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LW3DBH > ALL 16.03.15 16:33l 6 Lines 6002 Bytes #999 (0) @ WW
BID : 55782_LW8DJW
Read: GUEST
Subj: Antena Doble Bazooka......
Path: IW8PGT<I3LUG<IZ3LSV<DB0ERF<OK0NAG<OK0PBR<OK2PEN<CX2SA<LU7DBA<LW1DRJ<
LW8DJW
Sent: 150214/1630Z 55782@LW8DJW.#1824.BA.ARG.SA [Lanus Oeste] FBB7.00e $:55782_
From: LW3DBH@LW8DJW.#1824.BA.ARG.SA
To : TODOS@WW
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
Gracias Ea4eiu
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