Hello Joe, Consider the following -- in the far field (3 or 10 meters), a plane wave is monitored. In the near field, using either commercial or lab built near-field probes, either E field or H field emissions will be monitored separately. The E, H components will be isolated. The emission may be identified with a probe, but the effective radiator (culprit antenna) for that emission might be missed.
The above is an answer to the question. If you would like a real life experience describing the difference, read the example that follows. On a recent product we had a band of frequencies of non-compliant emissions that were somewhat polarity sensitive. We observed a particular signature of the emission (modulation on a pulse) at 3 meters using a bilog antenna. Using a direct contact E field probe, the pulse frequency showed up at high levels around the processor and DSP chip, but not with the signature. We were able to find a trace of the corresponding polarity that was suspect and had a similar signature, and at a lower level than we found around the processor and DSP chip. Looking at the schematic, we identified a reasonable fix. But that only helped part of the profile. We then sniffed with a non-contact magnetic loop probe and found another viable culprit. The fix implemented brought the product into compliance with reasonable margin. Neither fix by itself brought the product into compliance. Both were necessary, required a minimum amount of components and contributed to rationale "source suppression". We did not introduce "balloon squeezing", i.e., beat down an emission at one frequency and see it pop up at another frequency. This kind of isolation is more effective than monitoring the far field emission, hypothesizing the culprit antenna while analyzing the schematic. We have done it both ways. The near field approach takes a little more time to set up but saves time in the long run. Or maybe we were just lucky! Best regards, Don > ---------- > From: > [email protected][SMTP:[email protected]] > Reply To: [email protected] > Sent: Thursday, September 14, 2000 1:07 PM > To: [email protected] > Subject: Near Field Versus Far Field > > > > > I am having a difficult time answering the following question for a > non-technical person. Hopefully, someone can put the answer into a > language > that a non-technical person can understand. > > We have a 400 MHz clock and are failing radiated emissions at 10 meters by > 10 dB > at 400 MHz. We bring the product back to our lab and start making > modifications > on the clock circuit and taking measurements with a near field probe. > With > these modifications and measuring with a near field probe, we realize a 10 > dB > reduction in emissions at 400 MHz. Why would we not see the same > reduction when > taking the product back to a 10 meter site? > > Your help is appreciated. > > Regards > > Joe Martin > [email protected] > > > > ------------------------------------------- > This message is from the IEEE EMC Society Product Safety > Technical Committee emc-pstc discussion list. > > To cancel your subscription, send mail to: > [email protected] > with the single line: > unsubscribe emc-pstc > > For help, send mail to the list administrators: > Jim Bacher: [email protected] > Michael Garretson: [email protected] > > For policy questions, send mail to: > Richard Nute: [email protected] > > ------------------------------------------- This message is from the IEEE EMC Society Product Safety Technical Committee emc-pstc discussion list. To cancel your subscription, send mail to: [email protected] with the single line: unsubscribe emc-pstc For help, send mail to the list administrators: Jim Bacher: [email protected] Michael Garretson: [email protected] For policy questions, send mail to: Richard Nute: [email protected]
