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## Re: [TowerTalk] YOed older 4 el 20 meter CC Skywalkers

 To: , ,"Bill Tippett" Re: [TowerTalk] YOed older 4 el 20 meter CC Skywalkers "Tom Rauch" Tom Rauch Tue, 11 May 2004 09:28:33 -0400
 ```> Optimum stacking distance is a function of your antenna > pattern. Antennas with narrow beamwidths need wider stack > spacing to optimize gain. That's right. It's all a matter of pattern multiplication. To have significant gain increase in one direction or angle, you have to remove significant energy from another direction. We used to do this long hand by looking at patterns of individual antennas and arranging them so energy was removed from major lobes. You have to model the stack over real ground, and adjust spacing over real ground. Rules of thumb do not work well, and neither does setting optimum stacking spacing in freespace. You can prove this with simple dipoles or quad antennas. Let's assume we have a quad. A horizontally polarized quad is two stacked 1/4 wl long dipoles with 1/4 wl stacking height. The second element forces a shallow null straight up and down. At heights above earth where the individual sources in the quad element have a deep null at 90 degree elevation, like 1/2wl mean height, the quad has no gain over a dipole element. That's because the element spacing is trying to remove energy where there is already a deep null. At heights where vertical null turns into a significant lobe, like 3/4wl mean height, the quad element shows maximum gain (about a dB or so) over a dipole element. This is also why a multielement quad with several elements has no real gain advantage over a Yagi at any height. The use of multiple elements forces a vertical null, and the second set of elements stacked 1/4 wl away (created by the quad element's second current points) try to force a null where there already is no energy. Your yagis would behave the same. At some mean heights they would have much more gain at a given stack spacing than at other heights. A spacing that provides maximum gain in freespace would likely not be the optimum over earth, because the patterns of individual sources change over earth. >From what I have seen, very few stacks have optimum spacing and height. Most HF stacks really only provide the advantage of moving nulls around, rather than adding substantial gain. For optimum gain..... the narrower the lobes of each antenna or combination of antennas, the wider the spacing must be. The more arrays in the stack, the wider the spacing must be. The real difference most people see is they simply steer the elevation nulls in the forward lobe to less harmful angles, and that is what causes the "gain". You can see this effect in the gain curves of stacked dipoles at: http://www.w8ji.com/stacking_broadside_colllinear.htm Someday I'll correct the spelling of collinear in the link, hi hi. 73 Tom _______________________________________________ See: http://www.mscomputer.com for "Self Supporting Towers", "Wireless Weather Stations", and lot's more. Call Toll Free, 1-800-333-9041 with any questions and ask for Sherman, W2FLA. _______________________________________________ TowerTalk mailing list TowerTalk@contesting.com http://lists.contesting.com/mailman/listinfo/towertalk ```
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