Conformity Magazine - December 2007 - (Page 46) to the ground is different than that from the center pin to the ground. Some antennas, such as biconical antennas, employ baluns (short for balanced-to-unbalanced transformer) to overcome the imbalance. Basically, a balun provides a low impedance for differential current, and a high impedance for common mode current, which flow over the surface of the shield. A large amount of common mode current exists between the shield of the feed cable and the ground plane. An unbalanced antenna has different responses depending on which side is up when polarized vertically. This causes large measurement uncertainties. Bandwidth Bandwidth is “the range of frequencies within which the performance of the antenna, with respect to some characteristic, conforms to a specific standard” [1]. The definition is quite broad. It does not explicitly specify what the “characteristics” or “specific standard” are, so the term bandwidth is subjective. Depending on the application, the typical characteristics can include all or some of the terms discussed previously. Engineering judgments are needed to determine what is acceptable for your application. Typical EMC Antennas Several types of antennas can sometimes all satisfy the basic requirements of a measurement. The following list provides brief features, application notes and possible drawbacks of the typical EMC antennas to hopefully aid readers in selecting the best fit. Loop and Magnetic Field Coil Typically used in the frequency range from 20 Hz to 30 MHz for measuring magnetic fields. At these frequencies, measurements are for the most part within the near field region. Unlike in the far field, in the near field region, electric fields do not relate to the magnetic field by 377 Ω. The electric field cannot be derived simply from the magnetic field, although some regulations pretend that the far field relations hold. The designs of these antennas ensure they predominantly produce or respond to magnetic fields. Rod or Monopole Antennas Rod antennas are the counterparts of loop antennas. They are designed to respond to electric fields from 30 Hz to 50 MHz. Since rod antennas are so small compared to the wavelengths (at 30 Hz, the wavelength is 10,000 km), amplifiers within the antennas are sometimes necessary for small signals. Rod antennas are typically required for the GR-1089-core standard for network telecommunications equipment (where radiated emission and immunity tests for electrical field at 10 kHz are required). Dipole Antennas Dipoles are tuned to specific frequencies, from approximately 30 MHz to a few GHz. They are narrow band. To cover a wide bandwidth, they need to be tuned manually. Dipoles are often used as reference antennas because the dipole elements 46 Conformity DeCember 2007 performance can be theoretically calculated. Interestingly, Roberts’ dipoles, which are specified in the ANSI C63.5, have balun designs that are difficult to characterize, so performance of the Roberts’ dipoles is hardly calculable. Dipoles are seldom used in everyday measurements, due to the need for individual tuning at each frequency. Biconical Antennas Biconical antennas typically cover the frequency range from 20 MHz to 300 MHz. All wire-cage biconical antennas on the market have similar size and shape (approximately 1.36 m wide). This is because they are based on MIL-STD-461 specifications from the 1960s, which has become the de facto standard. Due to their small electrical size below 50 MHz, they have very high input impedance, resulting in high VSWR. Balun performance is crucial for biconical antennas. Common mode current can be easily induced on the feed cable (common mode impedance is no longer large compared to the input impedance of the antenna). Ferrite beads are often used on the feed cable to suppress the common mode. In addition, feed cables should be extended out a meter or more horizontally before the cable is dropped vertically to the ground to reduce possible interaction between the cable and the antenna. Log Periodic Dipole Arrays Log periodic dipole arrays (LPDA) typically cover the frequency range of 80 MHz to several gigahertz. As discussed previously, the phase center of a LPDA moves from the back of the antenna boom to the front as the frequency is increased. In ANSI or CISPR standards, emissions measurements are performed from the center of the log antenna boom. For immunity tests, EN 61000-4-3 requires measurements be made from the tip of the log antenna. The gain of a LPDA is typically around 5 dBi, which provides a good compromise between beamwidth and sensitivity. Bicon/Log Hybrid Bicon/log hybrid antennas are sometimes referred to by their trade names, Biconilog or Bilog. The hybrid combines the frequency range of a biconical antenna and a log antenna, which is approximately 20 MHz to several gigahertz. They have become increasingly popular, as there is no band break requiring an antenna change during a test. Just like biconical antennas at 20-50 MHz, hybrid antennas are electrically small at the lower end of the frequency range. To increase the transmit efficiency, some hybrids employ end loading techniques to compensate for the size. They typically have T-shaped or L-shaped bowtie elements. These antennas should only be used for immunity testing. The coupling between the loading elements and their surroundings are very strong, and they introduce large measurement uncertainties for emissions testing [3]. If these antennas are to be used for
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