Where does the far field start on a phased array?
 

First, I should state this is not a question about an amateur antenna
system, but I'm hoping someone may know the answer.

Consider a phased array of antennas. There are some 30 or so antennas
all in a line, as below, where "A" is an antenna and "-" indicates a space


A---A---A---A---A---A---A---A---A---A


The overall width of the array is D.

The radiation pattern varies as a function of distance from the antenna
until one is in the far-field. But where does the far-field start for a
phased array? Can one use the normal formula of

2 D^2 / lambda ?

If the width D is large (say 30 m) and the wavelength small (say 0.1 m),
then the far field does not start for

2 * 30 * 30 / 0.1

18,000 m
= 18 km

i.e. the radiation pattern is a function of distance until you are some
18 km (about 11 miles) from the antenna.

If anyone can give me a link to a professional reference on this,
scientific paper etc, that would be useful.


--
Dave (from the UK)

Please note my email address changes periodically to avoid spam.
It is always of the form:
Hitting reply will work for a few months only - later set it manually.

http://chessdb.sourceforge.net/ - a Free open-source Chess Database

Where does the far field start on a phased array?
 

On 21 abr, 10:32, "Dave (from the UK)" see-my-signat...@southminster-
branch-line.org.uk wrote:
First, I should state this is not a question about an amateur antenna
system, but I'm hoping someone may know the answer.

Consider a phased array of antennas. There are some 30 or so antennas
all in a line, as below, where "A" is an antenna and "-" indicates a space

A---A---A---A---A---A---A---A---A---A

The overall width of the array is D.

The radiation pattern varies as a function of distance from the antenna
until one is in the far-field. But where does the far-field start for a
phased array? Can one use the normal formula of

2 D^2 / lambda ?

If the width D is large (say 30 m) and the wavelength small (say 0.1 m),
then the far field does not start for

2 * 30 * 30 / 0.1

18,000 m
= 18 km

i.e. the radiation pattern is a function of distance until you are some
18 km (about 11 miles) from the antenna.

If anyone can give me a link to a professional reference on this,
scientific paper etc, that would be useful.

--
Dave (from the UK)

Please note my email address changes periodically to avoid spam.
It is always of the form:
Hitting reply will work for a few months only - later set it manually.

http://chessdb.sourceforge.net/- a Free open-source Chess Database

Hello,

Your formula for far field distance (Fraunhofer region) assumes a path
difference between the inner and outer antenna with respect to an
observation point of 1/16 lambda.

The 1/16 lambda (22.5 degrees) path difference is used by many experts
(for example page 39 of "Antennas for all applications" 3rd edition
[Kraus, Marhefka]).

Depending on your application and required accuracy, you can divert
from the 1/16 lambda rule. For example when you taper the power
distribution for your array (to get lower side lobes), the far field
distance is not that far. If you accept a 1 dB error in main lob
gain, you can go down to 25% of the far field distance formula.
However when you want to measure a dip in the radiation pattern of
(for example) -50 dBi, you might need a larger distance.

For your array, you can calculate the effect of distance on radiation
pattern in a spread sheet program.

Best regards,

Wim
PA3DJS

Where does the far field start on a phased array?
 

Wimpie wrote:

Hello,

Your formula for far field distance (Fraunhofer region) assumes a path
difference between the inner and outer antenna with respect to an
observation point of 1/16 lambda.

Someone has said that the formula I gave is not valid for a phased array.

His comment (about the 2 D^2/lambda) is below:

-----
That estimation does not apply in this case. It can be considered to be
valid for aperture antennas which is not the case here. It would only
require to have the transmitting antenna illuminating the pleased array
within its 3dB mainlobe which of course is by far the case at a distance
of 3000m or even more.
------


I'm not to bothered about the odd factor of two. I have seen a
derivation of the formula, but it was based on a rectangular aperture,
not an array of them. It don't know if that may mean the equation is
just not appropriate at all.


Using that forumal puts the far-field distance at about 10 km in my
case. Using someone elses idea, puts it at only a few hundred meters.
There is at least a factor of 10 difference.
--
Dave (from the UK)

Please note my email address changes periodically to avoid spam.
It is always of the form:
Hitting reply will work for a few months only - later set it manually.

http://chessdb.sourceforge.net/ - a Free open-source Chess Database

Where does the far field start on a phased array?
 

Dave (from the UK) wrote:

If anyone can give me a link to a professional reference on this,
scientific paper etc, that would be useful.



Hi all again


W4ZCB emailed me about this, but my attempts to reply have failed.
Anyway, W4ZCB suggested his paper, which has a nice physical (non
mathematical) explanation of the far field.

http://www.setileague.org/articles/hps_ham.htm
http://www.setileague.org/articles/ham/farfield.pdf

Unfortunately, it does not solve my particular problem.

--
Dave (from the UK)

Please note my email address changes periodically to avoid spam.
It is always of the form:
Hitting reply will work for a few months only - later set it manually.

http://chessdb.sourceforge.net/ - a Free open-source Chess Database

Where does the far field start on a phased array?
 

On Apr 21, 4:32 am, "Dave (from the UK)" see-my-
wrote:
First, I should state this is not a question about an amateur antenna
system, but I'm hoping someone may know the answer.

Consider a phased array of antennas. There are some 30 or so antennas
all in a line, as below, where "A" is an antenna and "-" indicates a space

A---A---A---A---A---A---A---A---A---A

The overall width of the array is D.

The radiation pattern varies as a function of distance from the antenna
until one is in the far-field. But where does the far-field start for a
phased array? Can one use the normal formula of

2 D^2 / lambda ?

If the width D is large (say 30 m) and the wavelength small (say 0.1 m),
then the far field does not start for

2 * 30 * 30 / 0.1

18,000 m
= 18 km

i.e. the radiation pattern is a function of distance until you are some
18 km (about 11 miles) from the antenna.

If anyone can give me a link to a professional reference on this,
scientific paper etc, that would be useful.

--
Dave (from the UK)

Please note my email address changes periodically to avoid spam.
It is always of the form:
Hitting reply will work for a few months only - later set it manually.

http://chessdb.sourceforge.net/- a Free open-source Chess Database

This sounds very much like an arrangement for a radar antenna that
operates a little above 1Ghz with 32 dipole assemblies space out over
approximately 10 meters. The far field is eatablished at about 2km on
this antenna.

Jimmie

Where does the far field start on a phased array?
 

JIMMIE wrote:

This sounds very much like an arrangement for a radar antenna that
operates a little above 1Ghz with 32 dipole assemblies space out over
approximately 10 meters. The far field is eatablished at about 2km on
this antenna.

Jimmie


No, it is not that at all, but if you have a reference to the antenna
you describe, I would be interested. This is wider and a higher frequency.



--
Dave (from the UK)

Please note my email address changes periodically to avoid spam.
It is always of the form:
Hitting reply will work for a few months only - later set it manually.

http://chessdb.sourceforge.net/ - a Free open-source Chess Database

Where does the far field start on a phased array?
 

On Apr 21, 8:40 pm, "Dave (from the UK)" see-my-
wrote:
JIMMIE wrote:
This sounds very much like an arrangement for a radar antenna that
operates a little above 1Ghz with 32 dipole assemblies space out over
approximately 10 meters. The far field is eatablished at about 2km on
this antenna.

Jimmie

No, it is not that at all, but if you have a reference to the antenna
you describe, I would be interested. This is wider and a higher frequency.

--
Dave (from the UK)

Please note my email address changes periodically to avoid spam.
It is always of the form:
Hitting reply will work for a few months only - later set it manually.

http://chessdb.sourceforge.net/- a Free open-source Chess Database

The antenna I was refering to is knon as an ATCRBS antenna or an SSR
antenna. I coouldnt find a lot of info right off but there were a lot
of references t oIEEE xplorer. If you belong to that Im sure you can
find some information.

Like chess? send me an email and I will connect you will a friend who
has been known to give a challenging game or two.

Jimmie

Where does the far field start on a phased array?
 

W4ZCB emailed me about this, but my attempts to reply have failed. Anyway,
W4ZCB suggested his paper, which has a nice physical (non mathematical)
explanation of the far field.

http://www.setileague.org/articles/hps_ham.htm
http://www.setileague.org/articles/ham/farfield.pdf

Unfortunately, it does not solve my particular problem.

--
Dave (from the UK)

Hmmmmmmmmmmm. Sorry about the one way skip, my ISP apparently doesn't like
your ISP. Happens occasionally. Glad you found Pauls article, was going to
be a bear for me to scan it for about the 10th time.

So, I give up, what IS your particular problem? As stated, the far field
depends on the antenna gain.

W4ZCB

Where does the far field start on a phased array?
 

On 21 abr, 23:30, "Dave (from the UK)" see-my-signat...@southminster-
branch-line.org.uk wrote:
Wimpie wrote:
Hello,

Your formula for far field distance (Fraunhofer region) assumes a path
difference between the inner and outer antenna with respect to an
observation point of 1/16 lambda.

Someone has said that the formula I gave is not valid for a phased array.

His comment (about the 2 D^2/lambda) is below:

-----
That estimation does not apply in this case. It can be considered to be
valid for aperture antennas which is not the case here. It would only
require to have the transmitting antenna illuminating the pleased array
within its 3dB mainlobe which of course is by far the case at a distance
of 3000m or even more.
------

I'm not to bothered about the odd factor of two. I have seen a
derivation of the formula, but it was based on a rectangular aperture,
not an array of them. It don't know if that may mean the equation is
just not appropriate at all.

Using that forumal puts the far-field distance at about 10 km in my
case. Using someone elses idea, puts it at only a few hundred meters.
There is at least a factor of 10 difference.
--
Dave (from the UK)

Please note my email address changes periodically to avoid spam.
It is always of the form:
Hitting reply will work for a few months only - later set it manually.

http://chessdb.sourceforge.net/- a Free open-source Chess Database

Hi Dave,

Whether or not the formula is applicable, depends on many factors as
mentioned in my previous posting.

For a broadside array, the formula holds with same accuracy as for
continuous aperture antennas. In my antenna courses I use the
broadside array approach to derive the 2B^2/lambda formula.

For an end-fire case, the situation is different. When you are
interested in main lobe gain only (so not the complete radiation
pattern), you can reduce the distance significantly. The reason for
that is that when you come closer to the antenna, the path difference
doesn't change; the amplitude contribution of each array element is of
importance now. However when you need to know the complete pattern
(including broadside directions), you need the large distance.

It is just a matter of change in path length difference amplitude
unbalance when you come too close to the antenna. If you keep this in
mind, you can figure out the measuring distance for you application
(for example with a spread sheet).

I would reserve the term "far field distance" for that distance where
the complete radiation pattern does not change with measuring
distance. In that case, the 2B^2/lambda formula is a good rule of
thumb.


Best regards,

Wim
PA3DJS

Where does the far field start on a phased array?
 

Wimpie wrote:

Hi Dave,

Hi Wim

Whether or not the formula is applicable, depends on many factors as
mentioned in my previous posting.

For a broadside array, the formula holds with same accuracy as for
continuous aperture antennas. In my antenna courses I use the
broadside array approach to derive the 2B^2/lambda formula.

My situation is very odd. As I said at the start, this is not an amateur
antenna.

The array of "antennas" are not designed to work as one nice antennas,
but are an essentially random(ish) collection of radiating centres.
(However, they are all energised from the same signal source). So they
can be considered like a phased array, as they are regularly spaced all
in one long line.

Hence my original diagram

A---A---A---A---A---A---A---A---A---A

accurately describes the situation. Each "A" is an antenna. The
amplitude and phase can be arbitrary.

I do *not* want them to behave as a nice phased array with decent gain
and low side-lobes! Each antennas is radiating an *unwanted* signal. But
the fact remains that the gain could conceivably be high under some
circumstances, which would create interference.

Hence I need to test this.

I would reserve the term "far field distance" for that distance where
the complete radiation pattern does not change with measuring
distance. In that case, the 2B^2/lambda formula is a good rule of
thumb.

In this case, I am interested in any direction. The direction of the
main lobe will be essentially random.

--
Dave (from the UK)

Please note my email address changes periodically to avoid spam.
It is always of the form:
Hitting reply will work for a few months only - later set it manually.

http://chessdb.sourceforge.net/ - a Free open-source Chess Database

Where does the far field start on a phased array?
 

"Dave (from the UK)"
wrote in message ...
Wimpie wrote:

Hi Dave,

Hi Wim

Whether or not the formula is applicable, depends on many factors as
mentioned in my previous posting.

For a broadside array, the formula holds with same accuracy as for
continuous aperture antennas. In my antenna courses I use the
broadside array approach to derive the 2B^2/lambda formula.

My situation is very odd. As I said at the start, this is not an amateur
antenna.

The array of "antennas" are not designed to work as one nice antennas, but
are an essentially random(ish) collection of radiating centres. (However,
they are all energised from the same signal source). So they can be
considered like a phased array, as they are regularly spaced all in one
long line.

Hence my original diagram

A---A---A---A---A---A---A---A---A---A

accurately describes the situation. Each "A" is an antenna. The amplitude
and phase can be arbitrary.

I do *not* want them to behave as a nice phased array with decent gain and
low side-lobes! Each antennas is radiating an *unwanted* signal. But the
fact remains that the gain could conceivably be high under some
circumstances, which would create interference.

Hence I need to test this.

I would reserve the term "far field distance" for that distance where
the complete radiation pattern does not change with measuring
distance. In that case, the 2B^2/lambda formula is a good rule of
thumb.

In this case, I am interested in any direction. The direction of the main
lobe will be essentially random.

--
Dave (from the UK)


Hi Dave

I'm curious about two things.

1 - Do you intend to actually make and record measurements of the
radiated field, or do you want to determine the minimum distance at which
the measurements can be made?
2 - What prevents the use of a computer modeling program to predict the
pattern?

Jerry

Where does the far field start on a phased array?
 

Jerry Martes wrote:

Hi Dave

I'm curious about two things.

1 - Do you intend to actually make and record measurements of the
radiated field, or do you want to determine the minimum distance at which
the measurements can be made?

yes

2 - What prevents the use of a computer modeling program to predict the
pattern?

nothing. I think that will be done. But a theoetical analysis would be
nice if possible.


Jerry




--
Dave (from the UK)

Please note my email address changes periodically to avoid spam.
It is always of the form:
Hitting reply will work for a few months only - later set it manually.

http://chessdb.sourceforge.net/ - a Free open-source Chess Database

Where does the far field start on a phased array?
 

On 22 abr, 12:25, "Dave (from the UK)" see-my-signat...@southminster-
branch-line.org.uk wrote:
Wimpie wrote:
Hi Dave,

Hi Wim

Whether or not the formula is applicable, depends on many factors as
mentioned in my previous posting.
For a broadside array, the formula holds with same accuracy as for
continuous aperture antennas. In my antenna courses I use the
broadside array approach to derive the 2B^2/lambda formula.

My situation is very odd. As I said at the start, this is not an amateur
antenna.

The array of "antennas" are not designed to work as one nice antennas,
but are an essentially random(ish) collection of radiating centres.
(However, they are all energised from the same signal source). So they
can be considered like a phased array, as they are regularly spaced all
in one long line.

Hence my original diagram

A---A---A---A---A---A---A---A---A---A

accurately describes the situation. Each "A" is an antenna. The
amplitude and phase can be arbitrary.

I do *not* want them to behave as a nice phased array with decent gain
and low side-lobes! Each antennas is radiating an *unwanted* signal. But
the fact remains that the gain could conceivably be high under some
circumstances, which would create interference.

Hence I need to test this.

I would reserve the term "far field distance" for that distance where
the complete radiation pattern does not change with measuring
[all text deleted]

Hi Dave,



I don't know what you are going to do with the array. As long as you
understand how a radiation pattern (whether within or outside the far
field distance) can be calculated based on the array elements, you
should be able to find a comfortable distance. I think references to
scientific documents will not help you any further, maybe a physics
book on electromagnetism or a specialized book on beam forming
antennas may help you.

If your organization is not able to do this in-house, you might hire
an expert.

Best regards,

Wim
PA3DJS

Where does the far field start on a phased array?
 

"Dave (from the UK)"
wrote in message ...
Jerry Martes wrote:

Hi Dave

I'm curious about two things.

1 - Do you intend to actually make and record measurements of the
radiated field, or do you want to determine the minimum distance at which
the measurements can be made?

yes

2 - What prevents the use of a computer modeling program to predict
the pattern?

nothing. I think that will be done. But a theoetical analysis would be
nice if possible.


Jerry


--
Dave (from the UK)

Hi Dave

Would you consider building a scale model of this array to allow a polar
orbiting satellite to be the illuminator?
I get some pretty good radiation pattern plots at VHF, using Patrik Tast's
SignalPlotter program and NOAA satellites.

Jerry

Where does the far field start on a phased array?
 

Jerry Martes wrote:
Hi Dave

Would you consider building a scale model of this array to allow a polar
orbiting satellite to be the illuminator?

It's not really practical to do that for various reasons. The structure
is not a nice conventional antenna that can be scaled up/down size.

I get some pretty good radiation pattern plots at VHF, using Patrik Tast's
SignalPlotter program and NOAA satellites.

Jerry

That's a concept I have not heard of before. But for me to work at VHF,
I'd need to make the structure larger by a factor of 10, so it would be
several hundred metres long.


--
Dave (from the UK)

Please note my email address changes periodically to avoid spam.
It is always of the form:
Hitting reply will work for a few months only - later set it manually.

http://chessdb.sourceforge.net/ - a Free open-source Chess Database

Where does the far field start on a phased array?
 

"Dave (from the UK)"
wrote in message ...
Jerry Martes wrote:
Hi Dave

Would you consider building a scale model of this array to allow a
polar orbiting satellite to be the illuminator?

It's not really practical to do that for various reasons. The structure is
not a nice conventional antenna that can be scaled up/down size.

I get some pretty good radiation pattern plots at VHF, using Patrik
Tast's SignalPlotter program and NOAA satellites.

Jerry

That's a concept I have not heard of before. But for me to work at VHF,
I'd need to make the structure larger by a factor of 10, so it would be
several hundred metres long.


--
Dave (from the UK)

Hi Dave

You are not restricted to VHF. It is possible that there is a polar
orbiting satellite transmitting continuously on the frequency you are
interested in. NOAA satellites transmit continuously at about half the
frequency range you refer to.

Jerry

Where does the far field start on a phased array?
 

Dave (from the UK) wrote:
Jerry Martes wrote:

Hi Dave

I'm curious about two things.

1 - Do you intend to actually make and record measurements of the
radiated field, or do you want to determine the minimum distance at
which the measurements can be made?


yes


An interesting problem. What you're presumably trying to do is
determine how far do I need to be to bound the uncertainty on a
measurement in an arbitrary direction. Or, another way, at what distance
is the collective effects of the phase error for each of the signals
(due to path length differences) smaller than your measurement
uncertainty (so you don't care anymore).

This can be quite challenging if you want to worry about -40dB nulls,
for instance, because a very small phase error can result in a -40dB
null becoming a -30 dB null.

Complicating it a bit is that what you're probably really concerned with
is a statistical problem.. you've got multiple sources, a random
direction of observation, (and practically speaking, some propagation
uncertainties between antenna and observation point).

You might want to look for a paper by Dybdal and Ott: "Coherent RF
Error Statistics", IEEE Trans MTT, v34,n12, Dec 86, pp1413-1420 which
discusses this in some detail, and, as well, provides some nice
approximations that are useful in practical systems.



2 - What prevents the use of a computer modeling program to
predict the pattern?


nothing. I think that will be done. But a theoetical analysis would be
nice if possible.

One can come up with a "bound" for the performance from analytical
means, and a Monte Carlo analysis can give you some statistics.

Jim

Where does the far field start on a phased array?
 

Jerry Martes wrote:
"Dave (from the UK)"
wrote in message ...

Jerry Martes wrote:

Hi Dave

Would you consider building a scale model of this array to allow a
polar orbiting satellite to be the illuminator?

It's not really practical to do that for various reasons. The structure is
not a nice conventional antenna that can be scaled up/down size.


I get some pretty good radiation pattern plots at VHF, using Patrik
Tast's SignalPlotter program and NOAA satellites.

Jerry

That's a concept I have not heard of before. But for me to work at VHF,
I'd need to make the structure larger by a factor of 10, so it would be
several hundred metres long.


--
Dave (from the UK)


Hi Dave

You are not restricted to VHF. It is possible that there is a polar
orbiting satellite transmitting continuously on the frequency you are
interested in. NOAA satellites transmit continuously at about half the
frequency range you refer to.

Jerry



One can also use Ku-band DBS satellites as an illuminator.

HOWEVER, if you're looking for gnat's eyelash precision, watch out for
atmospheric irregularities/inhomogeneity dominating the measurement.
When you start to get to many km as your "distance to adequately
replicate plane wavefront" the propagation uncertainty will probably be
the greatest source of error. You get "bubbles" of air with local
variations on the scale of meters.

A more practical approach (depending on the precision required) is some
form of near field measurement (or a measurement where the different
path lengths from the radiator to the observation point are explicitly
taken into account).

Where does the far field start on a phased array?
 

"Jim Lux" wrote in message
...
Jerry Martes wrote:
"Dave (from the UK)"
wrote in message ...

Jerry Martes wrote:

Hi Dave

Would you consider building a scale model of this array to allow a
polar orbiting satellite to be the illuminator?

It's not really practical to do that for various reasons. The structure
is not a nice conventional antenna that can be scaled up/down size.


I get some pretty good radiation pattern plots at VHF, using Patrik
Tast's SignalPlotter program and NOAA satellites.

Jerry

That's a concept I have not heard of before. But for me to work at VHF,
I'd need to make the structure larger by a factor of 10, so it would be
several hundred metres long.


--
Dave (from the UK)


Hi Dave

You are not restricted to VHF. It is possible that there is a
polar orbiting satellite transmitting continuously on the frequency you
are interested in. NOAA satellites transmit continuously at about
half the frequency range you refer to.

Jerry

One can also use Ku-band DBS satellites as an illuminator.

HOWEVER, if you're looking for gnat's eyelash precision, watch out for
atmospheric irregularities/inhomogeneity dominating the measurement. When
you start to get to many km as your "distance to adequately replicate
plane wavefront" the propagation uncertainty will probably be the greatest
source of error. You get "bubbles" of air with local variations on the
scale of meters.

A more practical approach (depending on the precision required) is some
form of near field measurement (or a measurement where the different path
lengths from the radiator to the observation point are explicitly taken
into account).


Hi Jim

Is the DBS satellite polar orbiting?

Jerry

Where does the far field start on a phased array?
 

Jerry Martes wrote:
"Jim Lux" wrote in message
...

Jerry Martes wrote:

"Dave (from the UK)"
wrote in message ...


Jerry Martes wrote:


Hi Dave

Would you consider building a scale model of this array to allow a
polar orbiting satellite to be the illuminator?

It's not really practical to do that for various reasons. The structure
is not a nice conventional antenna that can be scaled up/down size.



I get some pretty good radiation pattern plots at VHF, using Patrik
Tast's SignalPlotter program and NOAA satellites.

Jerry

That's a concept I have not heard of before. But for me to work at VHF,
I'd need to make the structure larger by a factor of 10, so it would be
several hundred metres long.


--
Dave (from the UK)


Hi Dave

You are not restricted to VHF. It is possible that there is a
polar orbiting satellite transmitting continuously on the frequency you
are interested in. NOAA satellites transmit continuously at about
half the frequency range you refer to.

Jerry

One can also use Ku-band DBS satellites as an illuminator.




Hi Jim

Is the DBS satellite polar orbiting?

Jerry

no.. Clarke orbit (e.g. for Dish Network or DirecTV)

There are some orbiting TV satellites in Molniya orbits for covering
Siberia that might be useful. They radiate in C band, and perhaps Ku?

The GOES polar orbiters radiate at 1.6 GHz, and I'm sure there's
others out there. GPS radiates at several L-band frequencies.

ISS radiates in S and Ku bands and is in LEO, as is the Shuttle when
it's up. I don't know if they would be useful as a source though,
because the antenna is directional and probably pointed at TDRSS or a
ground station.

Where does the far field start on a phased array?
 

Jim Lux wrote:

1 - Do you intend to actually make and record measurements of
the radiated field, or do you want to determine the minimum distance
at which the measurements can be made?



yes



An interesting problem. What you're presumably trying to do is
determine how far do I need to be to bound the uncertainty on a
measurement in an arbitrary direction. Or, another way, at what distance
is the collective effects of the phase error for each of the signals
(due to path length differences) smaller than your measurement
uncertainty (so you don't care anymore).

This can be quite challenging if you want to worry about -40dB nulls,
for instance, because a very small phase error can result in a -40dB
null becoming a -30 dB null.

I'm not really that bothered about the depth of nulls to any great
extent. Since this is radiating an unwanted signal, the concern is
finding where the gain is highest and how high it is.

Complicating it a bit is that what you're probably really concerned with
is a statistical problem.. you've got multiple sources, a random
direction of observation, (and practically speaking, some propagation
uncertainties between antenna and observation point).

You might want to look for a paper by Dybdal and Ott: "Coherent RF
Error Statistics", IEEE Trans MTT, v34,n12, Dec 86, pp1413-1420 which
discusses this in some detail, and, as well, provides some nice
approximations that are useful in practical systems.

In this case, it is certainly a statistical thing. As I said before, the
amplitudes and phases of the radiators are random(ish) and will be
changing all the time.

2 - What prevents the use of a computer modeling program to
predict the pattern?



nothing. I think that will be done. But a theoetical analysis would be
nice if possible.


One can come up with a "bound" for the performance from analytical
means, and a Monte Carlo analysis can give you some statistics.

Jim

I will look at doing some MC analysis.
--
Dave (from the UK)

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It is always of the form:
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Where does the far field start on a phased array?
 

"JIMMIE" wrote in message
ups.com...
On Apr 21, 8:40 pm, "Dave (from the UK)" see-my-
wrote:
JIMMIE wrote:
This sounds very much like an arrangement for a radar antenna that
operates a little above 1Ghz with 32 dipole assemblies space out over
approximately 10 meters. The far field is eatablished at about 2km on
this antenna.

Jimmie

No, it is not that at all, but if you have a reference to the antenna
you describe, I would be interested. This is wider and a higher
frequency.

--
Dave (from the UK)

Please note my email address changes periodically to avoid spam.
It is always of the form:
Hitting reply will work for a few months only - later set it manually.

http://chessdb.sourceforge.net/- a Free open-source Chess Database

The antenna I was refering to is knon as an ATCRBS antenna or an SSR
antenna. I coouldnt find a lot of info right off but there were a lot
of references t oIEEE xplorer. If you belong to that Im sure you can
find some information.

Like chess? send me an email and I will connect you will a friend who
has been known to give a challenging game or two.

Jimmie



The antenna I discred has a gain of about 20 Dbd and the far field doesnt
form up at 2Km like I said. Monitoring devices shuold be at least 2km awy to
make sure they are in the far field so my guess would be about 1/2 that
distance.