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This paper presents the development of a PC-based microwave five-port reflectometer for the determination of moisture content in oil palm fruits. The reflectometer was designed to measure both the magnitude and phase of the reflection coefficient of any passive microwave device. The stand-alone reflectometer consists of a PC, a microwave source, diode detectors and an analog to digital converter. All the measurement and data acquisition were done using Agilent VEE graphical programming software. The relectometer can be used with any reflection based microwave sensor. In this work, the application of the reflectometer as a useful instrument to determine the moisture content in oil palm fruits using monopole and coaxial sensors was demonstrated. Calibration equations between reflection coefficients and moisture content have been established for both sensors. The equation based on phase measurement of monopole sensor was found to be accurate within 5% in predicting moisture content in the fruits when compared to the conventional oven drying method.

The principle of the six-port reflectometer for measuring reflection coefficients was introduced by Engen [

Previously, we proposed the use of an open ended coaxial sensor to determine moisture content in oil palm fruits [

The measurement setup, shown in

The five-port circuit shown in _{r} = 2.2 and 1.5748 mm, respectively.

The widths of each arm and ring of the five-port were _{1} = 1.008 mm and _{2} = 1.272 mm. The radius of the ring was

The relationship between the complex reflection coefficient Γ of the unknown load and the power ratios w_{i} can be written in the form [_{i} are the powers measured by the three detectors, k_{i} are the unknown constants to be determined from the calibration procedure and q_{i} are the values of the calibration standards. The four calibration standards used in this work to determine the unknown constant k_{i}, x_{i}, and y_{i} were a precision 50 Ω load, standard open standard, offset 120° and offset 240° open calibration standards. Assuming perfect matched load and open standards, we obtained from

At port 1 (I = 1):

After some algebraic manipulation of _{1}, x_{1}, and y_{1} for port 1 can be determined from the linear equations system of

The same procedure can similarly be used to determine other k_{i}, x_{i}, and y_{i} values for ports 2 and 3.

From

For the device under test, u_{1} = u_{2} = u_{3} = u and v_{1} = v_{2} = v_{3} = v can be obtained from the following matrix:

A computer program has been developed to control, acquire, and save data using the Agilent VEE version 7.0 graphical programming software. The program was also used to implement all the calibrations and calculations of the reflection coefficients. As an example, the ADC 16 data acquisition module is illustrated in

The performance of the five-port reflectometer was tested by comparing reflection coefficient S_{11} values of eight different offset shorts using both the reflectometer and a commercial VNA. The results are listed in

The mean error in magnitude between the reflectometer and VNA measurements was 0.0202, whilst the phase mean error was 1.91°. The accuracy of the reflectometer was further tested by comparing the reflection coefficient S_{11} measurement results between the reflectometer and the VNA for several well known materials using both the monopole and open ended coaxial sensors. Again good agreement between the reflectometer and VNA results were obtained for all the materials listed in

More than 100 fruits in various stages of fruit ripeness were measured using both the coaxial and monopole sensors in conjunction with the five-port reflectometer. Abnormal, not fully developed, dry, and rotten fruits were not considered. The surface of the fruit was wiped dry to free excess surface moisture. All the reflection coefficient S_{11} measurements of the fruit samples using the five-port reflectometer were done at 26 °C. The samples were then dried in a forced-air oven for four days at 105 °C for moisture content determination on a wet basis [

The variations in the magnitude and phase of S_{11} with moisture content in oil palm fruits are shown in

However, for the monopole sensor shown in ^{2} shown in each graph were obtained by applying regression analysis. The determination coefficients for both magnitude and phase methods of the open ended coaxial sensor were almost similar. As expected, the highest correlation was obtained from phase measurements using monopole sensor. The phase of S_{11} is highly sensitive not only to the length of the extended the inner conductor of the monopole sensor but also sensitive to small variation in the complex permittivity of the samples due to different percentages of moisture content. The weak correlation for the magnitude measurement using monopole sensor was due to multiple wave reflection between the extended inner conductor and fruit.

A computer program was developed using the Agilent VEE software to predict moisture content in oil palm fruits by applying inverse relationships to the equations in

The results for a different batch of 100 samples of oil palm fruits using open ended coaxial and monopole sensors are compared with the actual values of moisture content obtained using standard oven drying method in

The five-port reflectometer represents a simple, cheap and efficient microwave network analyzer solution to determine reflection coefficients which in turn can be used directly to determine the moisture content in fruits. The reflectometer is especially useful for

Five-port reflectometer.

Five-port circuit.

Five-port reflectometer software.

Variation in magnitude and phase of S_{11} with moisture content using open ended coaxial sensor.

Variation in magnitude and phase of S_{11} with moisture content using monopole sensor.

Panel View of VEE Program to determine moisture content in oil palm fruits.

Comparison between predicted and actual moisture content.

Comparison between predicted and actual moisture content.

Magnitude and phase of reflection coefficient measured using reflectometer and commercial VNA.

0.9860 | −30.30 | 0.9931 | −31.22 | 0.0071 | 0.92 | |

0.9990 | −69.61 | 1.0070 | −72.13 | 0.0080 | 2.52 | |

0.9860 | −138.93 | 1.0210 | 137.40 | 0.0350 | 1.53 | |

1.0042 | −105.50 | 1.0200 | 107.20 | 0.0158 | 1.70 | |

1.0030 | 180.00 | 1.0410 | 177.10 | 0.0380 | 2.90 | |

0.9950 | 129.50 | 0.9850 | 131.40 | 0.0100 | 1.90 | |

0.9910 | 52.60 | 0.9750 | 50.20 | 0.0660 | 2.40 | |

0.0200 | 9.67 | 0.0027 | 8.25 | 0.0173 | 1.42 | |

S_{11} of several known materials using monopole sensor.

0.9845 | −30.00 | 0.9637 | −33.72 | |

0.9050 | −173.53 | 0.8980 | −169.77 | |

0.4712 | −80.53 | 0.5078 | −81.96 | |

0.7257 | −49.56 | 0.6939 | −52.15 | |

0.6773 | −154.56 | 0.7040 | −150.67 |

S_{11} of several known sample using open ended coaxial.

0.9829 | 138.53 | 0.9582 | 141.29 | |

0.8841 | 57.55 | 0.9143 | 58.50 | |

0.8091 | 127.24 | 0.8093 | 130.04 | |

0.7403 | 90.51 | 0.7701 | 90.40 | |

0.8939 | 133.08 | 0.8890 | 136.22 |