All-Printed Antennas Based on Transparent Substrates for RF Electronic Devices

Abstract

In future, flexible electronic devices requires connectivity on Bluetooth, WLAN, 2G, 3G and 4G-LTE standards and need support of printed flexible single and multiband antennas. Therefore, flexible and bendable antennas are necessary and unavoidable. The nature of flexible technologies dictates that the antenna should be flexible as well as mechanically robust such that it can tolerate high level of bending, rolling and flexing repeatedly. These challenges should be addressed because the performance of printed antennas degrades due to deformation in the flexible substrate under severe conditions. Therefore, it is critical and important to study and analyze the printed antennas for flexible electronic devices. The printed antennas can act as a radio frequency (RF) sensor to measure the permittivity of the liquids which can significantly detect the materials so, it is also important to analyze these printed antenna designs for RF sensors. To address future challenges of flexible antennas, this thesis presents various single and dual band flexible printed antennas on transparent substrates with different geometries for wearable electronic devices. In last decade, printing technologies such as screen printing, electrohydrodynamic (EHD) and inkjet printing are attractive pattering techniques which are gaining much interest for mass fabrication of various wearable electronics devices. The tremendous interest in these technologies is due to their features of one-step manufacturing process and vacuum free processing that fulfills the realization of eco-friendly, low-cost and flexible electronic devices and systems. New organic and inorganic solution processed materials are improving the performance of the electronic devices. By utilizing the solution process materials and printing technologies, this thesis addresses the challenges of flexible antennas and RF sensor on transparent substrates. Initially, a flexible single band monopole antenna design is presented for 1.8 GHz applications. The design of single band antenna is 2D projection of 3D helical antenna with a single turn as the 3D structures cannot be embedded in 2D flexible substrate. The mathematical modeling of the single band antenna design is presented. By using transmission line theory, the circuit model is presented which endorses the design of single band antenna and well matched with the simulation results in both circuit and full-wave electromagnetic software simulations. The straight and bent configurations performance analysis is carried out to optimize the design, and comparative analysis with other reported antennas is also presented. To confirm the reception capability of single band antenna, it was characterized on spectrum analyzer which showed the reception of single desired band. The antenna worked well and fully operational during flexibility test as the measured reflection coefficient was under -10 dB. A dual-band antenna design is described on a thin flexible and transparent Polyethylene Terephthalate (PET) substrate and characterized for wearable RF electronic applications. For printing both single and dual band antennas, the conductive ink based on Silver Nano Particles (SNPs) is utilized. The flexible antenna was fabricated on a very thin 40 micron substrate through inkjet material printer. To fabricate an optimal antenna on a thin substrate, the systematic procedure is given in detail. The results of fabrication process are confirmed through surface characterizations of the fabricated antenna. Next, a flexible and transparent inset-microstrip patch antenna based on conductive indium tin oxide (ITO) is presented. The patch antenna is fabricated on a Polydimethylsiloxane (PDMS) substrate and characterized with the help of Agilent network analyzer. The bendability and transparency tests are given for transparent wearable RF electronic applications. The presented flexible antenna design shows excellent performance over the considered band. Therefore, the proposed design provides an RF industrial solutions for future low-cost, eco-friendly and flexible wearable electronic market. Finally, a microstrip patch RF sensor is demonstrated for liquid identification. The relationship between the dielectric constant and resonance frequency provides the liquid sensing mechanism. The design procedure is given to find not only maximum frequency separation between two adjacent resonance frequencies but also lowest reflection coefficients at the resonance frequencies for all considered liquids. The fabricated sensor has 140 MHz minimum frequency separation and maximum -29 dB reflection coefficient, which gives large identification margin. By comparing with the existing sensors, the RF liquid sensor exhibits outstanding identification capability. Therefore, this type of patch sensor can be utilized in RF tunable liquid detection applications.