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Subject: [TowerTalk] Re: ANTENNA STACKS
From: "David J Rodman, MD" <>
Date: Fri, 15 Oct 2004 17:08:57 -0400
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Jose, I have had some experience looking at what you propose. Many people do this. I found the interaction between antennas in close proximity too much for my liking and physically too hard to put up. After considerable modeling, I found that the LOWER frequency antenna is LEAST affected by close proximity stacking or even common boom locations. The HIGHER frequency antenna will be MOST affected and will one notices degredation of gain and F/B the closer antennas become. The effects are partially due to electrostatic forces and the large ground plane of the larger antenna. The effect is frequency dependent and for 20m I could find degredation as close as 10 feet vertical spacing. If you move the higher frequency closer so that the elements are on the same boom careful optimization is necessary. The rule of thumb I determined was that the higher frequency antenna essentially loses the gain of one element as the spacing gets close. F/B can be optimized but this requires some time to model. You should be prepared to lose gain and F/B if you close space dissimilar yagis. I do not think there is any set distance whereby there is NO effect, it is just the closer one moves the second or third antenna, the interaction becomes more critical. I hope this helps and personally, I would choose a distance that was mechanically sound and physically allows proper and safe installation or maintenance. Good luck.


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