TOC: 1) Quick overview of my 300 Ohm impedance Folded Bazooka Antenna. 2) Quick info on building it. 3) Email response on performance, feed. Gang, I really don't have the time today, but just couldn't contain myself. My Bazooka antenna is now a Folded Bazooka, fed with 300 Ohm TV line to the Balanced output of my ant. tuner. Boy am I getting out! And actually copying QRP stations as never before. I have been getting sig reports 2 units higher than I have sent -- and to stations I couldn't have even heard before! Most of you recall I had a 40 meter dipole, improperly fed with 300 ohm TV line to minimize losses for multiband operation. I built the standard bazooka ant. and fed in exactly the same manner so the test comparison would be valid. The results were that the Bazooka ant. was slightly better than my dipole, and probably due to the larger conductor provided by driving the shield of the 100% dual braid high quality coax I used. Well, Saturday morning after the WSN-40 net, I went for the gold -- Add the extra wire for a Folded Bazooka and present a 300 ohm match to my feedline. WOW! As I said earlier. This puppy is hot. I knew there was a little loss due to the 75 Ohm to 300 ohm mismatch at the ant. terminals, but having eliminated that mismatch, everything is being delivered to the antenna. Feedline radiation is at an all-time minimum. We be in business, folks! I've had my Drake TR-3 ALC at about 40 dB compression just to keep my ears from ringing. As an added bonus, I appear (Deductively) to be able to use the Folded Bazooka on 80 meters. Last night my 5 watts was copied S9 from my Santa Rosa,CA QTH in Northern California to Los Angeles in the south. I won't be contesting in 80 meters, just a check if the added element would act as a 1/4 wave element driven from the ends, or at least inductively coupled to the primary bazooka feed. Apears to be about 60 percent efficient on 80 meters with the 40 meter folded Bazooka. Well, my previous dipole was unusable on 80 meters, so this is definately an improvement. Tests on 20 meters --- Forget it. I just had to share this. At the very least, make yourself a nice folded dipole with 300 Ohm feed and do some of your own experiments with a non-bazooka style folded dipole. Gleefully Submitted, Ed Loranger, we6w -- Recipient of coveted Samuel F. B. Morse Award, NTTC Pensacola, FL 1977. 72/73 de we6w qrp es CW ONLY; Member: QRP-L/ARCI/Norcal/ARS/AR http://www.qsl.net/we6w (From Non-Ham to Extra in one Day.) -------------------------------------------------------------------------- 2) Building a crude 300 Ohm input impedance Folded Bazooka Antenna. NOTE: I suggest extra effort be made to get the coax cut to the correct length, compensating for velocity factor. Use a Noise bridge or Antenna Analyzer. ---- This will get you in the ballpark for 7040 KHz (QRP Freq): Get some old coax with decent braid in it for the shield. Cut about 46 feet. Find the center, cut the braid but leave the center conductor and its teflon insulation intact. Pull about 2 inches of each side of the braid out and twist, forming the input connections for you 300 OHM TV twinlead. Lay this 46 feet on the ground like a dipole. Go to one of the far ends, pull off some of the plastic sheath, separate the center and shield and solder them together as a short. Repeat for the other side. Now, add 11 feet of wire to each end of the coax dipole. You now have a 66 to 67 foot long regular dipole but the coax forms the radiating portion for about 23 feet on each leg, then has a short to the center conductor which reflects a high impedance 1/4 wavelength away at the center,feed point. Now, Connect your SWR meter or Antenna analyzer DIRECTLY to the ant. feedpoint. Keep the swr meter close to the antenna. Using a rig with a VERY GOOD 50 Ohm output impedance (I.E. not a HW-8 or other rig having a TANK Capacitor in the output), Set the rig to transmit about 1 watt and at 7000 KHz if your license lets you go there. At the least, try BELOW 7040 KHz so you can prune the antenna ends and see improvement. The antenna will be optimized when you get 1.5:1 SWR Reading, that is: 75 Ohm resonant antenna on a 50 Ohm Rig. Prune each end carefully and in 1/2 inch increments. Cut a piece from one end, carry that cut piece to the other end of the antenna and use the cut piece to measure your next cut. This ensures you keep the line lengths the same. After you get a 1.5:1 VSWR at the Lower Frequency of say 7000 KHz, begin taking only 1/4 or 3/8 inch cuts from each end and as you get close to 7040 KHz, leave the rig there and get your 1.5:1 VSWR. That is the Basic standard Double Bazooka. If you are like me, you want more efficiency and less losses, and use 300 Ohm or greater transmission line. A folded antenna exhibits 4 times the impedance of the standard feedpoint impedance of 73 ohms. So your impedance of the folded bazooka will be about 300 Ohms. If you went to a Tri-folded Bazooka, you could use 650 Ohm line easily!!! Using the same wire you used for the dipole ends, measure the wire to equal the length of your Dipole. This wire will be soldered from one end to the other of the dipole and have 4 to 6 inch spacers every 4 feet or so. I passed the wire through holes drilled in the plastic block which held the center of the antenna too. HINTS: THE Coax length is ACTUALLY a huge variable and you should really cut the 1/2 wavelenth of coax with a noise source or antenna analyzer. Somehow, you have to find the true velocity factor of the coax. It is an electrical half wavelength and much shorter than a 1/2 wave dipole. But at resonance, the coax alone is too short for an antenna, that is why you extend the coax with wire. Off resonance, the coax shorted section transforms the antenna input impedance in an attempt to keep it at 75 ohms across the band. It does this rather well. The measurements I gave you can get you up and running with 85% VF Coax. If you use some other coax, you need to SEVERELY shorten the coax section!!!! Use the following formulas to get in the ballpark: #1) 492/F(MHz) = half wavelengthm in Free Space. 85% VF Coax length= FSWL * 0.85= Actual Coax length needed. 66% VF Coax length= FSWL * 0.66= Actual Coax length needed. Short the ends of the coax and then open the middle braid for feeding. Add wires to the ends such that the length = Half Wave in Free Space. (#1) Trim ends while monitoring VSWR until 75 Ohm Resonance obtained. I.E. VSWR= 1.5:1 Add folded wire element. Radiate Well! 72, Ed ------------------------------------------------------------------------ In response to an email about Folded Bazooka performance, I wrote: Yup, I'm really happy right now. By the way, all this success and my antenna is still only 35 feet up. To truely be optimised, It should be up 1/2 wavelength at 66 feet. But that's not possible at my QTH and rental unit. Antenna is Entirely Homebrew. It is the published design as described as a "Broadband Dipole" in the index of my 1974 Antenna Book. When you see it, and read, they state "Sometimes refered to a Double Bazooka Antenna." I did not use the open wire at the ends as depicted in the drawing. But I did add a single wire from one end to the other to form the Folded Bazooka. This is the trick for the 300 ohm feedline. It is transformer action. A single wire dipole has a 75 ohm (73) input impedance. Adding a second wire converts this to 300 Ohms. The turns ratio Squared* Zin=300 Ohms. so 2 wires * 2* 75=300 Ohms. If you had 3 wires then the input impedance is 9*75=675 Ohms, great for that large spaced OPEN WIRE Feedline. I'll do that some day since the losses for 600 Ohm line are even lower. More detailed construction information is in the Antenna Handbook. Be sure to measure the swr very carefully when you are tuning up. And Remember, Do the SWR measurements up in the clear as best as possible, and with the SWR meter Directly connected to the feedline. A good start with 100% braided, 50 Ohm coax is about 45 Feet, open the shield at the middle. Connect your 300 Ohm (75 for Single bazooka) feedline to each shield. Leave the Coax center wire alone. Short each far end of the coax to the center conductor at the coax end. Then solder a 12 foot piece of 12 AWG wire to each end. Trim the 12 Foot wires as you monitor SWR at 7.000 MHZ (The Low end) As you get close, Start reducing the cuts at each end to about 1/2 inch. The 300 Ohm feedline and folded bazooka may have a slightly better bandwidth than the 75 Ohm fed single bazooka dipole, but should be similar in signal performance at the design frequency. Of course If you have an antenna analyzer, you are WAY ahead of me! Wish I had one..... I'm thinking of building a Noise bridge for my antenna work. Soon. Need to get a scope too. Darn test equip. costs money, and me with 4 kids, spouse and IRS issues... Take care Friend. Review some of the QRP-l archives for last week if you want more info. I made some construction posts then. 72 my friend, Ed Loranger, we6w