Conformity - November 2008 - (Page 18) and internationally. This is because free-space AFs provide a good average value when these antennas are scanned over 1-4 m above a conducting ground plane. ANSI C63.5-1998 was published to harmonize with the CISPR standards. In the 1998 release, antennas can only be calibrated in the near-freespace setup (10 m separation/horizontal polarization/transmit antenna at 2 m height). It is called near-free-space because the SSM assumptions give rise to very small errors (less than 0.5 dB). This, however, becomes a big problem for users who adopted “geometry-specific antenna factors,” as no other geometries are allowed for calibration. An ad hoc group was quickly formed in 1998 to address the situation. The group later became a working group (WG 1-15.6) in Subcommittee 1 which reports to ASC C63®. It was noted that the “geometryspecific AF” is nothing more than a site-to-site comparison and, to make the method consistent, a “golden site” would be needed to complete the theory. There was a great deal of reluctance to the “golden site” concept because of practical concerns. Incidentally, SSM/NSA was embraced originally to avoid a site-to-site comparison method. A different approach was now preferred. As it turns out, Albert Smith had realized some limitations in his method. He addressed them by providing mutual coupling correction factors for dipole antennas in his seminal SSM/NSA papers. In ANSI C63.5-1988, correction factors for Roberts’ dipole were already included. Due to the precedent, and realizing the fact that broadband antennas are most widely used for site validation measurements, the working group adopted numerical correction factors for broadband antennas. The NEC2 code [7] was used for the numerical simulation, as its accuracy had been verified extensively in many scientific studies. These correction factors are referred to as geometryspecific correction factors (GSCF) which is not an antenna factor correction, but a correction to NSA. Site performance issues typically happen in the frequency range of a biconical antenna. EMC biconical antennas from various manufacturers have almost identical physical appearance and dimensions because they followed the original design in MIL-STD-461 (published in 1968) and restated in ANSI C63.5. This is convenient because unified correction factors can be provided. In the standards, some limits are provided on how much an antenna can deviate from these dimensions. Virtually all available biconical antennas meet the requirements. Additionally, GSCF depends on the impedance of the baluns. ANSI C63.5 provides corrections for both 50 ohm (1:1 impedance transformation ratio) and 200 ohm (4:1 ratio) baluns. These results were verified against a wide range of commercially available antennas [8]. A benefit of the GSCF approach versus the site-to-site comparison method is that only free-space antenna factors are needed for site validation measurements. Geometryspecific influences are all included in the GSCF. With a site-to-site comparison method, one would have to first find a “golden site,” and then perform reference measurements in all geometries (currently a total of eight geometries for 3 m and 10 m separations). With the GSCF method, antenna calibration is performed only in a single geometry that is the near-free-space setup (or any alternative method which produces free-space AF). This greatly cuts down the time and hence the cost associated with site validation measurements, as well as reducing the chance for and sources of additional measurement uncertainties. An often-cited criticism of the SSM, besides the listed shortcomings from the assumptions, is that, for calibrations using three antennas, one antenna is at a fixed height in one measurement, while scanned in height in another measurement. Since antenna factor is height dependent, we are trying to solve four unknowns with only three equations. This problem, along with the ones caused by other imperfect assumptions, is solved by the introduction of GSCF. Track Mount Shielding Distortion Free Installation ● Beryllium copper finger stock ● Gap range - .05" to .25" ● Security of mechanical assembly ● Ideal for bi-directional applications ● Wide range of plating options ● Economical and durable solution ● Installs with tape, plastic rivets or spot welding ● Single finger up to 24" ● Low force compression ● Immediate delivery Free Samples at www.tech-etch.com/shield ISO 9001:2000 REGISTERED TECH-ETCH, INC., 45 Aldrin Road, Plymouth, MA 02360 TEL 508-747-0300 • FAX 508-746-9639 Edmax is a theoretical parameter proposed by Smith for the purpose of calculating theoretical NSA above a conducting ground plane. It represents the maximum E field (dB µV/m) at the receive antenna position during height scanning for a half-wave dipole with 1 pW of radiated power. 1 18 Conformity november 2008 http://www.tech-etch.com/shield http://www.tech-etch.com/shield
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