Dave, per your request, below is the article I wrote describing and
presenting the construction details for the Double Bazooka antenna, also
referred to as the Coaxial Flattop antenna. It is considered as just a
single band antenna, but I have never tried to load it up on other bands
with a tuner. -- Ron, K8HSY

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How to Construct A Double Bazooka Antenna

By Ronald E. Milliman, Ph.D., K8HSY
Ham Since 1957


The double bazooka is reported to be broad-banded, which is a quality
especially attractive to those hams operating on the 75/80 meter ham band.
This broadband characteristic is probably do, at least in part, to the use
of a large conductor (coax) for the center part of the antenna.  This
Antenna will not transmit its second harmonic.  In addition, it does not
need a balun, and users say it is quieter than a regular dipole. 


Construction Details

This is meant to be a single band antenna. The center of the antenna is made
from 52 ohm, RG-58 coax. To find the length of coax needed, divide 325 by
the frequency in Mhz.  The coax forms the center part of the double bazooka
and a piece of number 12 wire on each end completes the antenna.  The length
of each of the end wires is found by dividing 67.5 by the frequency in MHz 

A simple, basic Double Bazooka for the 80/75 meter band cut for the center
frequency of 3.75 Mhz when completed would look like this from end to end:
an insulator connected to 18 feet of #12 insolated wire, which is connected
to 86 feet, 8 inches of 52 ohm, RG-58 coax; the feedline is connected to the
center of this 86 feet, 8 inch length of coax.  The other end of the 52 ohm,
RG-58 is connected to another piece of #12 insolated wire, 18 feet long,
which is terminated with an insulator.  This makes the total length of the
Double Bazooka for the 80/75 meter band, from end to end, 122 feet, 8
inches. Again, this is for the center frequency of 3.75 MHz. These
dimensions would, of course, need to be modified if you wanted your double
bazooka resonant for a different frequency. 

Specifically, here is how it is made.  At the very center of the length of
coax making up the main part of your Double Bazooka dipole, cut all the way
around the vinyl outer coating.  Then, remove about 1 1/2 inches of the
plastic covering from each side of this center point, exposing the shield.
Cut around the shield in the exact center point and separate it into two
parts, twisting each of these two braded shields together, making two pig
tails.  Be sure to not cut the dielectric or the center conductor.  Leave
the center conductor and its insulation intact. 

Next, on the feed-line remove about 3 inches of the outer jacket or vinyl
insulation, carefully slit and separate the braded shield from the insolated
center conductor.  Twist the braded shield together, making a pig tail.
Now, strip or cut away about 1 inch of the dielectric center insulation from
the center conductor, exposing about 1 inch of this center conductor. Next
you will connect the 52 ohm, coaxial feedline, by soldering the two exposed
feed-line conductors, the braded shield and the center conductor, to the
pigtails or the two pieces of the separated exposed shield of the double
bazooka's center. You need to take steps to heavily wrap all connections
with electrical tape or better yet, with a good quality rubber tape. It is
recommended that you wrap the entire center of the antenna and the feedline
to strengthen and reinforce the feed point, but also, it is very important
to seal the feed-point to prevent moisture from getting in. 

Next, go to each end of the length of coax forming the main, center part of
your Double Bazooka antenna, and carefully cut or strip back about 1 inch of
the vinyl outer jacket.  Now, you need to slit the braded shield and twist
it together, making a pigtail.  Then, remove the dielectric insulation from
around the center conductor.  Now, you will twist all together and solder
the braded shield, the center conductor, and the end wire.  Again, it is
important to wrap these connections heavily with electrical tape or rubber
tape to protect them from the weather, moisture, and to give them additional
support.   

Each of the very ends of the Double Bazooka are attached to insulators and
suspended in the air above the ground as high as possible.  Also, it is good
to give the center of your Double Bazooka some support by tying it up in the
air from a length of strong rope from a tree limb or some other supporting
point.  

Performance

I, personally, have had excellent results with this Double Bazooka antenna,
especially on the 75/80-m and 40-m bands.  It works well on the 20-10 meter
bands too, but of course, not nearly as well as my 5-element beam.  I have
never tried it on 160 meters, but I would think it will perform extremely
well on that band too.  The SWR averages around 1.5:1 or better across a
wide frequency range on the 75/80 band.  For me, it out performs any other
antenna I have tried on 75/80 and 40 meters, and I have tried about every
type of antenna, except for the full-wave loop, which is my next project.
  
Good luck with it. I hope it performs as well for you as it has for me!

Ron, K8HSY