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Asunto:[dxcolombia] Double Bazzoka
Fecha:Jueves, 8 de Junio, 2006  15:49:31 (-0400)
Autor:Alexis Deniz M. 4M5DXgroup <yv5ssb @...net>

 
        DOUBLE BAZOOKA/COAXIAL DIPOLE ANTENNA
                 -------------------------------------
                     
A typical, dipole antenna has bandwidth of about 50 to 100 KHz.  The
bazooka is  resonant in the center of the band and had an SWR of 2:1 or
less across the entire 80 meter band?
 
  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 in amateur radio 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.  
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.
 
 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 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. 
 
 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 juction
 to the coax.
             50 Ohm Coax feedline     
             at least 66 feet long.         
                                                       
                               shield side         center connection side              
     ___________   ________________|   |________________    ___________ 
    |___________|- [|__________________________________]- [___________]|
       Twin-Lead                    coax                   Twin Lead
                   <---------------L=325/F-------------> 
                                  coax length
    
    <----------------------------L=460/F------------------------------>
                                total length     
 
 
example:                3.888 kc
total length          118.3 feet
coax length            83.6 feet
twin lead length     34.7 feet