The method in which the frequency spectrum is currently allocated is unsustainable. An increasing number of devices are becoming wireless, overcrowding an already crowded spectrum (e.g., the ISM band). Therefore, future systems will be forced to move to higher frequencies in order to be allocated an unused slice of the spectrum and accumulate the desired/required bandwidth. Furthermore, with the continued desire to implement a multitude of sensors on unmanned aerial vehicles (UAVs), as well as the need for conformal small-cell repeaters for 5G communications, next generation systems will have to achieve unprecedented reductions in size, weight, power, and cost (SWaP-C).
In order for future systems to become practical, several fundamental technological hurdles must be overcome including the development of low loss and highly integrated components used to build next generation systems. The RF/microwave filter is of particular interest, as it is not only crucial for conditioning the signal for transmission and/or digitization, but can also affect critical system parameters based on it's placement in the system. Due to the increased attenuative nature of the environment at microwave frequencies, the systems dynamic range will have to be maximized requiring an exceptionally low loss filter if placed close to the antenna in the receiver (Rx) chain, which is necessary for defense and adaptive/re-configurable systems. While low loss microwave filtering can be easily achieved using waveguide design techniques, it is much more difficult in a highly integrated planar design due to increased radiation and dielectric losses. A promising solution which minimizes these losses and offers a planar solution is the suspended integrated stripline (SISL) filter.
In this research, a low loss fully-board integrated lowpass and highpass filter, using the suspended integrated stripline technology, are designed and studied, pushing the stat-of-the-art in planar filtering technologies. A multi-layer board stack-up, with internally buried hollowed cavities, is used to create the suspended stripline. The embedded filter is accessed through a co-planar waveguide-to-stripline vertical via transition and vice-versa. Simulated and measured results show that insertion losses of less than 1 dB are obtainable including the vertical via transition and associated trace losses. Compared to it's suspended substrate stripline (SSS) predecessor, the SISL filter is one order of magnitude smaller and lighter while achieving identical performance. Beyond the proposed filters, this technological solution can be applied to several other passive microwave components such as couplers, power dividers, and gain equalizers. The capabilities demonstrated in this research will be crucial to the design and integration of modern and next generation systems as it requires no mechanical housing, connectors, or assembly, resulting in a light weight, compact size, and low cost solution
