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DOUBLE BAZOOKA/COAXIAL DIPOLE ANTENNA
by Morris Lundberg, K4KEF


Eighty meters is 500 KHz wide, from 3.5 MHz to 4.0 MHz. Your
typical, standard dipole antenna will show a bandwidth of about 50 to 100
KHz. That means that you'll have to resonate it at the high or low end of
this band, to have a 2:1 or less SWR over the frequencies you wish to
operate. So you tune it to the CW or Voice portion of the band. Wouldn't
it be nice to have an antenna that was resonant in the center of the band
and had an SWR of 2:1 or less across the entire 80 meter band?

Enter the Coaxial Dipole Antenna (alias "Double Bazooka"). This
antenna is broadband; it will cover the entire 80 meter amateur band with
an SWR of 2:1 or less. The "Bazooka" antenna was developed by the staff
of M.I.T. for radar use. The original "Bazooka" used coaxial cable for the
entire radiating elements. The adaptation used by most amateur operators
uses coax only for the broadbanding portion of the antenna, while the
remaining portion of the elements are constructed of twinlead or ladder
line (see attached sketch). Ladder line is preferable for its inherent
strength.

This is a single band antenna. It will not radiate harmonics of
your operating frequency. In addition, there is very little feedline
radiation, which is great for those who have problems with TVI. Its
broadband characteristic makes it ideal for 80 meters and 10 meters. On
the other hand, a seperate antenna is required for each band. The Bazooka
antenna consists of a half-wavelength of coaxial line with the outer
conductor opened at the center and the feedline connected to the open
ends. The outside of the coax and the ladder line operate as a half-wave
dipole. The inside of the coax elements, which do not radiate, are
quarter-wave shorted stubs which present a high resistive impedance to the
feed point at resonance. Off resonance, the stub reactances change in
such a way as to cancel the antenna reactance, thus increasing the
bandwidth of the antenna.

In the attached sketch, the SWR curves are shown for two double
bazooka antennas; one for the 80 meter version and one for 40 meters. Note
that the SWR at resonance on 80 meters (3.75 MHz) is 1.2:1 and that the
SWR at the band edges is about 2:1. The curves were drawn from readings
taken with the DAIWA CN-620B power meter. The antennas were installed at
a height of about 25 feet (unfortunately, I have no trees for supports)
and the axis of the antenna elements were at 90 degrees with respect to each
other. The 40 meter antenna used TV twin-lead for the ends of the elements.
Both antennas used Tandy RG-58/U coax for the broadbanding elements and the
feedline. Apparently the Tandy coax I used had a velocity factor of 0.64,
since the resonant length turned out to be 84 feet on 80 meters (as
compared to the calculated 87 feet). More expensive coax may have a more
consistent velocity factor.

At the very center of the coax used in the elements, very carefully
cut away about one inch of the outer vinyl jacket. Then cut the exposed
shield all the way around at the center of the exposed area. Be careful
that you do not cut the dielectric material or the center conductor in the
process. Twist the two pieces of exposed shield into small pig-tails.
These are the feed-point terminals for the antenna. The center conductor of
the feedline is soldered to one and the shield of the feedline to the
other. Now solder the center conductor and shield together at each end of
the antenna element. Solder the two ladder line wires to the end of the
antenna element. At the other end of the ladder line, solder the two wires
together. The ladder line now appears to the antenna to be a very thick
extension of the radiating element, contributing to broadbanding the
antenna.

I've solved the mechanical strength problem by using a square piece

of plastic at the antenna center, drilling a small hole on each side of the
coax, wrapping a small wire around the coax and through the holes and
twisting the wire together on the other side. After this, a small amount
of quick setting epoxy secures the coax to the plastic support and prevents
the wire from untwisting. Once the antenna elements and the feedline are
secured to the plastic square in this way, the plastic square takes the
strain, protecting the delicate radiating element feed-point. A thorough
coating of silicone rubber or epoxy seals and protects the feed-point from
the weather. A similar technique at the point where the ladder line is
soldered to the shorted end of the coax provides strength and a
weather-tight seal.

I've constructed several of these antennas and they have all
performed as expected. The quality of the coax used seems to have little
effect on the antenna's performance. I've found that Tandy coax will work
well in this application. RG-58/U was the coax of choice for small size
and light weight. There is no reason, however, that RG-8 coax wouldn't
work as well or better. It's larger surface area would probably provide
better low signal level reception. Its large size would make it more
obvious to the neighbors and its larger weight might be a problem in your
installation. Its larger diameter conductors would, certainly, take more
stress and strain than RG-58. Eventually, the outer black vinyl coating on
cheaper coax will migrate into the inner dielectric material, contaminating
it. As is true with any cheaper coax, this changes the properties of the
coax, i.e., impedance, loss and velocity factor. Over the long term, the
exposure to the sun's untraviolet rays cause this contamination of coax
which hasn't been constructed to prevent it. For that reason, you may
wish to use non-contaminating coax, such as RG-141 or RG-213. After
several years of exposure to all kinds of weather, however, my Double
Bazooka's show little degradation in performance, using Tandy RG-58/U coax
(Radio Shack).

K4KEF.