I have been informed that my surmise was correct, that at least the FCC limit is flat above 1 GHz. Specifically the number was given as 54 dBuV/m out to 40 GHz. Now I don't think control is yet being exerted to 40 GHz, but consider the ramifications of what I said below. At 40 GHz, the effective height of a tuned dipole is 58.5 dB below 1 meter. With a 54 dBuV/m limit, that corresponds to -4.5 dBuV at the antenna port for a signal at the limit. If the victim has an IF bandwidth of 1 MHz, that level is just 2.5 dB above the noise floor of a perfect receiver with a zero dB noise figure.
To play devil's advocate for a moment, one could say that at 40 GHz a much higher gain antenna could be implemented than a tuned dipole or quarter-wave stub, and that therefore the antenna port signal would be higher than I calculated. But in order to justify a flat limit above 1 GHz, it wouldn't be enough to assume some value of fixed gain, the gain would have to increase with increasing frequency to offset the effective aperture dependence on wavelength, so that the horn used by a 40 GHz receiver would be the same physical size as the horn used at 1 GHz. That is not a realistic assumption at all. It seems to me, at least, that this limit requires closer examination. From: Ken Javor <[email protected]> List-Post: [email protected] Date: Thu, 22 Sep 2005 15:12:56 -0500 To: <[email protected]> Subject: Re: Above 1 GHz EMI testing with horn antenna The following doesn't answer the question, but it bears on the issue. I think the issue of transmission line losses would largely go away if the limit were more realistically placed. It is my understanding that limits above 1 GHz are set at the 1 GHz level. Even if they aren't, I believe they are flat as a function of frequency. If these assertions are incorrect and limit increase with increasing frequency, then the rest of this message may be consigned to the trash receptacle without further ado. Radio receivers protected by FCC/CISPR radiated limits are in all cases broadcast receivers, in the sense that they utilize relatively low gain antennas in order to facilitate signal reception from all directions. You can always find exceptions to broad statements like this, such as we used to have a roof-mounted Yagi-type antenna pointed at a particular distant television broadcaster, but in general, and specifically above 1 GHz, I believe the victims protected by FCC/CISPR limits are going to have no more antenna directivity than that of a quarter-wave stub. This means that for a given receiver sensitivity, the limit should increase monotonically with increasing frequency for a set amount of rfi coupled to the receiver antenna port. This is of course in stark contrast to what I premised the limits to be. If a horn or other fixed aperture type antenna is used to make the FCC/CISPR measurement, its gain increases with increasing frequency, meaning its antenna factor should be relatively constant. This doesn't work over the entire range of a broadband device such as a double-ridge guide horn, but even the DRG horn antenna factor is flatter than 6 dB per octave. But the point is that with a limit which increases linearly with increasing frequency, and a test antenna with much higher gain than that of the victim, the signal out of the antenna should be well above the analyzer or receiver noise floor. For a given coax, losses increase exponentially with increasing frequency, so it might not be possible to use the same RG-214 above 1 GHz that is used below, but there are any number of (very expensive) low loss coax cables on the market. Alternatively, a small battery-powered line driving pre-amp at the antenna output can be used to overcome line losses. Test expenses, as well as the cost of designing to EMI requirements, are minimized by an optimized radiated emissions limit. From: [email protected] List-Post: [email protected] Date: Thu, 22 Sep 2005 14:48:55 EDT To: [email protected] Subject: Above 1 GHz EMI testing with horn antenna Hello, A discussion has come up about the need to raise the horn antenna up to 4 meters when doing an EMI test above 1 GHz. ANSI states that the antenna height should be adjusted between 1-4 meters to maximize the emissions. Some labs are only doing this test at 1 meter or maybe 2 meters high, (probably to reduce cable loss issues). Normally, the emissions are worst case below 2 meters, but how can one be absolutely sure, without checking? What is the legal requirement for the antenna height? These questions apply to unintentional radiators, where measurement above 1 GHz is required. Regards, Tim Pierce - 2005 IEEE Symposium on Product Safety Engineering 3-4 October Schaumburg, IL http://www.ieee-pses.org/symposium ---------------------------------------------------------------- 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/listserv/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: Richard Nute: [email protected] Jim Bacher: [email protected] All emc-pstc postings are archived and searchable on the web at: http://www.ieeecommunities.org/emc-pstc - 2005 IEEE Symposium on Product Safety Engineering 3-4 October Schaumburg, IL http://www.ieee-pses.org/symposium ---------------------------------------------------------------- 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/listserv/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: Richard Nute: [email protected] Jim Bacher: [email protected] All emc-pstc postings are archived and searchable on the web at: http://www.ieeecommunities.org/emc-pstc
