Assembly
of the Double Bazooka Antenna
The Double Bazooka
Dipole is a very efficient single band antenna which is very quite, and does not
require the use of a balun. This antenna consists of coax (RG58) or other 50 ohm
type with the shield split at the center and the feed line attached to the open
ends. Do not break the center conductor. With the feed line attached directly to
the two open ends this acts as a half wave
dipole along with the open wire end sections. This double bazooka
can be cut for any operating frequency and is broad banded. It can be mounted as
a flat top or an inverted vee and will handle the legal limit. As an added plus,
it can be operated as a multiband antenna by using a suitable tuner. As with
most antenna projects, get the double bazooka up as high as possible.
Some tuning of the length for best swr may be required and you can use materials
that are easily obtainable.
On the cable ends
you do not need to use twin lead. You can make these antennas using a single
piece of 12 gauge copper wire or larger for each end or you can also use ladder
line etc.
I would advise
using heavy end wires for strength purposes. The weakest
part of this antenna is at the junction of the coax and the end wire or twin
lead. To prevent the joints from breaking especially for long lengths, I picked
up some 1/2 inch PVC pipe.
I cut
the PVC in half length wise so it would overlap a few inches on the joints.
Place the PVC at the joints in
"splint fashion" and secure with regular screw hose clamps. Put the clamps close
to the ends of the "splint", tighten snuggly and it should take the strain off
the joint and should give a strong joint.
Do this at the center using a " T
" support from PVC or use your own engineering. When designed for the
lower hf bands, this antenna can be a bit heavy since coax is used. It can be
supported along it's entire length with non-conductive cord, rope, cable, etc by
suspending and attaching the antenna from the support cable with nylon wire ties
every few feet.
This relieves the
tension and strain from the center and end
connections.
ED A DOUBLE BAZOOKA antenna is an extremely broad banded Half Wave Antenna which can operate efficiently across an entire Ham band with little change to the SWR. The BAZOOKA antenna design was developed by the staff of M.I.T. in the early 1940's for use by the U.S. Government as a radar antenna. It was modified for amateur radio use in the 1950's.
This unique design eliminates the need for antenna matching baluns and can be fed directly with 50 Ohm coax.
The DOUBLE BAZOOKA is 98% efficient and typically provides S.W.R. readings of less than 2:1 over the entire amateur band.
Since this antenna has no exposed metal wire static charges can not build up thus reducing noise by 6dB over antennas constructed of exposed wire.
The DOUBLE BAZOOKA antenna will handle full legal limit power with no effect to performance.
The DOUBLE BAZOOKA is recommended to be mounted in an inverted "V" configuration for optimum results. However the DOUBLE BAZOOKA can be configured horizontally with equally good results.
The 80 Meter DOUBLE BAZOOKA antenna will operate on 80 through 10 meters with the aid of an antenna tuner.
The following chart gives overall antenna lengths and recommended height placement above ground for a single DOUBLE BAZOOKA.
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A DOUBLE BAZOOKA antenna is an extremely broad banded Half Wave Antenna
which can operate efficiently across an entire Ham band with little change
to the SWR. The BAZOOKA antenna design was developed by the staff of
M.I.T. in the early 1940's for use by the
This
unique design eliminates the need for antenna matching baluns and can be
fed directly with 50 Ohm coax.
The
DOUBLE BAZOOKA is 98% efficient
and typically provides S.W.R. readings of less than 2:1 over the entire
amateur band.
Since
this antenna has no exposed metal wire static charges can not build up
thus reducing noise by 6dB over antennas constructed of exposed
wire.
The
DOUBLE BAZOOKA antenna will handle full legal limit power with no effect
to performance.
The
DOUBLE BAZOOKA is recommended to be mounted in an inverted "V"
configuration for optimum results. However the DOUBLE BAZOOKA can be
configured horizontally with equally good results.
The
80 Meter DOUBLE BAZOOKA antenna will operate on 80 through 10 meters with
the aid of an antenna tuner.
The
following chart gives overall antenna lengths and recommended height
placement above ground for a single DOUBLE
BAZOOKA.
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If
erected as a Dipole this antenna has horizontal polarization.
This
antenna can also be installed in an inverted V fashion "Center elevated, with 90
- 120 degrees between the legs" Then it will have vertical polarization and will
usually out perform a dipole type antenna at distances of over 500 miles due to
its lower angle of radiation.
The coaxial dipole uses the same type of coax for the feed line as the legs are made of or it will not work properly.
To build one of these antennas resonant at 3.920 Mhz. Measure out a 120 feet length of coax and cut,this will be the antenna. At the center of this cut off 1" of cover all the way around and remove. Now cut the shield all the way around and separate. Do not cut the dielectric. This is the point at which you will support the antenna so make a hanger from rope or other material.Take your feed line coax and cut back cover at one end so as to be able to separate the center conductor and shield. Attach the feedline center conductor to one side and the shield to the other side of the antenna coax shield at the center of the 120 foot long piece.
Next, measure down each leg 30' 6" from the center and cut a slot in the cover and shield, so as to be able to get to center conductor. At this point solder the center conductor to the shield on both legs. Now seal the center point and each leg where you soldered, with nonconductive silicon, so as to make weather tight. At the end of each leg strip the shield and center off and solder shield to the center conductor so it will not separate and seal.
The center conductor from leg to leg acts as a balun, thus making this antenna able to operate with a very low SWR across the whole band.
Double Bazooka Dipole Antenna Version #2 |
This adaptation used in amateur radio only uses coax for the broad banding portion of the antenna, while the remaining portion of the elements are constructed of twin lead or ladder line OR 12 gauge wire. 12 gauge wire 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 feed line radiation, which is great for those who have problems with TVI. Its broadband characteristic makes it ideal for 80 meters and 10 meters. The Bazooka antenna consists of a half- wavelength of coaxial line with the outer conductor opened at the center and the feed line 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. At the very center of the coax 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 feed line is soldered to one and the shield of the feed line to the other. Now solder the center conductor and shield together at each end of the coax 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 two
conductors at each end of each piece of ladder line should connect to each
other. Use 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. A small amount of quick setting epoxy secures the coax to the plastic support and prevents the wire from untwisting. A coating of silicone rubber or epoxy seals and protects the feed-point from the weather. Do the same where the ladder line is soldered to the shorted end of the coax. Short the center to shield at each end of the coax. Short the twin lead at each end and solder the coax/shield junction to the coax. Use 50 Ohm Coax feedline at least 66 feet long. _____________ _________________| |_________________ _____________[___________]- [__________________________________]- [___________]Twin-Lead * * * * * * * * * * * COAX * * * * * * * * * * * Twin Lead<---------L=325/Freq--------->coax length<-----------------------L=460/Freq------------------------->Total Overall LengthTotal Overall length minus Coax length = Total Twin lead length. Example: 3.888 Mhz. Total length 118.3 feet Coax length 83.6 feet Twin lead length 34' 8 1/2'' feet total or 17' 4 1/4'' on each end of the antenna. |
RICK A BERNARDI
SR
K4UUG /
AAV4XK