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#1
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Fractenna wrote:
I'm not taking credit. This is a minor adjustment of a Cebik design. Seems to fit the OP's rqmts, however: no folded-dopole feed, 50 ohm matched BW of 156-162 MHz and 5-6 elements. Regards, Wes This looks like a modofied approach to an NBS Yagi-Uda. The only special feature of the NBS designs (other than the prestigious mailing address) was a self-imposed restriction to equal spacings between directors. That turned out to be an evolutionary dead-end, and you won't find it in any modern optimized yagi design. The design proposed by Wes doesn't have that feature - fortunately - and is actually one of the OWA (Optimized Wideband Array) family. One of the special features of that family of yagis is a very close spacing between the driven element and the first director, which raises the feedpoint impedance to 50 ohms and allows a simple split-dipole driven element. Even this feature is not original to the OWA family. I don't know when (or indeed how often) it might have been invented, but it was first systematically used by DL6WU some 25 years ago. With additional help from computer analysis and optimization, the OWA family continued the development. Modern Y-U design allows for a higher feedpoint SWR. Matching loss is offset by higher gain and better consistency of F/B. If no matching, then a shaped, folded dipole is used. Typically a F/B better than 22 dB (sim) is acheived with a 10% bandwidth. We're now in the excellent position of having several alternative ways to design a yagi to meet each user's specific combination of requirements. That approach is certainly one of them. -- 73 from Ian G3SEK 'In Practice' columnist for RadCom (RSGB) http://www.ifwtech.co.uk/g3sek |
#2
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I can easily be mistaken, but I don't believe the NBS Yagi designs were
ever promoted as being optimal in any way. Their purpose, I believe, was to provide a set of easy-to-duplicate designs whose gains were well documented (although I understand an error was made in measuring the originals) and which anyone could construct as reference antennas with known gain. Roy Lewallen, W7EL Ian White, G3SEK wrote: The only special feature of the NBS designs (other than the prestigious mailing address) was a self-imposed restriction to equal spacings between directors. That turned out to be an evolutionary dead-end, and you won't find it in any modern optimized yagi design. The design proposed by Wes doesn't have that feature - fortunately - and is actually one of the OWA (Optimized Wideband Array) family. One of the special features of that family of yagis is a very close spacing between the driven element and the first director, which raises the feedpoint impedance to 50 ohms and allows a simple split-dipole driven element. Even this feature is not original to the OWA family. I don't know when (or indeed how often) it might have been invented, but it was first systematically used by DL6WU some 25 years ago. With additional help from computer analysis and optimization, the OWA family continued the development. Modern Y-U design allows for a higher feedpoint SWR. Matching loss is offset by higher gain and better consistency of F/B. If no matching, then a shaped, folded dipole is used. Typically a F/B better than 22 dB (sim) is acheived with a 10% bandwidth. We're now in the excellent position of having several alternative ways to design a yagi to meet each user's specific combination of requirements. That approach is certainly one of them. |
#3
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Roy Lewallen wrote:
I can easily be mistaken, but I don't believe the NBS Yagi designs were ever promoted as being optimal in any way. Their purpose, I believe, was to provide a set of easy-to-duplicate designs whose gains were well documented (although I understand an error was made in measuring the originals) and which anyone could construct as reference antennas with known gain. They were in fact described as "optimum designs" in the original paper, but it's clear that claim was made subject to a number of reservations. Most of the hype came from antenna manufacturers who subsequently picked up the designs from the published reports, and misused the "NBS" name to boost the credibility of their own products. With hindsight, the NBS designs were really quite good, and their forward gain is still competitive with more modern designs of the same boom length. But modern yagi designs are generally better, because there have been a further 35-40 years of development and optimization. In particular, the last 10 years have benefited from the availability of computer modeling techniques - you can now do more good work in a few evenings than the NBS program achieved in as many years. As a result of this development, you can now have a better combination of features - for example, cleaner patterns, wider bandwidth and/or greater tolerance to dimensional errors, and easier impedance matching - and keep the good forward gain as well. Many modern designs have been developed as complete 'families', with simple design rules that let you add or remove elements (changing the boom length accordingly) to create new yagis, each of which will be close to optimum for its boom length. The NBS yagis don't have that 'family' feature - each one is an individual design, and attempts to adapt them have generally not been successful. This lack of adaptability is a direct consequence of the original decision to use the same spacing between all elements; that's why I described it as an "evolutionary dead-end". It costs exactly the same to build a good, modern yagi as it does to build an older, inferior design - the only differences are in *where* you apply the hacksaw and the drill. Therefore there's not much point in building an inferior design... which is what the NBS yagis have now become. For many years, the greatest value of the NBS yagis was that they had accurate gain measurements, so they could be used as reliable benchmarks in antenna gain shootouts. (The known error in the gain measurements applies to the 2-element yagi only.) Even that use has now been overtaken by computer modeling. In summary, the NBS yagis deserve respect for their major contribution to the art and science of yagi design, but they are now mainly of historical interest. For examples of modern yagis, and tips on construction, see the 'VHF/UHF Long Yagi Workshop' on my website. -- 73 from Ian G3SEK 'In Practice' columnist for RadCom (RSGB) http://www.ifwtech.co.uk/g3sek |
#4
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Ian,
Thanks very much for the correction and additional information. Like a lot of other folks, I learn a lot from this newsgroup. Roy Lewallen, W7EL |
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