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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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.
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