I've been reading about companion planting. It makes sense and I plan
to try it.

I'm more concerned about the bad companions. They say beans and onions
don't like each other, but beans and carrots do (as well as carrots and
onions). I plan on planting a row of pole beans (along a fence line for
support), with a row of carrots beneath them. If I planted onions on
the other side of the carrots (so the carrots are between the beans and
onions) would that put the beans and onions far enough apart? The beans
and onions would be a foot or two apart with carrots in between.

--------------------------- Fence
B B B B B B B B B B B B B B Beans
C C C C C C C C C C C C C C Carrots
O O O O O O O O O O O O O O Onions

Also, how close do good companion plants (like tomato and basil) need
to be to gain benefits from each other? Does the magic happen
underground (root level, stuff exuded into soil) or it is only related
to the fragrance of the plants?

Thanks!