HI Grace I would agree that the use of an actual EUT should be avoided - actually I would say never used. They are simply too unreliable - sneeze and they change. They are far too susceptible to temperature variations, humidity variations, human variations on and on.
Also, are you doing a site comparison or are you comparing the whole system to another site and system. Too often I think we tend to compare a system to another system instead of comparing the actual test site itself. Remember, when doing NSA we are not trying to see what the antenna and other measurement instrumentation does, we are trying to see what the site itself is doing. This is one reason the use of tuned and matched dipoles for transmit and receive is much better during NSA measurements than bicons, bilogs or other varying types of broadband antennae. Consequently, if you want to compare the site itself, then the same equipment must be used on both (same signal source, same antennae, same cables etc). That way you remove the anomalies caused by instrumentation and you are only comparing the actual site itself. Also, if NSA was done correctly you should be within a max of 8dB from any other site also properly done (an accurate NSA is to be within +/-4dB. If your site is +4dB at one frequency and another site is at -4dB at the same frequency, then to difference should only be 8dB). Wishful thinking I know, but this is what should be in two properly done NSA's on two adequate sites when the sites themselves are compared. The 20dB or more sometimes seen in some round robin comparisons etc, is that they are not just comparing the site, they are also comparing how the test equipment figures in. This reintroduces all of the anomalies, errors and confusion factors caused by specific instrumentation. This is a good idea and does have merit. Consequently, before the comparison is done, you really need to determine exactly which you are trying to do - compare only the site, or compare the site with all associated measurement equipment. If the idea is to just do a quick comparison on a daily basis, then a comb generator and a broadband antenna might do the trick. However, while being a quick wide band approach, you may end up with antenna and other equipment problems that may become an issue if you are not careful. If you are wanting an as close as you can approach of how two sites compare, then a mini NSA approach would be good. Here you use a single frequency source set to one frequency at a time using a tuned dipole for each reading. You do not need to do a whole NSA, just a select number of frequencies and the same frequencies each time. Obviously this takes longer that the comb generator quick test, but is more reliable for accurate and usable information. Thanks Dennis Ward American TCB Certification Resource for the Wireless Industry www.atcb.com 703-847-4700 fax 703-847-6888 direct - 703-880-4841 From: [email protected] [mailto:[email protected]] On Behalf Of [email protected] Sent: Friday, November 20, 2009 5:30 AM To: [email protected] Subject: Fw: 3-meter Chamber Site Comparison A comb generator would be OK. I would not use an EUT. Bob Heller 3M EMC Laboratory, 76-1-01 St. Paul, MN 55107-1208 Tel: 651- 778-6336 Fax: 651-778-6252 ============================================================== From: Grace Lin <[email protected]> To: [email protected] Date: 11/20/2009 06:54 AM Subject: 3-meter Chamber Site Comparison Sent by: <[email protected]> Dear Members, I am asked to perform site comparison and would like to hear your comments. Should I purchase a signal source? If yes, do you have any reference? I do have a COM-POWER comb generator and don't know if it is good enough for the purpose. Should I use an EUT for site comparison? Thank you. Best regards, Grace Lin - This message is from the IEEE Product Safety Engineering Society emc-pstc discussion list. To post a message to the list, send your e-mail to < [email protected]> All emc-pstc postings are archived and searchable on the web at http://www.ieeecommunities.org/emc-pstc Graphics (in well-used formats), large files, etc. can be posted to that URL. 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Website: http://www.ieee-pses.org/ 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]> - This message is from the IEEE Product Safety Engineering Society emc-pstc discussion list. To post a message to the list, send your e-mail to <[email protected]> All emc-pstc postings are archived and searchable on the web at: http://www.ieeecommunities.org/emc-pstc Graphics (in well-used formats), large files, etc. can be posted to that URL. Website: http://www.ieee-pses.org/ 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]>
