Antenna Calibration Methods for Antenna Factor Measurements

Antenna Calibration Methods for Antenna Factor Measurements

L. Mescia (DEI-Dipartimento di Ingegneria Elettrica e dell’Informazione, Polytechnic University of Bari, Bari, Italy), O. Losito (DEI-Dipartimento di Ingegneria Elettrica e dell’Informazione, Polytechnic University of Bari, Bari, Italy), V. Castrovilla (ITEL Telecomunicazioni S.r.l., Ruvo di Puglia, Italy), P. Bia (DEI-Dipartimento di Ingegneria Elettrica e dell’Informazione, Polytechnic University of Bari, Bari, Italy) and F. Prudenzano (DEI-Dipartimento di Ingegneria Elettrica e dell’Informazione, Polytechnic University of Bari, Bari, Italy)
DOI: 10.4018/ijmtie.2012100104
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In the fields of electromagnetic interference and electromagnetic compatibility, it is important to measure the strength of the electric field originating from electric devices. For this purpose, knowledge of the antenna factor of a receiving antenna is necessary. According to international standards, the accurate measurement of the antenna factor involves the use of calibration test sites characterized by very large sizes of both the ground plane and the empty space volume above it. As a consequence, these setup conditions make the antenna factor measurements quite expensive for the customer. In this paper, the authors discuss the well know antenna-based and site-based methods as well as recently measurement method called Antenna Impedance Method as able to obtain the free-space antenna factor. Moreover, the authors investigate on the suitability of semi-anechoic chamber for reliable antenna factor calibrations. In particular, the experimental measurements of the antenna factor obtained by using the antenna impedance method were compared with Standard Field Method and the data provided by the manufacturer of three antennas (Biconical, Log-periodic and Horn antenna) founding an agreement with the international standard ANSI C63.5-2006.
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In recent years, wideband and ultra-wideband (UWB) technologies have gained notable progress. As a consequence, wideband systems are now essential parts of modern commercial, transportation, and military applications including airplanes, communication, traffic management or safety systems.

By considering that failure in electronic equipment could cause major accidents or economic disasters, the need to improve their electromagnetic interference (EMI) performance and to establish standards for electromagnetic compatibility (EMC) is an essential claim to satisfy. In fact, any new electronic product that is designed and manufactured today in Europe needs to undergo EMI/EMC testing to ensure that they meet the required EMC conformity declarations. To this aim, all manufacturers of electrical and electronic products in Europe use established standards describing the EMC requirements, measurement procedures, maximum emission and susceptibility limits of electronic systems and modules.

In EMI/EMC tests, broadband antennas are widely used for radiated emission and susceptibility measurements, radiated immunity testing, site qualification testing and human exposure tests. As a consequence, the accurate knowledge of the technical parameters associated with antennas is a fundamental requirement especially when calibrated antennas are required as probes for high-precision field strength measurements and as sources for the generation of specific electromagnetic field. Moreover, advantages in the antenna calibration methods make possible the reduction of uncertainty of EMC measurements, enabling saving in the design and manufacture of products as well as to achieve the required EMC protection.

In EMI testing, electric field measurements are necessary for determining compliance with most electromagnetic interference requirements, such as FCC Part 15, CISPR 22, CISPR 16, EN 55022, etc. In particular, with reference to the radiated emissions the electric field strength is measured by using a suitable receiving antenna connected to a receiver, such as spectrum analyzer, providing a voltage proportional to the incident electric field. A coaxial shielded cable is used for connecting the measurement instrument to the antenna. As result, to evaluate the electric field strength by using receiver readings (voltage), the antenna manufacturer has to provide the antenna factor versus frequency for both horizontal and vertical polarizations. The antenna factor value and the related uncertainty are the most critical parameters to be evaluated. Moreover, suitable setups and measurement procedures involving relatively high-cost instrumentation and specific test sites have to be used for an accurate evaluation of antenna factor. The antenna factor calibration of antennas used for radiated emission measurements of EMI from 9 kHz to 40 GHz is the subject of existing ANSI C63.5-2006 (ANSI C63.5-2006, 2006) international standard. In particular, it describes three main methods: the Standard Site Method (SSM), the Reference Antenna Method (RAM), the Equivalent Capacitance Substitution Method (ECSM). Additional methods are the Standard Field Method (SFM), the Standard Antenna Method (SAM), the Standard Transmitting Loop Method (STLM) and the near field three-antenna method (NF-TAM).

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