I built a Standard Double Bazooka Dipole. At resonance I expected a 75 or so ohm impedance, or 1.5:1 to my 50 Ohm Z-out OHR-100 Rig. I succeeded. At the end I have a email response I gave regarding my standard Bazooka. History: The bazooka antenna is for single Band use. Published documentation shows a 14% 3:1 bandwidth over a dipole, and 99% to 100% efficiency at the design frequency. It is about 92 % efficient as you approach the bandwidth edges. The antenna is electrically a 1/2 wavelength long, just like a typical dipole. The primary difference is the antenna is constructed with a 1/2 electrical wl of coax, shield opened at the center where it is driven, and the shield is extended off the ends with either single wires or a pair of wires extending to the end insulators. The ends of coax furthest from the center are shorted to the center conductor thereby providing an impedance transformation at frequencies OTHER than the designed center frequency. What I did: Estimated the coax to be 23'=1" but only had some 44 feet of good 50 ohm, 100% double braid coax at hand. So I settled for 21'-6 inches on each side and extended the braid and center wire with 14 AWG non- stranded wire. At each end I shorted the two wires and also added 11 feet of wire as well. Using my OHR-100 (Important it has a nice 50 Ohm output impedance, I connected a short section of 50 Ohm coax from the rig to the SWR Meter, then another coax, with choke balun from the swr meter to the antenna. I kept the driving coax to 3 feet. A resonant dipole should read approx 70 to 75 ohms so I was looking for a SWR reading of 1.5:1 at resonance. That was my goal. With center suspended by rope to the ceiling of my garage, I draped the antenna legs where I could, away from metal, and out the door, eventually hanging it along the top of the fence in both directions. I was surprized to learn that my wire extensions, (16 AWG Heater Cord), had a velocity factor of 85.36 %. I was in disbelief when I got the ends down to 6-3/4 feet! So to verify the antenna AND MY MEASURING TECHNIQUE, I decided to build another dipole, with the 16 AWG heater cord in its entirety. After getting the new dipole built, I calculated the Velocity factor, realizing my Bazooka was truely too long and I was going in the right direction after all... What a surprize! So I continued trimming the Bazooka antenna, eventually getting to 2:1 vswr. That was the best I could get. The QRG was moving up from 7000 KHz and I centered the 2:1 vswr length on 7040 KHz. I accepted this discrepency as caused by inexact phasing length of the coax portion. I had previously separated the center of the coax, and since it was too short to begin with, didn't cut it to proper length. Now if I had one of those Antenna Analyzers =8^), I'd be dangerous, (And building better/more antennas). NOTE: If you want the right Impedance, be absolutely careful and measure the 1/2 wavelengh of coax before construction. If you can. I was truely amazed, during the pruning operation I heard all of my QRP circle of friends. But the real test would be transmitting. That is where the antenna efficiency really comes into play. The Super Bowl football game was just beginning. I mulled over whether I still fit thru the bathroom window to reach the roof :) Yup. Darn. No excuse. OK. I brought the ant., soldering Iron, Extension cord, solder, Knife, diag. cutters, up onto the roof. Dropped the old ant. cut/re-attached feedline. Raised up Bazooka. I tried to get the Tree-anchored end of my old dipole down, and the darn rope broke! Couldn't find the slingshot, or heavy weight for the fishing pole, so I climbed the tree. I got the end anchored in just a few minutes. Wasn't too bad after all to climb the tree. Done. I watched the last 2 quarters of the football game. Afterward I had 4 qso's on the bazooka antenna. Using qrp stations as a better reference, I quickly realized I was giving 569's and receiving 579's. Shoot! The bazooka was really pumping out the signal. I'm happy! Comparison to my old dipole: My old dipole was also constructed with the 16 AWG heater wire and may have been a touch too long. It was not as carefully constructed as the bazooka antenna. It was very close though. The feedline hasn't changed except for the 2 inches lost when cutting the old antenna off. Height is about 2 feet lower for the Bazooka, and between 35 and 40 feet up. Orientation remains identical. The new antenna is stretched a little tighter though. I will continue to monitor and compare sent/received RST reports. Conclusion: This is a neat antenna and I can tune up the rig without a tuner on the entire 40 meter band. The test rig was my Drake TR-3, which has trouble loading when SWR > 2.5:1. The drake loaded well into the Bazooka. I am using my tuner anyway, and noticed the ant. tunes quite easily. Final Suggestions: It is a great single-band antenna, but bulkier than a dipole. High quality coax that I used is VERY Expensive. (Dual 100% braid!). If you desire to run without a tuner, this antenna is a "Excellent" solution. Efficiency is near-dipole 99% at the design freq, dropping to about 92 % at band edges. Those stuck with only ONE antenna and Multiband rigs will want something else for thier antenna. I However, look forward to a dedicated 40 meter antenna, and the opportunity to build a different and efficient antenna for other bands as I need them. Having Fun! (Just wish I had one of those ant. analyzers!) 72, Ed Loranger, WE6W --------------------------- --------------------------------- > Ed: > > Man it is *exciting* to keep up with your antenna explorations. Have > both learned a lot and personally benefitted from your expertise. Am > particularly interested in the current work with the bazooka. > > I too have erected a new antenna. It is a 66'-long centre-fed dipole up > 28' and fed with 300-ohm ladder line. Got it up last evening late...and > spent nearly all night answering QRZ from callers. Typical RST's were > consistently 569-589. Wow! I do have to use a tuner and the tuning point > is fairly sharp. (But once found, it is about 70Khz on 40.) The Impedance of a Resonant Dipole is 73 Ohms. Let's call it 75 for ease of use. It would really be a mojor improvement to feed with 75 OHM Open wire line. This will keep the standing waves off the 300 Ohm line. But, I'm like you, only have 300 OHm line and am feeding with it and letting the 4:1 Line SWR exist. Losses are still small, but as qrpers we should be working to eliminate this problem. An obvious solution is a 4:1 Balun. But this puts a MAJOR limitation on the antenna. Only can use on one band. So for multi-band antennas you should NEVER use a 4:1 Balun, or any balun. Use efficient OPEN-WIRE line and live with the SWR; Tuning at your Antenna Match unit. Hopefully the Antenna Matching unit is a Link-coupled type! (Am going to build one!) > > In your opinion, how much improvement might be noted with a bazooka at > this QTH? I have to keep it fairly omni-directional, of corse, given > this QTH. My other antenna is a GAP Titan DX at 7'. Omnidirectionality and Horizontal Dipoles is a contradiction in terms. I'll focus on comparing the Bazooka with a Standard Dipole. Compare first the field strength of 1/2 wavelength of 1 inch diameter aluminum tubing to 24 AWG copper wire 1/2 wl. antenna. The larger tubing has MUCH LESS loss. So your antenna is better with the tubing. So the larger diameter of the Coaxial portion of the Bazooka is more efficient -- simply due to it's physical diameter and lower ohmic loss. Someone mentioned yesterday on QRP-l that they noticed their antennas, when cut for a 1:1 match, actually produced a stronger MEASURED field strength at a frequency slightly off their intended design. This is because a properly resonant antenna, at 75 Ohms, should give you a VSWR of 1.5:1. So cut your antennas for a 1.5:1 at your design frequency and you will maximize signal there. That's what I did with my Bazooka. This is close enough to trim with your antenna match unit for sure, and you can still load up easily without a tuner. The big difference between the bazooka and regular dipole is the built-in phasing lines. These must be measured very carefully and are best done using a receiver and Antenna Noise bridge. (I need to Homebrew one of these too! I have plenty of photocopied articles.) The phasing line allows approximately 14 percent bandwidth advantage over a dipole. Radiation Efficiency is about the same, but with the bazooka used without a tuner, more input signal from the rig gets to the antenna. That is the big advantage of the Bazooka. DISADVANTAGE: You are stuck with only one band. Attempts to operate on other bands are fruitless. The phasing section puts a low impedance shunt at the feedpoint, not allowing any signal to enter the antenna. The standard dipole, even though not very efficient away from its design frequency, would still be usable -- with a tuner. I do believe that the Bazooka cannot ever be beat by a standard wire dipole when operated AT the design frequency. And the Bazooka definately wins on Single Band usability, being 92% efficient at band edges! ELITE ANTENNA: If your goal is to have the best Dipole possible, then put in a 3 inch diameter Aluminum tubing, center fed with a link coupled tuner and 73 Ohm feedline. But it will probably be only 1/2 dB better than the Bazooka, and probably .75 db better than a 12 AWG wire-constructed Dipole. And a lot less portable too! Summary: Forget about omnidirectional. Make two of them and put in a plug board where you can change directions. What I did here is build an top 'Tee' out of PVC pipe and mounted it at the top of the aluminum mast holding one leg of my antenna up. The horizontal section of the PVC is about 3 feet long. A rope from the end of the dipole goes through the Tee and at a 45 degree angle to the fence and tied off. When I need to change my dipole to a Horizontal Vee antenna pointing at the fox, I just untie the rope, lift the aluminum mast (Allowing the rope to slide at the top, and put the mast over a 4 foot broom handle stuck in the ground! Instant fox killer! A second guy rope about midway up the mast can be tied off easily too. But I found that the rope passing thru the Horizontal PVC pipe at the top keeps the mast from swaying really well. By the way, I used really good coax, it has Two braids and each is 100% coverage. Kind of like the threads per inch of a nice dense Chinese Decorative carpet. One sure sign that the Bazooka antenna has a stronger Field over my old 16 AWG dipole, my TV antenna amplifier is getting overloaded by the near field. The last time this happened was when my Vertical antenna was withing 15 feet of the TV Antenna. And this dipole is over 60 feet back! Feel free to copy the QRP-l list or distribute this message in its entirety. That is, if you think it might be useful... I hope so. -Ed Loranger, WE6W -------------------------------