MARTIN ALEXANDER. v2indd - National Physical Laboratory

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The reflectivity of the antenna supports needs to be as low as possible to make the lowest uncertainty measurements. • A source tower can be positioned to give.
Measurement of electrically small antennas via optical fibre Martin Alexander1, Tian Hong Loh2 and Andrés López Betancort* Communication Technologies Group, National Physical Laboratory, Teddington, UK 1 [email protected], [email protected] * guest worker at NPL from the University of Las Palmas de Gran Canaria, Spain

Abstract Electrically small antennas for wireless communications applications are prone to excite common mode currents on cables connected to them for measuring their performance. A miniature RF-optical transducer enables an optical fibre connection to the antenna, thereby eliminating the large distortion associated with the unwanted radiation from a coaxial cable. This opto-electric field sensor system operates from 300 kHz to 10 GHz. Results of a small UWB monopole antenna with coaxial cable and with optical fibre are compared.

Introduction

Experimental Findings

• Electrically small antennas inherently have wide-beam or near-cardioid radiation patterns • Accurate measurement of gain and patterns requires an antenna support that has minimal effect on the antenna radiation • If an external power supply or data cables are needed to test the antennas, a vital factor is the effect they can have on the measured radiation pattern, especially if the antenna has an inadequate balun

• To find out the deviation in the RF signal due to fibre movement, a 10 m long fibre was a) in a 0.5 m diameter coil, b) spread across the floor and c) twisted by rolling between thumb and finger. The variation in RF signal was up to 1 dB, see Figure 5(a). A feedback loop in the controller (detecting the level of returned light) reduces this to