Calibration of VHF-UHF-Antennas
Each antenna for EMI Field-Strength Measurement (or field generation) is individually measured for gain and antenna factor under near-free-space conditions. Antennas delivered to the user may be recalibrated after periods of one or two years using the most accurate method available, the "two-antenna-absolute gain measurement" under practically reflection-free conditions. In an antenna data comparison requested by the European Community the British National Physical Laboratory (an official calibration authority traceable to basic standards) and 15 participating official national (government) standards institutions and calibration test houses measured six antennas (4 of them commercially available, three of them Schwarzbeck models). The measurement uncertainty was published as 0.7dB (NPL and Schwarzbeck) up to 1dB, 1.5 dB or 2 dB with other laboratories. The results were published by the National Physical Laboratory (UK) by M. J. Alexander in a final report 26 September 1995 and in a contribution to the British IEE (Proc.- Sci. Meas. Technol. Vol. 143, No. 4, July 1996). Three of the participants (including Schwarzbeck) reached +/- 1dB. The highest score of 96 % of the measurements better than +/- 0.5 dB compared to the NPL average were achieved by Schwarzbeck with a new free-space calibration method of height-scanning and averaging. Typical differences to NPL were 0 dB to 0.3 dB with biconical, log.-periodic and dipole antennas in the range 30 MHz to l GHz. Test Antennas coming from production are individually tested with this near-free-space method by comparison to a pair of identical antennas of the same design, plotting the free-space attenuation and replacing one of the standard antennas by the A.u.T. sample in a 1dB-tolerance range. This continuous comparison (no frequency steps) shows even narrow deviations. If a customer requests a re-calibration, for example after 1 year, he may mention fixed frequency steps for individual point-to-point calibration. Normally 5-MHz steps are used in the lower vhf range, 10 MHz up to 300 MHz, and 25 MHz steps are recommended up to 1GHz or higher. The results are obtained from specialized calculating programs and printed in tables containing frequency, test separation (with. consideration of the variable phase centre of LOG antennas), wave length, free-space attenuation, isotropic gain, gain over the lossless half-wave dipole and antenna factor. The table data are also converted into gain and antenna factor graphical presentations. Also a VSWR individual plot and a Calibration Certificate are included.