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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) 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 |
#22
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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. |
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