Tim, I have some experience with the problem you are trying to solve. I specified a site that we used for calibrating EMC antennas. As I understand it, you can almost achieve the NSA to +/- 4dB without any metal on the ground at all, so don't worry too much about putting down metal that is the size of the CISPR ellipse. There is information in CISPR 16 or 22 about a "lollipop" shaped area that extends 1m outside the turntable, and 1m beyond the ends of the antenna.
I made a series of measurements before we built our site. The three worrying items were 1. Chain link fence. 2. Lampposts. 3. Row of trees along one side of the site. The way to do diagnostic work to evaluate these items is to make 10m site attenuation measurements with antennas at a single height, either polarization. You will see problems as a ripple on the NSA curve. The periodicity of the ripple corresponds to the path length difference between the reflected path and direct path. It was quite easy to make site attenuation measurements, and then get a tape measure out to verify which feature causes which problem. When worrying about reflections, you need to keep in mind the radiation pattern of the receiving and transmitting antenna. In vertical polarization, the antennas are omnidirectional, and reflecting objects all around the site will impact on the measurement. You will not be able to get rid of the chain link fence problem by clever orientation of the site.For horizontal polarization, radiation from the ends of the antenna (the sides of the site) is not relevant. Log-periodic antennas are more directional, so the reflecting objects around the site don't matter so much. The results of our investigations were 1. The effects of the chain link fence were barely measureable. 2. I couldn't see any reflections from the metal lamppost. I even did measurements with absorber fastened to it and could see no difference before and after. 3. The line of trees caused big problems. I developed some simple computer modelling to see how small the trees needed to be to get rid of the problems. It turned out that if we removed the bottom 2m of the trees then everything would be fine! It happens that the trees were on a fairly sharp slope (ex railway embankment), and I suspect that it was the earth and not the trees that caused the problem. This bank is 25m from the measurement point, but still causes ripples on the NSA. We worked out that about $30k of absorber could have been used to get rid of this reflection. I would highly recommend some diagnostic measurements. Don't get too hung up on height scanning but compare measured and theoretical NSA for fixed antenna heights. Try to get good agreement in the peaks and don't worry too much about the nulls. It is possible, but messy, to do the NSA calculations in Excel, because it can do complex numbers. Good luck! Dr Luke Turnbull Principal EMC Engineer TRW Conekt Stratford Road Solihull B90 4GW Tel: +44 (0)121.627.3966 Fax: +44 (0)121.627.4353 email: [email protected] web: www.trw.com/conekt/ >>> <[email protected]> 06/19/03 01:13am >>> Hello, I'm building an open area test site, and have some questions about the ellipse and ground plane. I'm following ANSI C63.7:1992. I have used several OATS and done site attenuation before, but this is the first time building one from the ground up. I'm building a 10 Meter OATS with a 14 foot turntable. Figure 1 in ANSI C63.7 says that the minor diameter of the ellipse shall be 17.3 Meters and the major diameter shall be 20 meters for a 10 meter site. For the ground plane, Table 1 in ANSI C63.7 says for a 10 meter site, the width shall be 12 meters and the length shall be 15.3 meters. The corners of the ground plane will be outside the ellipse. Is this ok, or should the ground plane be smaller, to fit inside the ellipse? What would be the minimum ground plane size I could use for this 10 meter site with a 14 foot turntable? Also, there is a chain-link fence running parallel with the proposed site, but the fence is just outside the ellipse. Would there be any problems with site attenuation? I can alter the ellipse so the site is not exactly parallel with the fence. (Antenna would be somewhat diagonal to the fence) Would that help avoid any problems? Thanks in advance for any input you may have. Tim Pierce EMC Engineer This message is from the IEEE EMC Society Product Safety Technical Committee emc-pstc discussion list. Visit our web site at: http://www.ewh.ieee.org/soc/emcs/pstc/ To cancel your subscription, send mail to: [email protected] with the single line: unsubscribe emc-pstc For help, send mail to the list administrators: Ron Pickard: [email protected] Dave Heald: [email protected] For policy questions, send mail to: Richard Nute: [email protected] Jim Bacher: [email protected] Archive is being moved, we will announce when it is back on-line. All emc-pstc postings are archived and searchable on the web at: http://www.ieeecommunities.org/emc-pstc
