Construction Considerations in Building a Permanent All-Weather Open Area Test Site

Abstract

The need for making radiated emission not only for final regulatory compliance testing but during product development has significantly increased the number of test sites being planned or under construction. Due to the ongoing need to test, the EMC team requires a site which can be operated during the entire year. While there is some information in the literature to help in designing a permanent all-weather enclosed open area test site, more is needed. (See references 1,2,3.) This paper fills much of this void by presenting not only technical guidelines but also construction considerations for building these very special sites. Especial emphasis is on such critical areas as the selection and construction of the allweather cover and the metallic ground plane. RF Technical Considerations The primary technical considerations for any open area test site is that the only reflecting surface is the reference ground used for the compliance measurement for radiated emissions. Generally, this requirement is met by: 1. Establishing a reflection free area around the site, and 2. Providing a conductive ground plane. The reflection free area can be bounded by many objects such as trees, buildings, fences, and the like. Within the obstruction free area, special structures can be built which do not scatter or reflect RF energy. An obvious special structure is an all weather structure covering the test area. These structures have been successfully made out of a broad range of building material that is transparent to RF energy up to 1000 MHz which is the highest frequency measured for FCC Part 15J compliance. Based on experience gained in constructing all weather test sites, we now review specific technical guidelines for the all weather structure above the ground plane, the ground plane itself, and the instrumentation support areas below the ground plane, Guidelines for Structure Above Ground The structure enclosing the test site must contain a minimum amount of reflecting material. Metal larger than a tenth of a wave length at the highest frequency of measurement is considered a good reflector. This length is 3 cm or about 1.2 inches at 1000 MHz. This generally rules out construction practices using significant quantities of nails, bolts, foilbacked insulation, steel beams, etc. Fortunately, the RF reflectivity or transparency of these materials and construction techniques can be measured. A simple experiment is to transmit a signal to a receiving antenna and see if there is any effect by placing the building material directly in the path of the transmitted signal. Smith in reference 4 describes such an experiment. Several building materials have been .investigated for the RF transparency. (References 4-6.) As one would expect, material comprised of metal exhibits too much attenuation or scattering to be selected for an open area test site. In fact, even normal residential building construction with wood studs 16 inches on center exhibits undesirable scattering at frequencies above about 100 MHz. (Reference 4.) Basic building materials which are good candidates for use in a RF reflection free rigid all weather structure are: • Wood