Study Of Patch Antennas For Strain Measurement

Reliable strain measurement is important for damage detection of mechanical and civil structures. Existing wireless strain sensing technologies require a high operating voltage, bulky signal conditioning systems, and an external wireless module to transmit the sensor output. These limitations make the strain monitoring unreliable and expensive. An innovative method of measuring strain using a patch antenna is investigated in this thesis to overcome the limitations of existing strain sensing technologies. The patch antenna is made of a thin sheet of low-loss insulating material, called the dielectric substrate. The antenna pattern, i.e. a metallic patch, is printed on one side of the substrate. A ground plane is coated on the opposite side of the dielectric substrate. The metallic patch and the ground plane form an electro-magnetic (EM) cavity. This EM cavity radiates at a resonant frequency that depends on the dimensions of the metallic patch. Strain changes the dimensions of patch antenna, resulting in a shift in the resonant frequency of patch antenna. First, single frequency and dual frequency antennas are designed using the transmission line model. The antenna design is then confirmed using an Electromagnetic (EM) simulation tool, Sonnet 11.5. Because the dual frequency antenna has two fundamental frequencies f010 and f010 corresponding to the antenna length and width respectively, it is sensitive to strains applied along the length and width direction. The single frequency antenna, however, has only one fundamental frequency f010. Therefore, it is only sensitive to strains applied along the antenna length direction. In another word, strains applied along the antenna width direction do not have an effect on the frequency f010. The effect of strain along the width and length direction of the dual frequency antenna and the width direction of the single frequency antenna on the antenna resonant frequency are simulated using Sonnet 11.5 and the strain sensitivity of the antenna is calculated for each case. The antennas are fabricated on a flexible Kapton substrate using conventional micromaching techniques. The effect of strain on the antenna resonant frequency is verified experimentally by bonding the antenna onto a cantilever beam and applying load at one end of the cantilever beam. The experimental and simulated results are in good agreement with each other.