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 cable.
Beyond being extremely easy to build, these
antennas have several nice features to recommend them:
Total cost under $10.00cdn
No coils needed.
No ground radials.
SWR under 2:1 across several megahertz
They are surprisingly rugged antennas.
Can be built as part of your feedline
Can be coiled up and put in your car trunk.
Performance like regular half wave antennas.
The plan for a Vertical Bazooka is on the
right. 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 wish.
The antenna 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 balconies.
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 SWR.
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.
Power handling 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 the coax.
The Vertical Bazooka is a fun and easy
antenna that can be built in an afternoon.
Building The Antenna
My weekend 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 antenna.
One preliminary 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.
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 the antenna.
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 plan.
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 antenna.
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 above.
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.
Fortunately 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 above
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 outer
insulation.
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.
My 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 needed.
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.
Trimming 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 time.
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 good.
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.
On the left above 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 affected.
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.
The last 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 well.
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.
The 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.
Some Suggested Modifications
Since originally posting this article
I've received a few emails with suggestions for improving the
design. Here are a couple of the better ones...
-
Use copper tube for the lower
section.
-
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 antenna.
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 Work
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.
Good luck with your Vertical Bazooka
dipole.
