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June 13th 04, 02:26 AM

On 12 Jun 2004 16:43:35 -0700, (David Harper)
wrote:

Hi Dave,

By the points asked:
how do the other side lobes get reduced and/or eliminated?
Through the addition or subtraction of phase;
Are the individual antenna transmitters/elements not omnidirectional
themselves?
This is not an imperative of design;
If not, what are the characteristics of their patterns?
They add or subtract according to phase offered;
how tracking radars can
send narrow beams in the desired direction without significant
secondary lobes to interfere with returns from the desired lobe.
How indeed, they are not immune from these issues.

Phase and distance are co-equivalent when you speak of multiple
sources. When one source excites resonant elements, those element's
create a phase lag/lead and combine with the original signal with a
phase lag/lead that is correlatable to physical separation from the
exciting source. The designer conspires to arrange all such
times/distances/lags/leads/phases to combine additively/subtractively
to create the desired pattern. Simple patterns like the figure 8 of
the dipole involve few components, but also reveal broad
characteristics for the same reason.

If you want a sharp beam with small fringing lobes, you have to invest
in quite a few times/distances/lags/leads/phases elements. Complexity
does not bring order easily. However money might. Those who build
radars are used to just such a lubricant. The dishes you see offer a
continuum of times/distances/lags/leads/phases with the added benefit
of fairly good shielding from a lot of their sources of grief.
However, they are constrained by physical necessity of being pointed
towards the area of interest.

Other radar designers using phased array sources, use massive
parallelism. Look at a picture of the USS Enterprise. Nearly the
entire surface area of the Island is radiating elements on all four
sides. Each and every source offers a very simple characteristic with
a variable delay line between it an free space - and of course, there
is some distance involved between each element and the next. Given
that those delay lines are addressable and individually setable, beam
steering is obtained through a rather simple concept.

If I recall my former trade correctly, some of the sources in the
array can be employed separate to the main combination - sort of like
picture-in-picture technology. :-)

73's
Richard Clark, KB7QHC

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