Analysis and implementation of 3D frequency selective structures using multi-layer PCBs

It is well-known that frequency selective surfaces operate as spatial filters for the incoming electromagnetic waves with different operating frequencies, polarizations, and incident angles [1]. Due to these distinctive characteristics, frequency selective surfaces are widely used in many applications of microwave and antenna systems, such as antenna sub-reflectors, radomes, absorbers, etc. [1]. Recently, a new kind of frequency selective surfaces, classified as 3D frequency selective structures (FSSs), has been reported to achieve superior performance [2–4]. In a 3D FSS, multiple resonators are constructed in each 3D-cavity based unit cell, providing a number of transmission zeros/poles at finite frequencies. Subsequently, it is very easy to obtain high performance (such as high selectivity, wide out-of-band rejection, and multi-band response) by controlling locations of these transmission zeros/poles. However, the main disadvantage of 3D FSSs is their complicated topologies, which will definitely increase the implementation difficulties when achieving high performance. In this work, a brief review of 3D FSS designs is first conducted to understand their operating principles and to identify their advantages and limitations. After that, multi-layer PCB technology is introduced to realize 3D FSSs, which can extremely release implementation difficulties of 3D structures. Two examples of multi-layer based 3D FSSs are proposed, as shown in Fig. 1. Operating principles of both examples are explained with the aids of equivalent circuit models and current distributions. Experimental verifications are also provided for both design examples.