Phased array antenna patterns
 

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?

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.

Thanks in advance for any insight!
Dave

"David Harper" wrote in message
om...
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?

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.

Thanks in advance for any insight!
Dave

Dave,

Actually phased arrays have existed in commercial broadcasting AM (day/night
patterns) as well as some FM.

The best way for you to visualize a phased array is to have 2, 3 or 4 stones
hit the water at the same time ....
you will see patterns emerge as cancellations happen.

http://home.earthlink.net/~jimlux/radio/antenna/phased/

http://mitglied.lycos.de/radargrundl...n/at18-en.html

Recent IEEE seminar
http://www.ieee-boston.org/edu/course_phased-array.htm

Hundreds of references available with a simple google search.

w9gb

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

"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?

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.

Thanks in advance for any insight!

=============================

To understand how things work it is necessary to be familiar with elementary
trigonometry. Its quite simple really.

Without trigonometry you just have to accept that's the way things are.

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

Phased arrays are fun to play around with (on the 2-meter band especially)
You can experiment using physically manageable components.
You can switch elements phasing by using different lengths of feedline
its interesting - a good hobby in a hobby

"David Harper" wrote in message
om...
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?

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.

Thanks in advance for any insight!
Dave


---
Outgoing mail is certified Virus Free.
Checked by AVG anti-virus system (http://www.grisoft.com).
Version: 6.0.690 / Virus Database: 451 - Release Date: 5/22/2004

"Reg Edwards" wrote in message ...
"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?

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.

Thanks in advance for any insight!

=============================

To understand how things work it is necessary to be familiar with elementary
trigonometry. Its quite simple really.

Without trigonometry you just have to accept that's the way things are.

Being an engineer, I am quite familiar with trigonometry. I believe
you didn't fully understand my question: How do the other side lobes
get reduced and/or eliminated? Theoretically, in some phased array
antenna configurations, some lobes have an undesirably high a gain. I
was wondering what filters / engineering work arounds were used to
mitigate this. Other posts have adequately answered this question,
however. Thanks anyway.

"David Harper" wrote in message
om...
"Reg Edwards" wrote in message
...
"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?

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.

Thanks in advance for any insight!

=============================

To understand how things work it is necessary to be familiar with
elementary
trigonometry. Its quite simple really.

Without trigonometry you just have to accept that's the way things are.

Being an engineer, I am quite familiar with trigonometry. I believe
you didn't fully understand my question: How do the other side lobes
get reduced and/or eliminated? Theoretically, in some phased array
antenna configurations, some lobes have an undesirably high a gain. I
was wondering what filters / engineering work arounds were used to
mitigate this. Other posts have adequately answered this question,
however. Thanks anyway.

David

I'd submit that your question is very difficult to explain within the
consraints of a newsgroup paragraph or two in a news group. The subject of
aperature distribution has been studdied for years and years by thousands
of high level engineers.
Although it is desired to minimize the sidelobe level from scanning flat
phased arrays, there is a limit to how low the sidelobes can be made. They
sure dont go to zero.
A side looking scanning antenna of phased elements will have individual
elements who's pattern is appropriate for whatever area is being scanned.
The pattern from each element wont be omnidirectional.

It might be a good exorcize for you to draw a plot on a piece of paper
with 10 antennas in a row. You can quickly see that the array will have max
gain at some power distribution and have sidelobes that change appreciably
when the distribution of power (and phase) is varied over the length of that
array.

Jerry

K7JEB wrote in message . ..
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


Thanks! You know any simple phased array configurations that are easy
to mathmatically model? I've played with a 4-element array all
located on the same axis just for fun to see what resulted (and to
make sure my equations were right), but I'd like to find some
real-world systems (hopefully without too many elements) to play with.
Any digrams exist on the net? (dimensional relationship between
elements, etc).

Thanks again!
Dave

It might be a good exorcize for you to draw a plot on a piece of paper
with 10 antennas in a row. You can quickly see - - - -

=============================

In other words - trigonometry! Or geometry!

Something to keep in mind is that phased arrays with a large number of
elements are simpler in one respect than ones with just a few elements.
In the first case, you can make the assumption with reasonable accuracy
that the feedpoint impedances of all elements are the same, since all
are in essentially the same environment with respect to the other
elements. For most simple phased arrays, you can't make this assumption.
So while pattern analysis of simple phased arrays is easy once you
assume equal element currents, actually getting those currents can be
more difficult than you might assume. If you're interested only in the
patterns and not how you'd actually get the currents you need, simple
trig is enough. But that's not enough to enable you to actually design
and build one that'll work as planned. Chapter 8 of the _ARRL Antenna
Book_ describes what has to be done to make simple arrays work properly,
as well as describing a few common simple arrays. There are also a few
examples of phased arrays with the free EZNEC demo (http://eznec.com),
where you can save yourself the math and immediately see the effect of
changing element currents.

Roy Lewallen, W7EL

David Harper wrote:

Thanks! You know any simple phased array configurations that are easy
to mathmatically model? I've played with a 4-element array all
located on the same axis just for fun to see what resulted (and to
make sure my equations were right), but I'd like to find some
real-world systems (hopefully without too many elements) to play with.
Any digrams exist on the net? (dimensional relationship between
elements, etc).

Thanks again!
Dave

Below

One method of eliminating* sidelobes from an array is to use 1/2 lambda
element spacing and in-phase, binomial power distribution to the elements.

This has been done in FM broadcast transmit arrays to reduce radiation
levels on the ground immediately adjacent to the tower. Paper 5 at
http://rfry.org shows some of the considerations for its use. Paper 10 at
the same site is a slide show wherein slides 20 and 23 also deal with this.

R. Fry

* in free-space theory, anyway
____________

"David Harper" wrote
I believe you didn't fully understand my question: How do the other side
lobes get reduced and/or eliminated? Theoretically, in some phased
array antenna configurations, some lobes have an undesirably high
a gain. I was wondering what filters / engineering work arounds were
used to mitigate this. Other posts have adequately answered this
question, however. Thanks anyway.

On 14 Jun 2004 11:53:37 -0700, (David Harper) wrote:

Thanks! You know any simple phased array configurations
that are easy to mathmatically model?...

In a follow-up to your post Roy, W7EL, suggested modeling
an array with his **FREE** EZNEC demo. I'll second that
motion. EZNEC is well worth learning to use if one is doing
any kind of antenna analysis, even back-of-the-envelope
guesstimates.

Here's what I would do to get a high-gain array with EZNEC:

Place a large number of 1/10-wavelength wires in a row,
end to end, and separated from each other by 1/2 wavelength.
Place a current source in the middle of each, up to the
maximum number allowed by the EZNEC demo version. Set each
current source to 1 amp at zero degrees. Force each wire
to consist of only one segment - this makes the current in
it uniform. Have EZNEC evaluate the pattern in the same
plane as the wires. This is a one-dimensional phased-array.

You can vary the amplitude and phase of the individual
current sources to experiment with beam steering. And you
can change the spacing for the individual elements out to
1 wavelength and beyond to see what that does. It's a lot
of fun to watch what happens.

This should be a relatively easy configuration to write a
far-field expression for if you are so inclined. Math never
was my strong suite, so I'm out of my element and can't give
you much help. (Hey, we're just a bunch of ham-radio operators
here).

Have fun!

Jim, K7JEB