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On Fri, 20 Nov 2009 08:52:25 -0800, Jeff Liebermann
wrote: On Thu, 19 Nov 2009 13:20:01 -0800, Richard Clark wrote: My model shows a more benign mismatch to a 72 Ohm load. Richard Clark, KB7QHC I built several models of the antenna folded dipole assembly. The simple rectangular rod folded dipole yielded about 300 ohms. A slightly better simulation of the rounded ends, but still using a round rod, was about 260 ohms. Converting it to a flat wire ended up about 280 ohms. I never got anything even close to 72 ohms. It's my http://802.11junk.com/jeffl/antennas/mfj1800/ EZNEC shows: Impedance = 73.13 + J 18.78 ohms which, I suppose, could have the reactance driven out if I shift frequency. Show my your NEC2 deck and tell me what I did wrong, and maybe I'll believe that it's 72 ohms. Incidentally, the possibility that I screwed up somewhere in the model is quite real: http://802.11junk.com/jeffl/antennas/mfj1800/mfj1800.txt (I'll convert this mess back to a macro form so it's easier to read maybe this weekend). I don't have an option of NEC2 deck. One thing you might check, and is something I reported about, is does your model have the loop symmetrical to the plane of the directors/reflector? I followed all of Mike's dimensions and I note that your lobe characteristics don't show his lack of driven element symmetry - mine do. Again, I have modeled only the three elements (Ref/Dr/Dir) as the additional directors are unlikely to perturb drive point Z as much as to push it from 73 Ohms up to your high 200s (triple?). Another point, as I have described, I used 1/4 inch diameter wire in place of larger flat sheet metal elements (which I note you try to replecate, but only once). True, 1/4 inch is not as big as any flat dimension, but as Roy reports on equivalence, flat is not the same as diameter, but flat performance is closer to a smaller diameter round wire. Hence the 1/4 inch. 73's Richard Clark, KB7QHC |
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