Printed tunable miniaturized Frequency Selective Surface with BST/polymer composite filled interdigital capacitors

Summary form only given. We present an electrically tunable Frequency Selective Surface (FSS) at X-band utilizing a novel version of Interdigital Capacitors (IDCs), in which the space between fingers are filled with a ferroelectric composite of Barium Strontium Titanate (BST) suspended in a polymer. We also introduce a low temperature, printed fabrication process on flexible films. The literature suggests the use of varactors based on ferroelectric ceramics as the electrically tunable element in FSSs. These varactors have low Q factor and are expensive to fabricate. The ceramics are brittle in nature and require very high processing temperature (~1100°C). On the other hand, tunable ferroelectric polymers such as Polyvinylidene Fluoride (PVDF) are lossy and slow in switching at microwave frequencies. However, a BST-polymer composite promises both excellent ferroelectric characteristics of the BST in its paraelectric phase and flexible processing of the polymer. A composite by mixing micro and nano size particles of sintered Ba_0.55Sr_0.45TiO₃ with a thermoplastic polymer, that has very low loss tangent at high frequencies, was prepared. It was found that when the key parameters - particle size and volume fraction - are tailored, high dielectric tunability up to 30% and low loss tangent are obtained for the composite, but the dielectric constant can be as low as 30 at microwave frequencies. To get around the low permittivity limitation, we introduced the filled IDC configuration that requires a dielectric constant as low as 20 for a frequency sweep range from 5.6 GHz to 12.8 GHz including full X-band (based on full-wave electromagnetic simulations), instead of the conventional IDCs with fingers on top of a BST ceramic layer that would require epsilon of few hundreds. Both computational and experimental results of the tunable filled IDC as well as the tunable FSS structure will be presented. Although, we demonstrate the implement- tion of this tunable filled IDC on FSS structures, this printable varactor can be utilized in various applications.