On Sat, 13 Dec 2008 01:56:27 GMT, Owen Duffy wrote:
I refer to the diagram at http://www.vk1od.net/lost/Fig6.png which is
from an article by the then VK2ZAB (now VK3EJ) on stacking Yagis.
Hi Owen,
This illustration seems to serve other commentary as it is filled with
odd eccentricities that are not very germane to the issue you raise
below. Eccentricities aside for the moment, I have to make a lot of
presumptions about an odd arrangement of 12 radiators. Some of them
are symmetrical by groups, but not all of them are symmetrical in
toto. I presume the groups are significant; but initially, what they
are significant of escapes me.
The + and - markings in the top tier four group, along with the
commentary, is suggestive; and I have to supply experience in the
matter to know that not ALL +s are connected together (and neither are
all -s connected together). I presume this top tier is a 4-Bay, but
there is nothing to support this except the graphical allusion.
Also from experience, I would presume that connections are not
horizontally placed, nor diagonally. The impression of incompleteness
is accruing.
I have highlighted two of the diagrams with a yellow background, and seek
opinions on them.
This implies (by your statement of "two" diagrams) that along this
middle tier of radiators, we have broken away from what might be a
4-Bay; and we are examining three pairs as choices put to an unstated
problem. Here, the eccentricity of what looks like an appendix
hanging from the folded element is further disturbed by what I can
only imagine to be an abstraction for a coax feedline. Incompleteness
is compounding.
If I am to pursue my forced presumptions, I would have to say that
this middle tier lacks many more alternatives in connections and
length variations. Incompleteness has reached saturation - which is
what I think you are responding to.
Referring firstly to the left hand one:
I suggest that the figure is in error because the scenario is not ALWAYS
wrong.
My contention is that at a single frequency, the phase inversion as a
result of the left to right swap of one driven element (DE) wrt the other
can be fully compensated for by ensuring that low loss feedline to one DE
is an odd number of electrical half waves longer than to the other.
Where the low loss feedline to one DE is an odd number of electrical half
waves longer than to the other, the Yagis are driven in phase.
The outcome being that the pattern at that frequency is approximately the
same as if equal length feedline branches were used.
Well, the original author does neglect to specify length, leaving it
to the reader's imagination to "presume" (have to say it) equal feed
lengths judged by eye. Unfortunately, the third example explicitly
offers this option, but only to those connections where phasing dots
are matched. Like I said, there are many missing alternatives.
Your imposition of an extra half wavelength in one feed may be
technically accurate, but it fights with the importance of their
length - which is to be found in the lost commentary, no doubt. I can
well guess, but that same commentary may illuminate these limited
choices and explain the eccentricities. I wouldn't want to slog
through that commentary, however.
Referring now to the right hand one:
I suggest that the figure is in error because the scenario is not ALWAYS
wrong.
My contention is that at a single frequency, that where the low loss
feedline to one DE is an integral number of electrical full waves longer
than to the other, the Yagis are driven in phase.
This would be a stretch of the imagination where application has
fallen into the ditch to serve argument. If the lengths drive
frequency to match to cable proportions in wavelength that do not
serve their loads, then such solutions are hardly useful.
The outcome being that the pattern at that frequency is approximately the
same as if equal length feedline branches were used.
Note that I am not trying to excite a purist discussion about branch vs
distributed feed arrangements for phased arrays.
Am I on the wrong track?
I am wondering why you are trying to resurrect this train wreck.
73's
Richard Clark, KB7QHC