>-----Original Message----- >From: Eric Penne [ mailto:epe...@olug.org] >Sent: Wednesday, August 06, 2003 8:44 PM >To: emc-p...@majordomo.ieee.org >Subject: Apples and ? comparison of dBuV/m > > > >I've been perusing some FCC test reports and had a question that I >couldn't find an answer to. I'm trying to brush up on my >skills so I can >get a job in the EMC industry again. Funny thing my former >employer told >me. I had my picture in the Fall 2002 issue of the IEEE EMC >newletter 4 >times at the Minneapolis EMC Symposium. Unfortunately I was laid off 1 >week after the show. :) > >Anyway, on a couple of the reports, measurements were taken to >verify that >a transmitter didn't exceed the limits of a band next to it. The EUT >including antenna was tested in a chamber and the maximum >value in average >and peak testing was found in dBuV/m. The unit is operating >above 1GHz. >To verify the band edge measurements the transmitter was directly >connected to a spectrum analyzer and power measurements were >taken to find >the overall peak and the peak in the restricted band. The >difference of >the direct connection Spec Analyzer test was subtracted from >the maximum >average value in the radiated emissions test to say that the >EUT passed in >the restricted band. > >What I couldn't understand was how an average value from the >RE scan with >a Res BW of 1MHz could subtract a peak value from the power >scan that has >a Res BW of 100kHz? > >Is an average value with 1MHz RBW equal to a Peak value with >100kHz RBW? > >Thanks, >Eric
Eric: I'm more concerned that the conducted measurements do not take into account the efficiency of the antenna at out-of-band frequencies. It is one thing to measure the RF power at the fundamental and other frequencies in a constant 50-ohm system. It is quite another to measure the field strength created by that RF power when radiated by a real antenna. Real antennas usually exhibit wide variations in efficiency and radiation pattern when you depart from the design frequency. While measuring in a controlled 50-ohm system, with various detectors and bandwidths may be interesting, the requirement you defined was suppression of RADIATION at an adjacent frequency. Unless you exhaustively define the characteristics of the antenna (and apply that as a correction factor to the 50-ohm power information), the only right way is to really measure the fields. Regards, Ed Ed Price ed.pr...@cubic.com WB6WSN NARTE Certified EMC Engineer & Technician Electromagnetic Compatibility Lab Cubic Defense Systems San Diego, CA USA 858-505-2780 (Voice) 858-505-1583 (Fax) Military & Avionics EMC Is Our Specialty
