I have been dabbling in antennas (mainly vhf/uhf) on an amateur basis for many years. Here are a few things that I use for guidance: Formulas (generally) only work for resonant antennas. Antennas do not multiply or generate power, they only focus it. Antennas are influenced by extraneous items. Do not underestimate the effect of resonant objects. Don't expect to understand everything you observe. The 48 element collinear array that I built for ATV on 439mhz was theoretical 48 dbd but best efforts to measure it was more like 20 dbd! Everything affects everything to some extent! John Ferrell 6241 Phillippi Rd Julian NC 27283 Phone: (336)685-9606 Dixie Competition Products NSRCA 479 AMA 4190 W8CCW "My Competition is Not My Enemy" ----- Original Message ----- From: "Sean Breheny" To: Sent: Tuesday, January 14, 2003 8:36 PM Subject: [EE]: Antenna question > Hi all, > > I've got a somewhat advanced antenna theory question and I'd appreciate > it if anyone here can help me out with it (the piclist rarely disappoints!). > > If you have an antenna array, it is a relatively straight-forward matter > to determine the pattern (pattern multiplication, the array factor times > element factor, for identical elements). However, to determine the actual > gain, you need to know the total power radiated. Often people assume that > this would just be the sum of the radiated powers of all the elements. > Technically, it has to be (by conservation of energy) HOWEVER, the E > field contribution from each element DOES NOT have to be the same as it > was in the individual, isolated case. > > It is easy to show this (take two isotropic elements phased 180 deg apart > and draw a graph of the total output power as you move them closer and > closer together using the assumption that they produce the same E field > as when they ran individually - you will get a curve that varies between > 0 watts and four times the power of a single element - you can make it > violate conservation of energy in either direction, and it doesn't only > happen when they are really close, it happens even several wavelengths > away, although if you get many wavelengths away it settles down to less > and less variation, approaching a constant line at twice the power of an > individual element, as expected). > > In a crazy way, this makes sense, because we usually estimate the E field > of a single antenna, theoretically, by integrating the poynting vector > over a sphere around the antenna. There is no reason why we should assume > that we can use this same value if there are other antennas around, since > their contribution would have to be taken into account in this > integration, too. > > After the longwinded intro, my question is this: Is there a well-known > way of estimating how much the far-field E field of an antenna must > change when other elements are placed in its vicinity? This would seem to > be a very fundamental quantity that you would need to know because it > affects the estimated gain of an antenna array. > > Textbooks seem to ignore this and make the assumption that the elements > do not interact, but they MUST interact or else energy will not be > conserved. You may get the correct pattern shape under that assumption, > but not the correct magnitude. > > Thanks for any help you can provide in understanding this, > > Sean > > -- > http://www.piclist.com hint: The PICList is archived three different > ways. See http://www.piclist.com/#archives for details. -- http://www.piclist.com#nomail Going offline? Don't AutoReply us! email listserv@mitvma.mit.edu with SET PICList DIGEST in the body