[TowerTalk] Re: ANTENNA STACKS

[David J Rodman, MD]

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.