Conformity Magazine - December 2007 - (Page 49) performance. For log antennas, there is currently no correction table provided. These major factors contribute to errors: • Non-stationary phase center with respect to frequency (distance between antennas is vague) • Large deviation of the antenna pattern to that of a dipole (note that the gains are approximately 5 dBi or more) It is not as straightforward to derive correction factors for log antennas as for bicons. The biconical antennas are similar in mechanical design while log antennas vary by make and model. Research is in progress to develop a new method, which is based on a complex fit normalized site attenuation scheme. Interested readers can refer to [5] for more details. Reference Antenna Method This is another method specified in ANSI C63.5. It is basically a substitution method. The responses between two known antennas are measured (specifically two Roberts’ dipoles), and then one is replaced by the antenna under test. The antenna factor is derived from the difference. Mutual coupling between the two standard antennas and the antenna under test, as well as ground plane effects, can be significantly different, leading to significant errors. Other Methods There are other calibration methods, such as those used for loop antennas and rod antennas. Some labs use variations of the ANSI C63.5 method to calibrate log, dipole and bicon antennas, such as standard field method, or standard antenna method with precision dipoles etc. One important note is that, even if a perfect free-space antenna factor is obtained, one should still apply the correction factors provided by ANSI C63.5 for normalized site attenuation test. This is because normalized site attenuation (NSA) tests are not in free space. The correction factors are used to correct the influences from the test setup, i.e. the differences between free space and the specific geometry of an NSA setup. Calibrations for High Gain Antennas High gain antennas such as horn antennas have narrow beamwidth. They do not “see” the ground plane when placed in close proximity. In that case, the calibration is in free-space condition. Use of Antennas for Radiated Emissions Testing Radiated emissions tests are defined in ANSI C63.4 in the U.S. or the equivalent EN standards in Europe. The test setup is very similar to the antenna calibration setup. A large flat, unobstructed metal ground plane is used. The equipment under test is set on a low dielectric table, which is 80 cm high. The table is placed on a turntable, which can provide a full 360o scan. The receive antenna is scanned from 1 to 4 m for both horizontal and vertical polarization, and the maximum readings are recorded and compared to the standards. For emissions measurements, it is recommended to use free-space antenna factor. This is despite the fact that the measurements are not performed in a free space environment. The true antenna factors in the specific test environment are height dependent. The free space AF provides a good compromise. Use of Antennas for Radiated Immunity Testing Most immunity tests are performed per European standard EN 61000-4-3. It requires the establishment of a uniform field plane where the EUT would be. The typical setup is illustrated in Figure 2. The antenna power handling capability is an important parameter in such a test. Many antennas are designed to have superior performance in balance and impedance matching, but are not designed to handle high powers required in an immunity test. For an immunity antenna, the balance is not as important, since the purpose is to establish a known electromagnetic field. In the setup shown, the isotropic field probe is the key in setting up a calibrated field. A field probe is a special kind of antenna. It consists of three independent broadband antennas, which are oriented orthogonally. The field levels are measured and reported digitally through a fiber optical link to a readout unit or a computer. The total field is summed as RMS values of the December 2007 conformity 49 http://www.pseinc.com http://www.pseinc.com
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