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>You are correct. The only reasons one sees 40M beams mounted BELOW >higher frequency ones are the structural considerations and the greater >difficulty of doing it right. >By the way, because of the harmonic relationship between 40M and 15M, >you should mount them at 90 degree orientation tominimize interaction. The 90-degree turn isn't necessarily required, though I did it on mine, which are spaced 6 feet apart, because the SWR on the C-3E (below the EF-240S) was perturbed with them pointed in the same direction. I know that a lack of SWR perturbation doesn't necessarily mean no interaction, but it is probably less if there is no effect on SWR, and the tribander seems to work well on 15 now. One consideration worth thinking about is that a 2-element 40m yagi has its elements at opposite ends of the boom, far from the mast. For an antenna like the EF-240S that can require considerable tweaking of element electrical lengths and the hairpin, this may be awkward to do unless you can work out some way to easily tilt the antenna until you get it right. At 6 feet above my tribander, my 40m yagi can't be tilted down far enough to reach the feedpoint or the reflector tuning from the tower; yet anything much more than 6 feet spacing and you won't be able to reach the boom-to-mast mount to tilt it over. I haven't done this yet, but if I ever decide that I must adjust the antenna, or have to repair something at the feedpoint, I think I will have to loosen the U-bolts and slide the boom 3+ feet to one side before tilting it down on the short side (which won't be easy with the preponderance of the weight on the long side). If I were doing it again, I think I would mount the 40m yagi on the mast first, at about 3-4 feet above the tower, with only the center bolt of the F12 mount in place, Then I would do my tilting and tuning. Once it was where I wanted it, I would install the rest of the bolts, slide the antenna up the mast as high as it would go, and tighten it. Then and only then would I add the tribander. I'm sure there are other ways to do this -- for example, I might be able to turn the 40m antenna so that its elements are vertical and then rotate it down through the elements of the tribander far enough to reach. Happily, all this is theoretical, so far. The antenna works fine with the cookbook dimensions, but I put that down to dumb luck. 73, Pete N4ZR The World HF Contest Station Database was updated 23 Feb 03. Are you current? www.pvrc.org/wcsd/wcsdsearch.htm -------------------------------------------- Force12Talk mailing list provided as a service by Force 12 Antennas, Inc. Force12 Web Site: http://www.force12inc.com To Submit Message to the List: Force12Talk@qth.com To unsubscribe and view the Message Archive: see http://qth.com/force12/list For problems with the list: contact n4zr@qth.com |
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