Fix a couple errors
> >
> > The spacing also determines how many nulls or main lobes
> > you'll have and where they are at. It is very unlikely
> > you'll be able to synthesize patterns with nulls and lobes
> > where you want, unless you carefully plan element locations.
> >
> > When receiving, you could move nulls around at will within
> > limits of what the element locations allow, but you'd have
> > to live with main lobe locations. You could pick main lobe
> > directions, and take what you got (through luck) for null
> > locations. When transmitting, you could do the same EXCEPT
> > you would almost certainly give up gain and efficiency.
>
> In general, assuming that your elements are suffienciently close (about <
> 0.5 wavelengths apart), you can form N-1 nulls if you have N elements, at
> arbitrary angles. It falls out of basic matrix math, and is the basis of a
> whole raft of adaptive beamforming techniques. There are some limitations
> on how close two nulls can be, and still be distinguished (related to the
> overall extent of the array). Nulls (unlike lobes) can be very, very
> narrow. In fact, the real challenge is in making broad nulls with a
limited
> number of elements (because you don't have enough nulls to can't just
stack
> up a bunch of narrow nulls next to each other). There are also some
> limitations from symmetry (e.g. if all your elements are in a straight
line,
> you'll get matching/mirrored nulls on both sides of the line).
>
> On TX, who cares where the nulls are.. you want to push the maximum power
in
> the desired direction. If the elements are "far" apart (i.e. low mutual
Z),
> then the optimum is generally to phase for time delay matching the spacing
> normal to the desired direction.
time delay matching the spacing "in the desired direction", not normal to
it. [ In math terms, phase = 2*pi/lambda *dotproduct(element position
vector, unit vector in look direction)]
> If the elements are close enough together
> to couple, you can get superdirectivity, as in a Yagi. There's a limit
there
> too (superdirectivity means you're storing more energy in the reactive
> field, and that increases dissipative losses).
> >
>
> Jim, W6RMK
>
_______________________________________________
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