Antenna Calibration
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.