It appears to me that everyone is making this way more complex than necessary. Certainly the computations can become complex for some irregular geometry, but the concept is simplicity itself.
As I said in an earlier posting, the far field is equivalent to a plane wave, which implies a point source radiator. Practically that means that if the point of observation or measurement is to be in the far field of the source, that means that the rays traced from the point of observation to any portion of the source radiating element must be electrically equidistant, that is, have negligible phase front differences. If one chooses lambda/116 as a criterion for "negligible" this results in the familiar 2D^2/lambda criterion. To make a blanket statement that a certain number of wavelengths suffices violates the Einstein rule: "Everything should be made as simple as possible, but not simpler." Consider the difference between a Ku band dish antenna one meter in diameter and an electrically short dipole. The dish operates at 15 GHz. 2D^2/lambda yields 100 meters for the far field, which is 5000 wavelengths. By comparison, a 1 m dipole radiating at 30 MHz achieves the far field at 0.2 m, or about 0.02 wavelengths. This is why you use a 1.37 meter tip-to-tip biconical for 1 meter and 3 meter measurements at 30 MHz as opposed to a 5 meter long tuned dipole. And the statement that antennas used for EMI measurements are calibrated in a 377 Ohm field is inaccurate, or misleading. That may be the case some of the time, but is a consequence of the spacing involved, not a driver for the spacing. Antenna factors are determined for the distance(s) at which antennas will be used. If an antenna were only to be used in the far field, only one antenna factor (gain) would be necessary. But since we are often not in the far field, antenna factors for 3, 10, and 30 meters are often necessary. In my experience, 10 and 30 m antenna factors are the same. but there are variations (dependent on frequency and antenna physical aperture) between 3 and 10 meter antenna factors. And for those antennas which are also used at 1 meter, the factors change dramatically from those measured at 3 meters, again dependent on antenna type and frequency. The critical factor in the far field calibration of an antenna is that the gradient of the field across its physical aperture be zero; the field intensity is constant over the antenna aperture. At 1, 3, or 10 meters this is often not the case and it is noted that antenna factors are therefore determined using identical antennas for transmit and receive, else there is no standardization, because the field impinging upon the physical aperture of the receive antenna is not constant, and is in fact dependent upon the type of antenna used to create the field. As opposed to a far field calibration of a receive antenna, where it is entirely immaterial what kind of antenna is used to establish the field. - This message is from the IEEE Product Safety Engineering Society emc-pstc discussion list. Website: http://www.ieee-pses.org/ To post a message to the list, send your e-mail to [email protected] Instructions: http://listserv.ieee.org/request/user-guide.html List rules: http://www.ieee-pses.org/listrules.html For help, send mail to the list administrators: Scott Douglas [email protected] Mike Cantwell [email protected] For policy questions, send mail to: Jim Bacher: [email protected] David Heald: [email protected] All emc-pstc postings are archived and searchable on the web at: http://www.ieeecommunities.org/emc-pstc ______________________________________________________________________ This email has been scanned by the MessageLabs Email Security System. For more information please visit http://www.messagelabs.com/email ______________________________________________________________________
