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#14
February 7th 05, 03:31 AM
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Howard wrote: On 6 Feb 2005 18:10:11 -0800, wrote: snip It was an unusual use of the YO program, in that we used two reflectors and NO directors, to get the shape we wanted. We actually couldn't use the optimizer, because it would actually try to narrow the beam too much. Remember that you must keep the pattern about 180 degrees, and the F/B ratio should be no more than about 11 dB. Slick Slick, Could you explain the use of two reflectors? I've heard that if you are building a two element yagi it's best to use a radiator and reflector but haven't really learned anything about using more than one reflector. Have seen antennas that use 3 reflectors, but they were mounted perpendicular to the plane of the elements and there were several director elements - if memory serves me it was Cushcraft that did that. Haven't seen them added behind the first reflector and am curious as to what characteristic of the antenna's performance it is meant to enhance. For this particular design, it was required in that we needed something with a 180 degree pattern, and not to much F/B ratio. I can't tell you much more other than we had to manually tweak the variables to get a decent result...we couldn't use the program's optimizer, it would have made the forward lobe too small. So what ends up happening in the program, is that you end up using two directors, but in the H-plane the lobe goes to the rear (see link). So in essence, you flip the whole deal around, and your two directors are actually reflectors. Interesting, eh? Slick |
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