Hi Larry, Thanks a lot for your insightful comments. Yes I'm familiar with the setup and with the antenna 3dB beamwidth. My question aims to help me estimate how much is it worth the investment in a mast with tilting or boresighting capability for our 3m semi-anechoic chamber. Most of the tests we perform are to CISPR standards. In CISPR 16-2-3 antenna directivity and beamwidth are covered, but there is no explicit requirement for antenna alignment with the EUT as in ANSI C63.4.
We use a Schaffner CBL 6141A bilog antenna in the 30M-1G range and a Schwarzbeck BBHA9120D horn antenna in the 1 - 18 GHz range. Most of the EUTs we test are table-top, with maximum height of about 2m. Semi-anechoic 3m setup below 1GHz, fully-anechoic (absorbers added on the floor between antenna and EUT) above 1GHz. As you mentioned, this subject is more relevant in the > 1GHz range, where antenna and source beamwidths are narrower. However, the analysis and numbers I found in standards and literature so far are mainly related to the below 1 GHz frequency range. Antenna directivity: the uncertainty budget contribution is per CISPR 16-4-2 (Annex A) with our setup (3m SAC, Hybrid antenna) as shown below: Vertical polarization, 30 - 1000 MHz range: 1.37dB max improvement in measurement uncertainty with tilting for > 200MHz. Total expanded (2-sigma or 95% confidence level) expanded uncertainty improvement of 1.06 dB with the tilting. Horizontal polarization, 30 - 100 MHz range: 0.29dB max improvement in measurement uncertainty with tilting for > 100MHz, no difference below 100 MHz. Total expanded (2-sigma or 95% confidence level) expanded uncertainty improvement of 0.10 dB with the tilting. 1 - 18 GHz range (both polarizations): 0.87 dB directivity difference contribution to measurement uncertainty. No mention of tilting effects. There is a work by Austrian Research Center a few years back. See abstract at the link below: https://ieeexplore.ieee.org/abstract/document/4652052 "For a test distance of 3 m the measurement uncertainty can be reduced from 5.62 dB to 5.43 dB with antenna tilting. With a bore sight antenna tower a reduction to 5.31 dB is feasible. The bias and uncertainty given by CISPR 16-4-2 are not adequate." This work shows a best-case improvement of just about 0.3 dB of measurement uncertainty in the 30M-1G range with a 3m measurement distance. My feeling so far is that investing in a tilting/boresighting mast is not so worth it, although someone above me already decided to buy it... Thanks again and best regards, Paolo On Wed, Mar 29, 2023 at 7:25 PM Larry K. Stillings < la...@complianceworldwide.com> wrote: > Paolo, > > > > I’m going to go on some assumptions here that you are familiar with the > measurement setup and process in Part 15 and ANSI C63.4:2014. If you look > in section 8.2.4 there is a discussion on antenna beamwidth > > > > You need to keep the antennas within their 3 dB of radiation or reception > also known as its half power beamwidth. This typically does not come into > play from 30 MHz to 1 GHz since most if not all standard EMC antennas are > very wideband receivers in multiple directions. Although you should look at > the datasheets of your measurement antennas to be sure. > > > > Where this does come into play (mainly) is above 1 GHz. You will find when > you study the datasheets of various antennas used above 1 GHz, that their > reception gets narrow. > > > > Let’s take for example the EMCO / ETS Lindgren 3115 antenna and there are > many other similar antennas from ARA, Com-Power, etc. that follow this > design which ironically was only designed for usage from 1 to 12 GHz. Here > is a copy of the 3115’s ½ power beamwidth, you will observe that it has > good E plane reception until about 14 GHz, and then drops of significantly > from 15.5 to 17.5 GHz to 15 degrees or so. > > > > By doing some math you can figure out at a given measurement distance (say > 3 meters) that a 15 degree half power beamwidth will only stay in the > antennas reception area at some given height above the floor. You have a > product on an 80 cm table, and then you are raising and lowering the > antenna from 1 to 4 meters. At some point up the mast you are going to get > out of the receiving beamwidth of the antenna and technically won’t be > measuring any signals from the product at a given distance. > > > > > > This is really what became termed as bore-sighting although I believe that > language has been removed from the standards at this point, as staying > within the cone of reception or ½ power beamwidth is a much more accurate > description of what needs to be done. An antenna mast that supports > bore-sighting will fulfill the requirements as it will “point” the antenna > towards an 80 cm high turntable at a 3 meter distance so it is within its ½ > power beamwidth at all times as it travels up and down the antenna mast. > > > > Larry K. Stillings > Compliance Worldwide, Inc. > *Test Locally, Sell Globally and Launch Your Products Around the World!* > *FCC - Wireless - Telecom - CE Marking - International Approvals - Product > Safety* > 357 Main Street > Sandown, NH 03873 > (603) 887 3903 Fax 887-6445 > complianceworldwide.com > > *Follow us on social media* > > linkedin.com/company/compliance-worldwide-inc > twitter.com/complianceww > > Privileged/Confidential Information may be contained in this message. If > you are not the addressee indicated in this message (or responsible for > delivery of the message to such person), you may not copy or deliver this > message to anyone. In such case, you should destroy this message and kindly > notify the sender by reply email. Please advise immediately if you or your > employer do not consent to Internet email for messages of this kind. > Opinions, conclusions and other information in this message that do not > relate to the official business of my firm shall be understood as neither > given nor endorsed by it. > > > > *From:* Paolo Roncone > *Sent:* Wednesday, March 29, 2023 1:06 PM > *To:* EMC-PSTC@LISTSERV.IEEE.ORG > *Subject:* [PSES] Antenna tilting or boresigjting for radiated emissions > > > > Hi all, > > > > Is there an FCC / ANSI requirement, or maybe just recommendation, for > antenna tilting or boresighting wrt radiated emissions in a semi-anechoic > chamber ? > > > > If yes, can you please get me the reference section in FCC Part15 or ANSI > std or somewhere else? > > > > > > Thanks a lot in advance! > > Paolo > > - > ---------------------------------------------------------------- > > This message is from the IEEE Product Safety Engineering Society emc-pstc > discussion list. 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