It is hard to imagine a culprit radiator that emits fields of 20v/m (or even 30v/m) into a hospital ward or a local clinic from 3m or more away. It is true enough that the airwaves are full of signals from WLAN, wifi, bluetooth, etc. But these are all low power emitters.
The real culprit and hence the real threat is the ubiquitous cell phone in close proximity (placed on top of the medical equipment). Being at such a short distance away, this means by definition the medical equipment is within the near field of the threat emitter. So why test against this threat with a source 3m away?
By the way, when does a cell phone emit the greatest field?
Answer – at switch on. The power is at maximum while the phone identifies all available base stations in its vicinity. The power is then turned down to a level conducive with the distance to the selected base station.
In the next posting we will do the engineering calculations permitting a valid comparison between testing at 3m and 1m.
I think the real question here is valid for al testing. Do we test according a repeatable verfiable and implementable and above all safe an secure way or do we test an EUT as in real life conditions? f.i. all signals can be present at one time blue tooth, GSM, Wifi radio communcation and emitting at various levels and intervals creating different harmonics in time and even combined hamonics.And this all within a proximity of less than 10cm of the EUT. How can you test this kind of sitation?
Reply from EMC Zone (Tom). Dear blog reader, thank you for your interesting comments.
I believe the power levels of bluetooth and wifi are limited by the FCC and equivalent authorities to a few hundred milliwatts, so I don’t see these as a significant threat. Also, the drive in medical equipment today is wireless connectivity, so modern equipment will be immune to signal levels from its integrated antenna. The power level of a cell phone is 2000 milliwatts, significantly higher than WLAN levels. However I see your point on multiple simultaneous threats. My thoughts on this are that though not ideal, testing one threat at a time seems to work. For instance, military aircraft are subjected to high test fields a single frequency at a time, and these aircraft tend not to fall from the sky. An argument I shall be making later on is that the tests should check susceptibility to real cell phone signals, not susceptibility to simple amplitude modulated signals that in no way represent the real threat waveform.