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coaxial dipole
Hello Bill, I can understand that the coax dipoles you built are somewhat more broadbanded than a simple dipole. However, I suspect that you are unaware of the reason for the broadbanding. Sorry to tell you, but it is achieved only by the resistive losses in the coax, and not by the reactance obtained by the two sections of shorted coax, as incorrectly stated in several published articles. I have made extensive measurements and calculations that prove my statement above. I have reported these measurements and calculations in both QST and in my book 'Reflections'. The QST reference appears in the Technical Correspondence, September 1976 issue, and in Chapter 18 in Reflections. You can read Chapter 18 from my web page at www.w2du.com. On the other hand, realistic broadbanding, without the loss introduced by the resistances in the coaxial dipole, can be obtained by the 'cage' dipole, using several parallel wires separated by spacers, as mentioned other posts appearing in this thread. Walt, W2DU Hi Walt et al, I was going to point out the same flawed thinking. I believe Frank Witt also published the analysis for the flawed reasoning of the bazooka stubs correcting for the reactance of the dipole off resonance. He also published coaxial stub designs that did work in QST and several of the ARRL compendia. It is amazing to me that the bazooka is still used given the alternatives. One sight where thay are sold claims more gain too! Dale W4OP |
#2
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coaxial dipole
"Dale Parfitt" wrote in message news:k7jHi.550$6o2.304@trnddc05... Hello Bill, I can understand that the coax dipoles you built are somewhat more broadbanded than a simple dipole. However, I suspect that you are unaware of the reason for the broadbanding. Sorry to tell you, but it is achieved only by the resistive losses in the coax, and not by the reactance obtained by the two sections of shorted coax, as incorrectly stated in several published articles. I have made extensive measurements and calculations that prove my statement above. I have reported these measurements and calculations in both QST and in my book 'Reflections'. The QST reference appears in the Technical Correspondence, September 1976 issue, and in Chapter 18 in Reflections. You can read Chapter 18 from my web page at www.w2du.com. On the other hand, realistic broadbanding, without the loss introduced by the resistances in the coaxial dipole, can be obtained by the 'cage' dipole, using several parallel wires separated by spacers, as mentioned other posts appearing in this thread. Walt, W2DU Hi Walt et al, I was going to point out the same flawed thinking. I believe Frank Witt also published the analysis for the flawed reasoning of the bazooka stubs correcting for the reactance of the dipole off resonance. He also published coaxial stub designs that did work in QST and several of the ARRL compendia. It is amazing to me that the bazooka is still used given the alternatives. One sight where thay are sold claims more gain too! Dale W4OP Hello Walt, Dale, et al I thought we worked this out years ago. The parallel-resonant network of the stubs lowers the SWR some very near resonance by oscillating at the driven frequency and storing the small amount of energy that would be reflected back otherwise, but a driven circuit won't oscillate very far from resonance, and it does nothing for bandwidth. That's due to loss, as Walt proved long ago. Increasing the diameter of the radiator, as Walt pointed out, does work. There was a good recent QST article on just such an antenna. 73 H. NQ5H |
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