Calibration of dipole-like antennas, 100 Hz to 5 GHz
NPL advises customers on the most appropriate calibration for their application, and carries out calibration of a wide variety of antenna types, including monopole, tuneable dipole, biconical, log-periodic dipole array, biconical-log hybrid antennas, conical log-spiral, loop and horn antennas. Such antennas are used for electromagnetic compatibility (EMC) testing, for example to meet the requirements of EMC Directive 2004/108/EC.
NPL calibrates most types of antennas, for example those used for radio and TV reception, wireless communications applications, EMC testing and EMC test site validation. Antennas have to be calibrated in order to know the uncertainty of measurement of electric or magnetic field strength, or of the transmission loss between two antennas: the appropriate measurement parameters antenna factor or antenna gain.
Other properties of antennas that should be measured are:
- Reflection coefficient (in order to calculate mismatch uncertainties)
- Balun balance
- Cross-polar rejection
- Phase-centre position
- Radiation patterns (if the measurement involves transmission or reception at angles other than the antenna boresight direction, depending how broad the mainbeam is)
Antennas used for EMC conformance testing must be traceably calibrated to national standards for the products to be eligible to carry the CE mark. NPL holds the UK's national standard for field strength, which is based on a calculable dipole antenna, which in turn was proved on one of the largest and flattest outdoor ground planes in Europe, for the purpose of antenna calibration. The ground plane of dimensions of 60 m x 30 m is built from seamlessly welded steel plates, and is flat to within ±6 mm over 95 % of its area. The site is available for hire.
Calibrations are also performed in a fully ferrite-tiled anechoic room down to 30 MHz and in an anechoic chamber up to 18 GHz.
Over the frequency range 20 MHz to 5 GHz NPL measures antenna factor of wire antennas to an uncertainty of ±0.5 dB; or, at lower cost, to uncertainties in the range ±1.0 dB to ±1.5 dB. Free-space antenna factor is measured, according to the CISPR international standards, but measurements can be made in proximity to the ground plane, including height scanning, according to customer requirements, for example to US standards ANSI C63.5 and, for 1 m separation, to SAE ARP 958.
NPL advises customers on the most appropriate calibration for their application as well as offering consultancy for measurement of non-standard antennas and for antenna range design and validation.
Also see the NPL Good Practice Guide No. 73 on Antenna Calibration
The electric field strength of radiating sources is also measured, above a ground plane or in an anechoic chamber. Such sources are commonly used to validate a measurement test site. Common frequency ranges are 30 - 1000 MHz and 1 - 7 GHz.