Miniaturized frequency selective surface using the double layered convoluted structures

Miniaturized frequency selective surfaces (FSSs) have been intensively investigated because of the space limitation in practical applications, such as wireless communication system working at low frequencies. Loading lumped capacitor is an effective miniaturization method to achieve the unit cell size as compact as λ
0
/15 (λ
0
refers to the resonant wavelength in free space) [1, 2]. But this approach requires bulk components which increase the complexity and cost of fabrication. Also, the tolerance of lumped elements makes it a challenge to maintain a stable resonant performance. Another effective method of reducing the unit cell size is to adopt convoluted elements which perform well in conforming to curved surface [3–5]. This method not only improves the angular stability of the frequency responses, but also moves the operating band away from the grating region which is determined by the periodicity of the array. The convoluted structures can reduce the FSS dimension because of their space filling property [5]. Therefore, their miniaturization potential will be restricted by the element area determined by the periodicity. To address this problem, the double layered convoluted structures, which make use of the element areas on both sides of substrate, are presented in this work to further miniaturize the FSS unit cell. Fig. 1 shows the geometry of the proposed bandpass FSS, which consists of a convoluted square slot loop and a convoluted capacitive element arranged on each side of the substrate. The top layer acts as a parallel LC resonator and the bottom layer acts as a capacitor. The LC resonator and the capacitor are connected in parallel. Therefore, the proposed bandpass FSS operating at 2.5 GHz provides the unit cell dimension as small as λ
0
/23 because of the increased capacitance of the unit cell. On the other hand, a double layered convoluted structure for the bandstop FSS is also proposed as shown in Fig. 2. It is seen that the top and bottom layers are connected by metallic via to construct a closed loop, which results in an increased inductance of the unit cell. Consequently, the element size of the presented bandstop FSS resonating at 2.5 GHz is only λ
0
/22. In addition, all the presented FSSs exhibit good performance of resonance stability with respect to different polarizations and incident angles.