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

I had a simple question in regards to phased array antenna patterns.
If a phased array is trying to send a narrow beam in a specific
direction, how do the other side lobes get reduced and/or eliminated?
Are the individual antenna transmitters/elements not omnidirectional
themselves? If not, what are the characteristics of their patterns?

There's really nothing special about a phased-array antenna.
It is a radiating aperture, just like the mouth of a horn
or the front of a parabolic "dish". The only difference
is that the phase and amplitude of the excitation across
the aperture can be much more finely adjusted to "squint"
the beam.

But the essential tradeoff remains: the larger the aperture,
the narrower the beam. And, like horns and reflectors, one
can reduce the sidelobes by not exciting the outside elements
as strongly, but with a corresponding reduction in gain from
the maximum obtainable for a given aperture size. This is
called illumination tapering and is done on all antennas
in one form or another to work the gain/sidelobe tradeoff.

I ask this because I'm trying to understand how tracking radars can
send narrow beams in the desired direction without significant
secondary lobes to interfere with returns from the desired lobe.

Most radars go for wider beams with lower sidelobes because
the overall system performance is better. But the sidelobes
are only reduced, not eliminated entirely.

Oh, yeah, and about the patterns of the individual elements,
they have to be as non-directional as possible. The deal is
that the overall antenna pattern is the result of the PRODUCT
of the array factor with the pattern of the individual elements.
If the individual elements have reduced gain off boresight, the
entire antenna gain is reduced off-boresight as well, limiting
the angles at which the beam may be "squinted".

Jim, K7JEB