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#11
January 26th 05, 02:14 PM
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Thanks Roy,
You covered every aspect Art "Roy Lewallen" wrote in message ... Modeling of wires connected at an angle is a problem only with MININEC-based programs such as AO or ELNEC, not with NEC-2 based programs like EZNEC or most other modern programs. The problem with wires connecting at an angle is that MININEC "cuts the corner" -- it acts like a wire extends between about the middles of the corner segments, rather than going clear to the corner as they should. The problem gets less and less as the segment length gets shorter and shorter near the junction (since the distance "cut" is less), or as the angle gets more obtuse. There's no magic threshold where the problem suddenly occurs (same reason). Its seriousness depends entirely on how serious a small length error is -- it'll just shift the resonance slightly of a single loop modeled with a moderate number of segments, but it can seriously alter the pattern of a quad because of the sensitivity of the parasitic element(s) to small wire length changes. ELNEC had a way of correcting for this, called "segment length tapering", where it would automatically convert a wire into a series of wires with increasing segment lengths. That way you could have a very short segment at the corner, but not have to use a large number of very short segments for the whole antenna. That feature is still in EZNEC. It's no longer needed for this purpose because NEC-2 doesn't have the problem, but it's occasionally useful at other times. You can find out more about this phenomenon in "MININEC: The Other Edge of the Sword", in Feb. 1991 _QST_. Roy Lewallen, W7EL art wrote: My understanding of antenna modeling is that sharp corners or connecting points should be avoided as they can create errors. The question I ask is what constitutes a sharp angle ? I am assuming that it is the capacitance at the connection is the problem. If so is the corner of a quad a problem or is a delta feed point on a dipole a problem ? I see many instances of quads being modeled and also " V" wire antennas each without a correcting "gusset" at the corners. So when is an angle termed to be sharp? Regards Art |
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