In this paper, a circular microstrip antenna for stress evaluation is studied. This kind of\nmicrostrip sensor can be utilized in structural health monitoring systems. Reflection coefficient S11\nis measured to determine deformation/strain value. The proposed sensor is adhesively connected\nto the studied sample. Applied strain causes a change in patch geometry and influences current\ndistribution both in patch and ground plane. Changing the current flow in patch influences the\nvalue of resonant frequency. In this paper, two different resonant frequencies were analysed because\nin each case, different current distributions in patch were obtained. The sensor was designed for\noperating frequency of 2.5 GHz (at fundamental mode), which results in a diameter less than 55 mm.\nObtained sensitivity was up to 1 MHz/100 MPa, resolution depends on utilized vector network\nanalyser. Moreover, the directional characteristics for both resonant frequencies were defined, studied\nusing numerical model and verified by measurements. Thus far, microstrip antennas have been\nused in deformation measurement only if the direction of external force was well known. Obtained\ndirectional characteristics of the sensor allow the determination of direction and value of stress by\none sensor. This method of measurement can be an alternative to the rosette strain gauge.
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