ANSI C63.5:2001 pdf free download – Draft Standard – American National Standard for Electromagnetic Compatibility
4.2 Calibration Measurement Geometry
Accurate antenna calibrations require restrictions on measurement geometry. The antenna separation distance R shall be large enough to ensure that near-field effects and antenna-to-antenna mutual coupling effects are minimized. Antenna heights (h 1 , h 2 ) shall be great enough to minimize antenna-to-ground plane mutual impedance. The separation distance, R, between antennas shall be measured between points projected vertically from the antenna to the ground plane. This is equivalent to the horizontal distance between antennas when the antennas are at the same height. For dipole and biconical dipole antennas, the separation distance shall be measured from the midpoint of the dipole elements. The separation distance between log-periodic array antennas shall be measured from the midpoint of the elements along the longitudinal axis of each antenna. For horn antennas, the separation distance shall be measured from the front face of the antennas.
4.3 Test Site and Instrumentation
The test site used for antenna calibrations shall be within ± 2dB of an ideal site when tested for site attenuation in accordance with C63.4-2000. The normalized site attenuation test shall be evaluated over a volume (e.g., as an alternate test site) for the measurement distances the site will use to calibrate antennas. Measurement instrumentation should be located beneath the ground plane or at least 20 meters from the edge of the ground plane to reduce site and system contributions to uncertainty (see Annex I). All test instrumentation including signal generators, radio noise meters, spectrum analyzers, tuned voltmeters, receivers, etc. shall have a nominal 50 Ω impedance. Refer to ANSI C63.2-1996 or CISPR 16 Part 1 – 1999 for radio noise meter specifications. An impedance mismatch at the output of signal sources or at the input of receivers, and at antennas may result in reflections that could cause antenna factor measurement errors.
I for the formula to compute this error term). The signal sources should provide sufficient power to produce a signal at the receiver input of at least 16 dB above the equivalent receiver noise at the receiver input. A signal plus noise to noise ratio of 16 dB reduces this error term to 0.11 dB, significantly below the expected measurement uncertainty. This 0.11 dB must then be subtracted from the measured value. Power amplifiers may be used at the signal source output to raise the signal above both the ambient and receiver noise. See Annex I for further information on this minimum signal plus noise to noise ratio. Preamplifiers may be used at the receiver input to raise both the signal and ambient above the receiver noise and meet this requirement. The source power amplifier and/or preamplifier gain requirements will depend upon receiver and/or preamplifier sensitivity, antenna factors, cable losses, ambient signal level and measurement distance. Care shall be taken to avoid overloading pre-amplifiers or receivers with signals that are close to or higher than their maximum allowed input. Filters may be used to prevent out-of-band signals from saturating a preamplifier or receiver. Lossy ferrite material shall be applied to at least the first meter of both the transmitting and receiving antenna cables whenever dipole, biconical dipole or biconical hybrid antennas are calibrated, to serve as a common mode choke and to minimize induced currents on the cable shield from antenna coupling.