Good morning, Dick.
Regarding how many elements are "active on each band" in a multi-band Yagi
style antenna, the answer is: every element on the boom. Since the elements
are in such close proximity, it is not possible to isolate them and every
element carries at least some current, regardless of the frequency of the
energy applied to the antenna. Consequently, it is extremely important that
all of the elements work in concert to produce a positive outcome: forward
gain. This is not isolated to multi-band, single-boom Yagis, it is a primary
consideration when stacking monoband Yagis on a single mast, as well as
having any combination of antennas within close proximity to each other.
Knowing how many elements and the boom length in a multi-band Yagi will
provide a very broad clue as to the possible forward gain potential, but it
does not mean the antenna achieves that figure, nor does it mean the antenna
has any forward gain at all. If the elements are all tuned properly, if all
the elements are in the proper positions, if all the losses are known, then
one can make some deternination of the potential forward gain of a
particular antenna. Having a certain number of elements does not guarantee
any certain amount of forward gain. All it means is that there are a
"certain number of elements." The important item is how much actual gain the
antenna truly has.
Hope that helps answer your question.
Have a good day and 73,
Force 12 Antennas and Systems
(Home Page http://www.QTH.com/force12 )
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