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

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

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

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

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

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

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

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

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

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