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#20
April 27th 05, 02:35 AM
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"Roy Lewallen" wrote in message ... wrote: . . . Now Roy I have a problem with what you are saying here I spend hours modelling an array to lower the TOA or angle of max radiation which directly controls the main lobe dimension both in width and height. And, as I've said quite a few times in one way or another, it's largely a waste of time. O.K. Roy let's get down to the nitty gritty and look at this question. Your background in antenna programs makes you a person of choice to declare if what I do is a waste of time and where I must default to an experts judgement. A month or so ago I gave a description of the antenna that I modelled and for your interest I used the AOP program by Beasely a person that you have collaborated with in the past. One can design a yagi antenna with 8 elements say on a 60 foot boom and then note the gain and the elevation angle of maximum gain. This can be done using the most basic antenna program available. The next step is to apply this same antenna to a program that is capable of changing dimensions to obtain a desired function ,which in this case can be "Gain". There is reference to a NEC program on this group during the past week or so that I believe is capable of doing this, that is on the web and also free to all. With the use of variable dimensions which includes best x,y and Z positions for various pulses or physical positions the program will procede to do as asked. This test is about as simple as it gets to show how the angle of max radiation can be changed as well as the envelope of the new angle range to achieve a 3 dB window of radiation You will see that the computor program will immediately remove itself from a Yagi design to obtain a better gain and form an array that consists of one driven element and where the rest are all reflectors! Though the final shape appears to represent a dish it is not, it is simply a design with multiple dimentional reflectors in the best coupling mode. The result is a gain figure that will exceed the original design, which is what we requested of the program i.e.allow it to make changes of choice to achieve a higher gain than the initial yagi design With the above. one can change the elevation angle for maximum gain which has now dropped to a 11 to 10 degrees or even 9 degrees if one is willing to sacrifice some gain. This can be also be achieved by allowing the driven element to deviate from a straight dipole to a vee shape tipped in such away to helps control reactance swings of the total array. The above is quite simple to duplicate, where anybody can place a 8 element yagi with a long boom of 60 ft placed over real ground and challenge the program to devise a way of increasing gain. In my case the program changes to a non director mode without any prodding, other programs may well need some prodding. Changes to elevation for maximum gain will change automatically and one can expect to easily devise an array with a 10 degree angle where a gain of 16 dbi is attained as well as a broader lobe than can not be accomplished with a Yagi design. If you find that you cannot repeat the above results in a short space of time then it surely reflects a misuse of programs on my part. Please note that propagation has no part in forming the shape of the main lobe to the best of my understanding but you would know better than most as to what the program parameters actually are I look forward to your response or any other program users response that shows my findings are a waste of time so I can direct my experimentation in a more fruitfull direction. Best regards Art I model an antenna array such that it emulates in a way a "stacked" array where as low as a 9/10 degree TOA. The 3 db gain window is broader in width and narrower in height than say the normal array. It is this "TOA" that determines what window we have and where it hits the ionesphere which thus determines its point of arrival on the earths surface snip Is this the error of my ways where any change I make to an antennas pattern is rendered of no use because I must first find a way to manipulate propagation where all the action is really at? . . . Close. Sometimes two or more propagation modes are possible, such as snip any case, it doesn't matter how much I'm radiating at 1, 5, 7, or 15 degrees, or what my antenna's maximum angle is. All that counts is how much I'm radiating at 3 or 12 degrees. Snip I also want to make it clear that I appreciate your post which I see as an attempt to clarify matters that are presently being discussed ie.It is propagation and not the antenna that determines the TOA. Or "antenna pattern is determined by propagation" so that we don't get hung up on the term TOA snip I suggest downloading the excellent, free, and easy to use propagation software by Shel Shallon, W6EL, http://www.qsl.net/w6elprop/. In a few minutes, you'll be able to see what angles are supported at a given time and frequency for a given path. Best regards Art Roy Lewallen, W7EL |
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