Multi-layer frequency-selective surface

1 . A multi-layer frequency-selective surface, comprising:

a first layer comprising a first array of first metallic elements,

a second layer comprising a second array of second metallic elements, adjacent ones of the second metallic elements being distanced by gaps from each other, and

a third layer comprising a third array of third metallic elements,

wherein the second layer is arranged in-between and adjacent to the first layer and the third layer;

wherein the first array and third array comprise of a metallic filling fraction of at least 70% and the second array comprises of a metallic filling fraction of less than 15%;

wherein a frequency response of the multi-layer frequency-selective surface comprises at least one stop band and a passband offset from each other in frequency domain; and

wherein the second layer includes gaps in between adjacent ones of the second metallic elements configured to shift the frequency of the at least one stop band or the at least one passband without affecting the other.

2 . The multi-layer frequency-selective surface of claim 1 , wherein the at least one stop band is at higher frequencies than the passband.

3 . The multi-layer frequency-selective surface of claim 1 , wherein a metal filling fraction is lower for the second array than for the first array and the third array.

4 . The multi-layer frequency-selective surface of claim 1 , wherein the second metallic elements are loop-shaped.

5 . The multi-layer frequency-selective surface of claim 1 , wherein the second metallic elements are cross-shaped.

6 . The multi-layer frequency-selective surface of claim 1 , wherein the second array has a twofold rotational symmetry.

7 . The multi-layer frequency-selective surface of claim 1 ,

wherein the gaps include first gaps and second gaps,

wherein the adjacent ones of the second metallic elements are distanced by the first gaps along a first in-plane direction of the second array,

wherein adjacent ones of the second metallic elements are distanced by the second gaps along a second in-plane direction of the second array, and

wherein the first gaps are wider than the second gaps.

8 . The multi-layer frequency-selective surface of claim 1 ,

wherein adjacent ones of the second metallic elements are distanced by the gaps along a first in-plane direction of the second array,

wherein adjacent ones of the second metallic elements are joined together along a second in-plane direction of the second array.

9 . The multi-layer frequency-selective surface of claim 1 , further comprising:

tunable capacitors arranged in one or more of the gaps.

10 . The multi-layer frequency-selective surface of claim 9 , wherein the tunable capacitors comprise PIN diodes.

11 . A system, comprising:

the multi-layer frequency-selective surface of claim 9 ,

a voltage source configured to apply a bias voltage to the tunable capacitors, and

a control unit configured to control the voltage source to apply the bias voltage based on control data indicative of a frequency of a stop band of a frequency response of the multi-layer frequency-selective surface.

12 . The system of claim 11 , wherein the voltage source is configured to apply the bias voltage to a series connection of multiple ones of the tunable capacitors.

13 . A wireless communication device, comprising:

a cover,

an antenna configured to transmit or receive electromagnetic waves,

the multi-layer frequency-selective surface of claim 1 attached to the cover adjacent to the antenna.

14 . A computer-implemented method, comprising:

obtaining control data indicative of a frequency of a stopband; and

controlling a voltage source to bias tunable capacitors arranged in gaps between elements of an array of a frequency-selective surface;

wherein the array of a frequency-selective surface is a layer of a multi-layer frequency selective surface,

wherein the multi-layer frequency selective surface comprises a frequency response of the multi-layer frequency-selective surface having the stopband and a passband offset from each other in frequency domain;

wherein the tunable capacitors are arranged in a series connection and are configured to shift the frequency of the stopband without affecting the frequency of a passband.