Conformity Magazine - December 2007 - (Page 48) both emissions and immunity testing, the end plates should be removable for emissions testing configurations. Conical Log-Spiral Antenna The distinctive difference between the conical log spiral antenna and most other antennas is that the electric field is circularly polarized. The circularly polarized field eliminates the need for horizontal and vertical measurements separately. It is mostly used for MIL standard measurements. The frequency range is typically from 100 MHz to 1000 MHz. Note that, when measuring the gain of a circularly polarized antenna with a linearly polarized one, the gain appears 3 dB lower because of the polarization mismatch. Broadband Ridged Waveguide Horn These versatile and broadband ridged waveguide horns can cover 200 MHz to 40 GHz. The horns for low frequencies can be physically large. For example, the horn that covers 200 MHz to 2 GHz is approximately 37x39x29 inches. Since gain for these antennas are generally larger (around 10 dBi), the beamwidth is narrower. One should make sure that the beamwidth meets the measurement requirement. Standard Gain Horn Antenna These are very similar to dipoles or calculable bicons in that the gains can be theoretically computed. Compared to the ridged waveguide horn, they are narrow band. Many standard gain horns are needed to cover a broad frequency range. Antenna Calibrations There are several methods to calibrate an antenna. Much confusion exists for free-space antenna factors and antenna factors obtained in a specific setup (sometimes called geometry-specific antenna factors). Understanding how AFs are arrived is important in choosing the right antennas and their associated AFs. Standard Site Method The standard site method is specified in ANSI C63.5. This method is best suited for “dipole-like” antennas, such as dipole, bicon, log, and hybrid antennas. The site attenuation, or the insertion losses between the transmit and receive antennas, are measured. The basic setup for a standard site method includes a large, flat, and unobstructed conducting ground plane (made of metal). One antenna is set to be at a fixed height, while the other one is scanned from 1 to 4 m in height. The maximum response between the two antennas is recorded. Typically, three antennas are needed to perform such a calibration, and they are measured in three pairings. Calculations are then performed to derive the antenna factors. Although a ground plane is used, the aim of the C63.5 standard site method is to obtain free-space AF by theoretically removing the ground plane effect. The conducting ground plane is there to establish a repeatable calibration environment. There are some important assumptions made in the standard site method. First, the calculation to remove the ground plane assumes the antennas under test have radiation patterns of a point dipole (i.e., a donut shape pattern - uniform in H-plane and figure “∞” in E-plane). Second, no coupling exists among antennas and the metal ground plane. And third, antennas are in the far field so that the physical size of an antenna has no effect (i.e., the antennas are immersed in a uniform field). However, these simple assumptions are not always acceptable. The errors for a single bicon antenna factor derived from the standard site method can be as large as 2 dB [4]. This means if these AFs were used for normalized site attenuation test, the total error would be 4 dB (because an antenna pair is used for the site attenuation). ANSI C63.5-2004 standard addresses this limitation by providing correction factors. The correction factors are based on numerical simulations, and the baluns are assumed to be either 50 Ω or 200 Ω [4]. Note that this is not the perfect solution either, since some commercial antennas have balun impedances that vary drastically with frequency. Even a well-made one does not have a perfect match. The balun impedances have significant influences on the correction factor. The best approach is probably to use the calculable biconical antennas. The baluns are individually calibrated, and no approximations are made regarding their electrical 48 Conformity DeCember 2007 http://www.okaya.com http://www.okaya.com
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