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Scaling of Yagi design
Jim,
Thanks for your input of caution. I'm sure that along with all the horror stories there are a multitude of success stories. My belief is that antennas have been around for years and millions have been constructed on almost every frequency imaginable. In my case I am new to antenna design and green when it comes to determining the parameters that would best suit my applications. My requirements are not critical with regard to front/back ratio or the last drop of gain available for a specific design. I find there are always such a range of responses to a request. Some of these are the theorists that love to explain how black holes in the universe some 10 million light years away might effect free electrons floating on the end of a copper wire. Then we have the ones that are not particularly experienced but like to have a say on every topic. They would normally ask why you want it in the first place. "Oh, you want blue, why not have red". Between these extremes are a bunch of very kind people who have "played" with antennas and gained practical experience that they are prepared to pass on to others. Modeling programs have come a long way and my understanding is that as long as you model the unit correctly, your actual results should not be too far off. (I don't know how to model yet so hence the reason to ask for help). I imagine there are still some disasters when engineers build new cars but I believe this would mainly occur when something is overlooked or they are trying to achieve something new. I am not trying to break any new ground here, just looking for around 7 dBi gain from a Yagi at 921MHz. Regards David Huisman Jim - NN7K wrote: And, consider: even then, It might not work as it does on another frequency. Have a friend (this years ago), who modeled the Komosko- Johnson yagi for 2 meters (a 13 el, antenna , that was emperically designed), long before modeling programs, and then antenna range measured. This was a staple of the old VHF handbooks, and the ARRL handbooks for years, and seemed to deliver exceptional results. He had pull at his local company (not to name them)! and had an EXACT copy made for 1296, and then entered it into a antenna contest- it failed misserably! (this not only was element length/Dia, spaceing, ,but BOOM diameter/length)!! It was (and is) a work of ART, but it wont make his retirement check (or Contest Score) any higher! For those that wish to try it- look for those names, or for W2NLY-W6QKI in the ARRL handbooks mentioned. But it must leave to the builder one of the following conclusions: 1) It was great on 144, but a dog on 1296, or: 2) It wasn't the greatest design , for even 144 (tho it compairs favorable with the range patterns that were shown in the VHF handbook, for 144)! As info, Jim NN7K David wrote: Ian, Thanks for that. I did previously look at your site and bookmarked it. I am going to make up the element cutting jig and bending jig (for when I want to experiment with folded dipoles). Great site. Regards David Ian White GM3SEK wrote: David wrote: Hi, I have a design for a Yagi antenna at 436MHz and would like to scale it to 921 MHz. The calculations I used from the understanding I have obtained from reading various articles appears below. Would someone be kind enough to advise me if this procedure and values look correct ? [...] Regards David Huisman The basic principle of scaling antennas according to wavelength is that you must make an *exact* scale model. That means scaling every dimension, and not adding any new or different 'features'. Often that isn't practical for mechanical engineering reasons, so then you need to apply corrections. However, you should still aim to keep those corrections as small as possible. For example, simple scaling is likely to produce odd, unavailable values for the element diameter, so it's OK to use corrections to change to the nearest commercially available diameter. But if you also decide to double the diameter, the correction is more risky - you're entering the territory of a completely new design. Sorry that I don't have time to check your particular dimensions in detail, David, but you can find resources for scaling and constructional techniques on my 'VHF/UHF Long Yagi Workshop' pages. There is a downloadable program which leads you through the relevant questions, and also a link to an online Javascript version. (From about 1200utc I will not have access to this newsgroup for the next few days, so I hope the above will keep you going.) |
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