Tim,
There are many different ways to build a ground plane. If it is close to grade, ground rods don’t have much impact (at least not in my experience). They primarily help with electrical safety. I’ve built 3 OATS facilities in my career. Size matters. Drainage helps. #1 was a 30 meter OATS near Hollister, CA. Worked for Tandem Computers at the time. CKC Labs now owns the site. 40 x 140 foot ground plane. Galvanized after weaving wire mesh with continuous seam soldering over a poured concrete base. Perfectly level. Won’t do that again – doesn’t drain when it rains. The continuous seam soldering was to keep maintenance to a minimum, not for performance. Took the sub-contractor 22 days to lay the wire and do the soldering. He bid it assuming 2 days. Lost his shirt on that contract. That and I specified 18 gauge wire mesh and he bid based on 22 gauge. I didn’t budge. The side rode the 1989 Loma Prieta earthquake without any problems. Control building was a double wide modular building on a poured concrete foundation with the frame welded to steel inserts in the concrete. It didn’t move. The building over the turntable was made with fiberglass beams for a frame, covered with plywood, 6 inches of Styrofoam insulation and then a fiberglass coating. Plexiglass windows on either side of the FRP door on the centerline. Turns out that this bent the wave leaving the EUT, resulting in having to place the mast a bit off center to meet NSA. Three different materials, depending on which part of the wall you looked at, each with a different propagation velocity. A nice dielectric lens. #2 was a 10 meter OATS at Intel in Hillsboro, OR. 57 x 67 foot ground plane. This is large enough to keep the entire ellipse in the standard on the ground plane. Wire mesh for most of the ground plane with sheet metal under the dome covering the turntable (for the entire area needed for a 3 meter OATS – thinking about using it well above 1 GHz at that distance. Ground plane on a pressure treated wood deck to make it level over a sloping site. Control building just off a back corner of the ground plane. Ground plane about 30 inches above grade down one site, maybe 4 inches above grade on the other. Trees and other items in the general area cause this site to “just” meet NSA at 10 meters. #3 is a 10 meter OATS at Intel in DuPont, WA. Same ground plane dimensions and construction, but on a level site. Much larger clear area (at least double the dimensions in the standards). Ground plane is about 4 to 6 inches above grade, all the way around. Meets NSA with plenty of margin. Lessons learned: 1. Make sure the ground plane can drain water when it rains. 2. Make sure the clear area is much larger than called out in the standards. You can’t make it too large, but you sure can make it too small. 3. Continuous seam soldering is great for minimizing maintenance, but is overkill and costs lots of money. 4. Watch out for the cover over the turntable. It can really give you some screwy results if you’re not careful. Have fun. Ghery S. Pettit Intel Corporation From: [email protected] [mailto:[email protected]] Sent: Thursday, June 19, 2003 1:00 PM To: [email protected] Subject: Re: Question about OATS Hello everyone, Thanks for your input. I can go somewhat diagonal to the fence with the test site. With this angle, the EUT end will be closer to the fence than the antenna. I can also change the chain link fence to a wooden fence, but this fence is built on top of a 2 foot concrete retaining wall. I would like the site to be parallel to the fence for space saving reasons, although I know its not the best for an OATS. What do you think would be best for this site? I plan to use wire mesh over a concrete slab, with about 8 grounding rods 8 feet deep, evenly spaced. (4 on each side). Are you saying to also coil the wire mesh in an 8 inch coil and bury it into the ground on all the sides? If so, how deep do you bury it? I was also thinking about using aluminum plates, rather than mesh, because sometimes the mesh tears. I'm concerned that moisture would build up under the ground plane, changing the characteristics of the site. I appreciate any help you may have. Tim Pierce
