STUDY OF NEAR FIELD ANTENNA FOR MICROWAVE TOMOGRAPHY APPLICATION

• Main Author: Oktafiani, Folin
• Format: Dissertations
• Language: Indonesia
• Online Access: https://digilib.itb.ac.id/gdl/view/71836
• Institution: Institut Teknologi Bandung

In certain fields, it's essential to be able to generate an object's internal structure without harming it. An imaging method termed microwave tomography allows for object detection without damaging the object itself. This method makes utilisation microwaves to inspect an object's internal structure. One of the crucial components in microwave tomography system is antenna. The antenna capability to transmit and receive signals that penetrate through any objects are the determining factor that affect the image reconstruction process. The required specifications for microwave tomography antenna are different from other microwave applications. On microwave tomography, the antenna is located closed to the object so that the antenna is within the near field area. Based on the literature, the parameters that significantly influence the performance of near field antennas are bandwidth, radiation pattern, input impedance, efficiency, and antenna polarization. Some near field antenna designs have been proposed in previous studies, such as dipole, horn, bow-tie, monopole, tapered slot antena (TSA), and log periodic antenna. Dipole, monopole, and bow-tie antenna are frequently used due to the simplicity on the design and fabrication process. However, those types of antenna have several drawbacks, including a limited band and omnidirectional radiation pattern. Log periodic antenna meets the requirement in term of bandwidth, however, the dimension is tipically too large. By considering the aforementioned parameters, the antennas that are suitable for microwave tomography application are horn antenna and TSA. According to field measurement, horn antenna has lower noise level and higher contrast level than TSA. In the imaging process using a microwave tomography, the near field antennas are arranged to cover the object. The more antennas arranged around the object, the more sampling data could be generated so that it can increase the performance of image reconstruction process. To meet such configuration, a near field antenna with compact dimension is required. Other than the dimension, a high resolution imaging is needed for the object detection accuracy. A wide antenna bandwidth is required to acquire a sufficient resolution. The wider the bandwidth, the better the resolution. The operating frequency selection related to the bandwidth must cover the low frequency so that the low frequency signals could not be easily attenuated when penetrating the object. Thus, the signal can be detected by the antenna and processed to be reconstructed into an image. One of the drawbacks of a horn antenna is having a large dimension for low operating frequencies with limited bandwidth. A method for increasing the bandwidth of horn antenna towards a lower frequency while maintaining a compact dimension is by inserting ridges into the horn part. The insertion of ridges basically causes a capacitive effect that it plays a role in reducing the cut-off frequency which can widen the bandwidth of the horn antenna. The analysis of the effect of ridge insertion to the cut-off frequency is carried out by designing a horn antenna with a ridge and theoretically identifying it based on a lumped element equivalent circuit and a transmission line circuit. The quad-ridged horn antenna (QRHA) and the double-ridged horn antenna (DRHA) are two ridged horn antennas that were observed. The performance analysis shows that the horn antenna with four additional ridges outperforms the horn antenna with only two additional ridges. Based on the investigation of the effect of the ridge profile with four shape variants: linear, quadratic, exponential, and Gaussian shows that the quadratic profile produces the best bandwidth and sidelobe level parameters among the other profile variations. Based on the theoretical approach, it is known that the cut-off frequency of the ridged horn antenna is not only influenced by the width of the waveguide and the height of the waveguide, but also the width of the ridge and the space between the ridges. The next step is design a horn antenna with a QRHA type and a ridge shape utilizing a quadratic profile after proving the performance and conducting theoretical analysis. A CNC milling technique based on aluminum and a 3D printing technique based on PLA were used to manufacture the results of QRHA design. The QRHA prototype utilizing these two methods generates a bandwidth as wide as 7.920 GHz with an operating frequency of 3.500 GHz - 11.420 GHz and 8.260 GHz with an operating frequency of 4.440 GHz - 12.700 GHz for the PLA-based and aluminum-based QRHA, respectively. The radiation pattern shows that the two antennas produce radiation patterns that have almost the same pattern when being compared to the simulation results. The QRHA antenna was then implemented for microwave tomography applications. In this research, the QRHA antenna is used in the data acquisition process in the object detection process. The data acquisition process is performed by placing two antennas around an object. Furthermore, the data of transmission coefficient (S21) is taken to be reconstructed into an image. Signal processing method used is compressive sensing technique. The object detection results show that the designed QRHA antenna can be used to detect objects with fairly good reconstruction results.