Research Paper The analysis and design of multi-layer microstrip moisture sensor for rice grain Fariba Jafari*, Kaida Khalid, W.M. Daud W. Yusoff, Jumiah Hassan Microwave Laboratory, Dept. of Physics, Faculty of Science, University Putra Malaysia, 43400 UPM, Serdang, Selangor, Malaysia article info Article history: Received 25 August 2009 Received in revised form 25 January 2010 Accepted 12 April 2010 Published online 23 May 2010 The analysis and optimal design of a multi-layered microstrip sensor for measuring moisture content of rice grain are described. The microstrip sensing structure consists of three layers: substrate, protective layer and semi-infinite grain medium. The effective dielectric constant, characteristic impedance and attenuation or insertion loss of this structure are calculated at various moisture contents with respect to the thickness of the protective layer thickness. The sensor is operated at 9 GHz and is suitable for a broad range of moisture contents (MC) ranging from 10% to 30% (wet basis). This study investigated the optimal thickness of the protective layer for suitable sensitivity, the thickness of grain medium which can be consider as semi-infinite thickness, the effect of kernel density and loading method. The experimental results for the attenuation of the signal at various moisture contents agree reasonably with the theoretical prediction however for better accuracy, the compactness of grain medium should be observed. The effect of air inside the medium and compactness of the material under the test has been studied and the errors of 0.16 dB have been calculated for the measured reflected attenuation of the sensor. The overall accuracy of the sensor is about 1.5% moisture content as compared to oven drying method. ª 2010 IAgrE. Published by Elsevier Ltd. All rights reserved. 1. Introduction Performing fast, accurate moisture content measurements is of great importance in agriculture, manufacturing, process- ing, storing and trading of most products and raw materials. Moisture content (MC) of granular and particulate materials is generally the most important characteristic used for quality assessment and process control (Kaatze, 2005; Kraszewski, 1996). The moisture level of grain is also very important during harvesting, trading, during transportation and storage. As define by standards, long term storage is safe for MC below 13% (free from microbial degradation). However over-drying the grain can decrease its nutritional and reproductive values and contribute to increased breakage during handling (Mohamed, Roman, Nelson, Kraszewski, & Barwicz, 1999). In the late sixties, microwave techniques emerged as a promising alternative for rapid and non-destructive sensing of moisture. However, the expected widespread commercial use never materialised because of cost, calibration and accuracy concerns. In recent years, the cost of microwave components has dropped drastically because of a surge in demand from the wireless telecommunications sector (Trabelsi & Nelson, 2007). Microwave techniques have attracted a lot of interest in the agricultural industry especially for measuring MC in grains. * Corresponding author. E-mail addresses: fariba3117@gmail.com (F. Jafari), kaida@fsas.upm.edu.my (K. Khalid), wmdaud@fsas.upm.edu.my (W.M.D.W. Yusoff), jumiah@fsas.upm.edu.my (J. Hassan). Available at www.sciencedirect.com journal homepage: www.elsevier.com/locate/issn/15375110 biosystems engineering 106 (2010) 324 e331 1537-5110/$ e see front matter ª 2010 IAgrE. Published by Elsevier Ltd. All rights reserved. doi:10.1016/j.biosystemseng.2010.04.005