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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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)

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It is always of the form:
Hitting reply will work for a few months only - later set it manually.

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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.