DP Tool Club 15/CD_ASCQ_15_070894.iso/vrac/hod005.zip/FILE1.005

Text File | 1994-05-02 | 6KB | 100 lines

                        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.