Methods for measuring RF radiation properties of small antennas
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Doctoral thesis (monograph)
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Helsinki University of Technology Radio Laboratory publications. Report S, 250
AbstractIn this work significant improvements for measurements of the radio-frequency (RF) radiation properties of small antennas have been proposed and investigated. The main focus is on electrically small antennas as used in mobile communications systems. The methods proposed and evaluated in this thesis allow the minimisation of the dimensions of measurement chambers, and the methods also allow pattern measurements with a minimised error from the RF feed cable and thus lower measurement uncertainty. The first two parts of the work relate to measurements performed in especially small chambers. The aim is to provide an alternative measurement environment to large, fully anechoic chambers in the special case of small antenna calibrations. The use of small chambers such as GTEM cells and small anechoic chambers is proposed. Both options have been constructed and investigated by both simulations and measurements. The results show that a GTEM cell allows the reliable measurement of the radiation pattern and 3-dB bandwidth of small antennas with a low directivity and a dynamic range of less than 20 dB. A small anechoic chamber with the largest dimension of 2.5 m was built during the work for this thesis. The results obtained when measuring the 3-D radiation pattern, efficiency and the gain of a small handset antenna in that small anechoic chamber show further that far-field measurements in such a small anechoic chamber does not result in greater measurement uncertainty than results obtained in conventional large anechoic chambers. Finally, the influence of the RF feed cables on the radiation characteristics of a small antenna under test has been reduced by a novel method. This method is based on a multi-frequency balun that efficiently suppresses the propagation of leakage and parasitic currents on the shielding of the RF feed cable. The effect of the balun has been thoroughly investigated by means of computer simulations and measurements with a prototype. Both the far field and the near field have been analysed to yield a comprehensive set of figures of merit, showing that the presented balun decreases the measurement uncertainty much better than other commonly used measures against cable effects, such as ferrite beads. The balun cannot only be used in radiation-pattern measurements in large or small anechoic chambers, but also in radio-channel measurements, in near-field scans and in Standard Absorption Rate (SAR) measurements.
small anechoic chamber, dual-band balun, mobile communications, radio frequency, antenna measurements, RF-cable related effects