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#9
March 28th 05, 03:10 AM
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"Dave Platt" wrote in message ... In article P2G1e.110904$Ze3.11791@attbi_s51, wrote: By all means put me on your kill file. I asked a simple question and you want to reply to a different question of your liking and place your question as one preferable to mine. Art, in cases like this, you keep asking "simple questions" which imply, by their very wording, a whole bunch of assumptions about how things work which just ain't so. The fact that you keep getting answer after answer, from a lot of knowledgeable people, which you either don't understand or "blow past" or that you feel evade the point of your question, ought to be saying something to you: that there's something wrong with the questions you ask. No one has been able to supply the answer to my question, Using your words ,what steers the pattern away from a circular form from a natural circular form. That depends on what you mean by "a natural circular form." If you're referring to the fact that the main lobe of a dipole tends to look circular on many of the commonly-used plots, then the pattern isn't "circular" in any cosmic sense of the word. It's just as correct to say that it's elliptical, or bumpy, or squashed, because that's exactly how it will look on plots which use different circular axes (linear, logarithmic, etc). To try it again, though: I thank you for that you're asking why the pattern appears to be compressed, as the gain increases. Fundamentally, it's due to the fact that the antenna is sending more power out in the desired directions (more gain), at the expense of sending less in other directions. Fully agreed to This is done by creating multiple radiators, which are offset in power and location and phase so that their individual radiation wavefronts reinforce in the desired directions, and cancel in the undesired directions. Accepted as long as you can agree that a similar vector analysis with multiple radiators can also create a non focussing pattern When we plot the resulting RF strengths, the RF in the desired direction is stronger (we got the gain that we want). Let's assume that (as is common practice) we continue to plot the signal in the strongest direction on the outer circle of the graph. Now, one of two things will have to be true: [1] Every other direction in the main lobe had its power "scaled up" by the same amount... the increase in gain worked the same for all directions within the main lobe. In this case, the shape of the main lobe will not change at all. O.K. this would /could be the case I am thinking of In this case, the additional power required to achieve the increase in gain in the main lobe will have had to come for somewhere. Agreed Since it didn't come from the main lobe, it will have had to come either from the sidelobes, or from the rear half of the antenna's pattern. Agreed for over all gain but not necessarilly for the lobe becomming focussed which is the crux of my question There's a limit to how far you can take approach [1]. It stops working when your sidelobes and rear half of the pattern drop to zero... and it becomes rather ineffective some time before that, when the largest of the side/rear lobes is maybe 10-15 dB down. Beyond that point, there just isn't enough power left in those backlobes to be useful. Using a antenna computor program the main lobe at 10 degrees does not deviate from a circle even if the F/R is more than 30 db ( note F/R vs F/B) and this is comprised of vector addition mode as with a yagi design. [2] The other possibility is that you didn't manage to boost the gain, uniformly, in the entire main lobe. Hum! In this case, if you're still plotting the strongest signal on the outermost circle of the graph, you'll notice that the shape of the main lobe has changed. No, not always, only with a yagi design in my opinion Any direction in which the gain increase was less than the maximum you achieved, will be closer to the center of the circle than before. [Another way of looking at this is that by increasing your maximum directional gain, you've "enlarged the circle" on which you're plotting it, but that some points didn't move outwards by the same ratio.] Agreed thus my question In the common case of a Yagi, when you boost the gain (say, from 10 dB to 15 dB) there just isn't enough power available in the side and rear lobes to make up this gain... you can't 'rob' enough directivity from the sidelobes and rear lobe. Instead, you 'rob' the power from the outer edges of the main lobe, and shift it in towards the center. Now we are getting closer to my quest. How do we "rob" from the outer edge of the main lobe is the underpinnings of my question. You do this, most commonly, by adding additional parasitic elements, whose location and phasing are such that their radiation reinforces that moving in the "forward" direction, and interacts destructively with (cancels) radiation moving outwards at an angle. Agreed if we are adding or subtracting on a constant plane. Could you by any chance referring to the closing vector of the vector analysis to consist of two vectors and where one of these vectors is the force at right angles to the main lobe and which deforms it. If so I am beginning to see the light! When you plot the resulting pattern, and scale it so that the strongest signal is on the outer circle of the plot, you find that the main lobe looks narrower. Part of this is due to the actual redirection of power, and part of it is due to the fact that you've re-scaled the graph.a I am lost here but if we agree on my interpretation of what you said then I am at a point where I can generate vector diagrams of different arrays and forecast the width of the resultant lobe .Does anybody else agree that the main lobe width can be forecast via vector analysis. Seems like from past posts that vector analysis is not now in vogue for electrical engineers in the U,S and only creates blank stares when mentioned/ .. kes several wrathful deities...] Art, if you continue to ask "simple questions", and you continue to get back complicated and detailed answers, it really ought to convey to you the possibility that your "simple" question is oversimplified. Or, perhaps, that you've been given the actual (simple) answer three or four or five times already, have rejected it, and people are trying to explain to you why it's actually correct. Goodbye, Art. This is/was my last attempt, I think. I doubt I'll try again. But David, nobody pointed to vector analysis and the particular facet that you referred to. You are to be congratulated in pointing to a trail of logic that could well be the direction I was looking for. Nobody but you presented in real terms an analysis that leads to serious consideration and I thank you very much for that. Best regards Art KB9MZ.....xg g the Jade Warrior home page: http://www.radagast.org/jade-warrior I do _not_ wish to receive unsolicited commercial email, and I will boycott any company which has the gall to send me such ads! |
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