Bill sure has it right. We should always model the antennas to pick
We used to do this on paper before it was common to use personal
computers to model antennas. You can do something like this:
1.) You look at the pattern of one antenna by itself.
2.) You look at the pattern of two non-directional radiators with the
same polarization spaced various distances apart.
3.) You either adjust distance between the two non-directional
radiators so the pattern they make causes a null where the real
antenna has a major lobe that you do not want (minimum sidelobe
stacking), or you adjust the distance so the stacking of the two
non-directional radiators shows a null area extending about half-
way up the main lobe (maximum gain distance) of what would be
the pattern of a real antenna.
So you see, stacking distance has absolutely nothing to do with
boom length! Optimum stacking distance is a function of how
narrow the original pattern is....or where the minor lobes are that
you want to get rid of.
Rules of thumb do not work.
Take two freespace dipoles for example. The boom length is zero.
Optimum gain stacking distance is just barely under 3/4
wavelength, because that forces four deep nulls in the main lobe,
removing the most possible energy from unwanted directions. Zero
boom length = 3/4wl stacking distance!!!
Make it two two element yagi's in freespace, and optimum gain
stacking distance is slightly wider than the dipoles no matter how
long the booms of each yagi are.
Make it two antennas of any type that have 20 degree beamwidth,
and stacking distance has to be so wide that it forces a null over a
good part of the main lobe...otherwise you don't get maximum gain.
Stacking distance would be well over 1-wl no matter how long the
Stacking distance relates only to how wide the individual antenna
patterns are (over earth), and if you want high gain or good side
lobe suppression (you can't normally get both). The narrower each
antennas pattern, the wider stacking distance becomes. It has
nothing at all to do with boom length.
73, Tom W8JI
FAQ on WWW: http://www.contesting.com/FAQ/towertalk
Administrative requests: towertalk-REQUEST@contesting.com