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Abstract
The traditional frequency selective surface (FSS) needs further improvement with the development of stealth technology, and the design of multifunctional FSSs is essential. In this letter, an active absorptive FSS (AFSS) has been designed based on the absorption structure of the spoof surface plasmon polariton (SSPP) and the switching activity of the active FSS. The active FSS embedded with PIN diodes realizes the shift of two transmission/reflection frequency bands by controlling the bias voltage of the feed network, which switches from one band-pass response (at around 3.06 GHz) to the other (at around 4.34 GHz). And when one of the transmission windows switches to the other, the original transmission window closes. The upper plasmonic structure achieves a continuous and efficient absorption band from 6.31 to 8.34 GHz. A sample was also fabricated and carried out to verify the numerical simulation, and the experimental and simulation results are consistent. This work provides new ideas for the design of active AFSS and promotes its application in common aperture radome, antenna isolation, and electromagnetic shielding.
© 2021 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
1. Introduction
The frequency selective surface (FSS) with band-pass or band-stop characteristics is composed of a two-dimensional (2D) or three-dimensional (3D) periodic array of meta-atoms, which is usually printed on a substrate or etched on a conductive surface. FSS has been a research hotspot for decades due to its wide range of applications, including performance enhancement of resonant cavity antennas [1–2], antenna reflectors [3], electromagnetic compatibility [4], and stealthy radome [5–7]. Traditional FSS constructed in a specific shape can reflect out-of-band signals in a direction away from the incident wave, but it cannot eliminate the threat posed by bi-static radar. Therefore, compared to scattering, the structure that can realize absorption [8–10] not only achieves radar cross section (RCS) reduction but also ensures that the radome has better defense performance.
However, in order not to affect the transmission of operating signals, the absorptive FSS (AFSS) has been widely studied, which can absorb signals outside the frequency band and transmit signals within the frequency band [11–20]. The composite structure of AFSS is composed of two layers, the upper layer is the absorber structure, and the bottom layer is the band-pass FSS. In general, band-pass FSS is used to replace the grounding of the absorption structure to achieve the transmission property. The upper absorption structure absorbs the energy of electromagnetic (EM) waves by loading lumped resistor elements, patterned resistive film arrays, or magnetic loss materials. The position of the transmission band can further classify the AFSS: The transmission band of the first type of AFSS is lower than the absorption band [11–12]. The transmission band of the second type of AFSS is higher than the absorption band [13–15]. The third type of AFSS has one transmission band within two absorption bands [16–19]. The fourth type of AFSS has two transmission bands within two absorption bands [20]. Recently, AFSSs with one switchable transmission/reflection band characteristic have been proposed [21–22]. Nevertheless, these FSSs have a common limitation, that is, the transmission band is fixed. For antennas in different operating frequency bands, the radomes based on the above FSSs lack versatility. Consequently, it is a very urgent need to control the switch of the transmission bands actively.
Motivated by the requirement, it is introduced the concept of AFSS with two alternately switchable transmission/reflection bands, as shown in Fig. 1. The upper absorptive layer is a trapezoidal metal structure driven by spoof surface plasmon polariton (SSPP) [23–28]. Due to the strong localized ability of EM waves in the metal-dielectric interface and the progressive k-vector matching performance, the plasmonic structure can achieve broadband and efficient transmission in the low-frequency [29]. In addition, when the strongly localized EM field is used in combination with a high-loss substrate (FR4), the plasmonic structure can directly achieve high-efficiency broadband absorption at high frequencies [30–31]. The lower structure is active FSS. The design principle of active FSS is illustrated with the help of its equivalent circuit (EC), thereby physically explaining the performance of the active FSS in different bias states of the PIN diode. The sample of the proposed AFSS was tested in an anechoic chamber after being manufactured. The AFSS proposed in this paper overcomes the limitations of traditional AFSSs and shows excellent potential in stealth technology.
2. Design and analysis of the plasmonic metamaterial absorber
A two-layer composite structure is used to attain high-efficiency absorption at the high-frequency band and achieve transmission windows switching at the low-frequency band. By loading the metal blade structure on the dielectric substrate, absorption is achieved, and when the frequency below the cut-off frequency, the structure has good transmittance. Therefore, the plasmonic structure can be used as a top-layer structure. As for the bottom structure, active FSS realizes switchable transmission/reflection bands by changing the PIN diode state. In this part, two independent structures and the integrated structure are analyzed.
2.1 Dispersion relation of SSPP on the plasmonic structure array
The SSPP mode works in the microwave region, and the energy is wholly localized near the plasmonic structure surface or material and propagates along the structure surface or the material. To illustrate the EM waves onto the metallic fishbone structure are transmitted in SSPP mode, the dispersion curves of EM waves in vacuum and EM waves on the metallic blade array structure are calculated. And the results are simulated by the CST Eigen-mode solver, whose periodic boundary conditions are along the x, y, and z directions. As shown in Fig. 2(a), the plasmonic structure is composed of one layer of FR-4 dielectric substrate ($\varepsilon $r = 4.3, tan$\delta $=0.025) and a metallic blade array. And the thickness of the substrate is d1 = 0.5 mm. The width of the FR-4 dielectric substrate is a, and the dimension of the metallic blade along the y-direction is l. The width of the metal wire is b. The width of a sawtooth of the metallic blade is m, and the gap between two sawteeth is n. The optimization of structural parameters is performed after being integrated with active FSS, and the optimized dimensions are: a=30 mm, b = m=n=0.2 mm. When the working wavelength is much larger than the period (m + n) of sawtooth structure, and the gap width n, the dispersion equation of a single metallic blade is [32]:
kz is the propagation constant of the EM waves on the surface of the plasmonic structure. And k is the wave vector of the incident y-polarized wave. Figure 2(b) shows the dispersion curves of EM waves on the plasmonic structure are below the one in the vacuum, which indicates that the metallic blade array can highly confine EM waves on the surface of the meta-atom. It is observed that the dispersion curves contain solid and dashed branches, corresponding to even and odd SSPPs modes, respectively. [27] Consequently, the plasmonic structure can propagate SSPPs modes in the sub-wavelength region for the y-polarized wave. When the frequency below the cut-off frequency, it can ensure that the transmission of SSPP is high-efficiency. Therefore, a high-efficiency passband can be obtained below the cut-off frequency. The plasmonic structure can enhance the k-vector matching of the ground active FSS at low frequency and achieve the broadband absorption at high-frequency. And with the increase of the l, the cut-off frequency of SSPP would not remain at the original frequency, and it moves to a lower frequency. Therefore, the dispersion of the plasmonic structure by changing the length l of the blade. Equation (1) has given the dispersion equation of a single metallic blade, from which it can be inferred that the equation for the cut-off frequency is [32]:
Fig. 2. (a) The schematic diagram of the metallic blade array structure. (b) The dispersion curves of waves on the metallic blade array structure. (c) Simulated reflection and transmission spectra of the proposed plasmonic structure under normal incidence and schematic diagram of the plasmonic structure. (d) The schematic diagram and simulated absorption spectra of the metallic trapezoid structure.
The absorption of EM waves is achieved by the high dielectric loss of non-magnetic media, which follows the equation P=1/2($\omega \varepsilon ^{\prime\prime}$+$\sigma $)|E|2 [28], where $\omega $ is the angular frequency, $\varepsilon ^{\prime\prime}$ is the imaginary part of permittivity, $\sigma $ is the conductivity and E is the total electric field. Absorption can be achieved by using a high $\varepsilon ^{\prime\prime}$ and $\sigma $ material substrate or through local electric field enhancement. At the cut-off frequency of the metallic blade, the electric field is strongly confined to the plasmonic structure surface, which makes the electric field strength sharply increase. Consequently, high absorption performance can be obtained at the cut-off frequency, and metallic blades of different lengths absorb EM waves at different frequencies. By arranging metallic blades of different lengths, the absorption band can be superimposed.
As shown in Fig. 2(c), the plasmonic structure (trapezoidal metal structure) composed of series grooves with linearly varying depths is designed to absorb EM waves in a high-frequency band. The plasmonic structure period is p = 30 mm, and the height of the plasmonic structure and the height of the metal trapezoidal structure along the z-direction are both h = 20 mm. The shortest and longest metal wires along the y-direction are l1 = 8.14 mm and l2 = 14.93 mm, respectively. Based on this structure, the low reflection of the entire working frequency band and efficient transmission below 5.75 GHz can be achieved synchronously. By comparing Fig. 2(b) with Fig. 2(c), the cut-off frequency region is consistent with the working frequency band where both reflected and transmitted waves are suppressed. Consequently, the plasmonic structure exhibits efficient and continuous absorption performance in the cut-off frequency region, and the result is given in Fig. 2(d). In order to prove the working principle of the absorbing structure, the electric field Ey distributions of the proposed plasmonic structure in the y-z plane are monitored at the frequencies of 4, 7, and 9 GHz in Fig. 3, respectively. The enhanced electric field Ey is always concentrated on the metal-dielectric interface of the plasmonic structure. At 4 GHz, due to the incident wave being highly coupled to the structural surface, the EM wave can be transmitted to the free space at the other end. At 7 GHz, since most of the incident EM waves are absorbed or reflected by the plasmonic structure, the EM waves cannot be transmitted to free space along the surface of the structure. At 9 GHz, the wavelength of EM waves is close to the period of the trapezoidal metal structure so that it can maintain a high transmittance. Above all, as the frequency increases, stronger electric field Ey gradually appears at a higher position of the structure, consistent with the relation between the cut-off frequency and the length of the metal blade.
Fig. 3. The simulated electric field Ey distributions of the metallic trapezoid structure under y -polarized wave normal incidence at 4 GHz, 7 GHz, and 9 GHz.
2.2 Analysis of the active FSS
The dimensions and layouts of a meta-atom of the active FSS are depicted in Figs. 4(a), 4(b). The active FSS layer is printed on a 300 × 300 mm F4B sheet with a dielectric constant of 2.6. The band-pass characteristics are achieved by incorporating an array of circular apertures on one side of the F4B sheet, which has a thickness of d2. The annular aperture width is |r1- r2|, where the outer diameter is r1, and the inner diameter is r2, respectively. Cross DC bias lines are employed on the back of the dielectric substrate as the negative side, while the positive DC biasing is realized from the printed side of the FSS structure. The width of the bias line is t. A hole of diameter t is drilled through the center of each bias line to the center of the circle on the other side of the substrate, and an electrical connection is made by connecting a pin of diameter t through the hole. The optimized parameters of active FSS are: p=30 mm, d2=1 mm, r1=26 mm, r2=25 mm, t=0.5 mm.
Fig. 4. The layout of a meta-atom of active FSS: (a) front side and (b) reverse side. The comparison of (c) reflection curves and (d) transmission curves with or without a DC bias line when PIN diode is in the ON or OFF state.
The working mechanism of the bottom active FSS can be explained by the equivalent circuit model shown in Fig. 5. On the front side, the circular patch inserted with the diode is equivalent to the parallel connection of the diode Zd and the shunt capacitor C1, in series with the inductor L1 of the external metal grid. The dielectric substrate is regarded as a transmission line, and since the length is much shorter than the wavelength of operating frequency in the low-frequency region, it is almost negligible. On the back of the switchable active FSS, the bias line is equivalent to a shunt inductor L2. The PIN diode is modeled according to its datasheet [33]. When the PIN diode is in the ON state, small resistance RON = 1.5 Ω, small inductance Ld = 1.5 nH; When the PIN diode in the OFF state, high capacitance COFF = 0.1 pF. The equivalent impedance of the equivalent circuit can be expressed as follow:
By substituting the equivalent model of the diode in the two states into Eq. (3), the equivalent circuit of the active FSS loaded with PIN diode can be obtained. And the equivalent impedance in the two states can be expressed as follows:
In the ON and OFF state of the PIN diode, the resonant frequency can be calculated as [22]:
It can be seen from the equivalent circuit model that when the diode is in the ON or OFF state, the equivalent capacitance and inductance exists, causing the circuit to resonate. At the resonant frequency, the parallel resonant circuit provides a high impedance (ideally infinite), allowing most EM wave energy to pass through it. Since the equivalent parameters of the diode in the two states are different, the resonance frequency changes with the bias voltage changes. As indicated in Fig. 4(b), one narrowband transmission window with insertion loss of 0.87 dB is observed at 4.34 GHz in the ON state of the PIN diode. And in the OFF state of the PIN diode, another narrowband transmission window with insertion loss of 0.48 dB is observed at 3.06 GHz. Therefore, by changing the biasing state of the diode, the designed active FSS realizes switching in two transmission windows.
DC bias lines have a significant influence on the EM response of active FSS. Figures 4(c), 4(d) shows the effect of the DC bias line on the reflection and transmission characteristics in the two states of the PIN diode, respectively. It can be seen from Fig. 4(c) that when the PIN diode is in the OFF state, the resonant frequency of the active FSS with a DC bias line is 3.06 GHz and the depth is -17.32 dB, while the resonant frequency of the active FSS without a DC bias line is 2.74 GHz, the depth is -39.62 dB. When the PIN diode is in the ON state, the resonant frequency of the active FSS with DC bias line is 4.34 GHz, and the depth is -34.60 dB, while the resonant frequency of the active FSS without DC bias line is 4.34 GHz, the depth of it is -22.80 dB. It can be found that when the PIN diode is in the OFF state, the DC bias line has a slight impact on the resonant frequency and affects the resonant depth. When the PIN diode is in the ON state, the DC bias line has almost no effect on the resonant frequency.
2.3 Analysis of the proposed active AFSS
The proposed active AFSS design has been shown in Fig. 6(a). The active AFSS was simulated using a CST Time Domain Solver with magnetic boundary along x-direction, electric boundary along y-direction, and open boundary condition along z-direction. Whether the diode is turned ON or OFF, the bottom active FSS behaves as a band-pass filter. The parallel resonance makes the bottom plane transparent to the incident EM wave at the resonant frequency. At the two resonant frequency points, ZON and ZOFF both approach infinity. As shown in Figs. 6(c) and 6(d), when the diode is in the OFF state, a passband with an insertion loss of 0.48 dB is observed at f1=3.06 GHz, which can be used as the communication window of the AFSS. When the diode conducts through the current provided by the feed network, the passband at f1 is closed, and a new passband appears with an insertion loss of 0.87 dB at f2=4.34 GHz.
Fig. 6. (a) Schematic of active AFSS. (b) Simulated absorption of the active AFSS for both diode states under the normal incidence of the transverse magnetic (TM) polarized wave. Simulated S parameters of the active AFSS under the normal incidence of the TM polarization: (c) Diode OFF (d) Diode ON.
However, at the high-frequency band (f3≤f ≤ f4), the integrated structure behaves similarly to a traditional EM absorber [Fig. 6(b)], and a simultaneous decrease in reflectance and transmittance can be observed. Furthermore, in both states of the PIN diode, the structure exhibits high reflection (close to 0 dB) and small transmission (less than -10 dB) throughout the stopband frequency. Therefore, by changing the biasing state of the PIN diode, the transmission/reflection band can be switched without affecting the absorption spectrum (6.32-8.36 GHz) of the active AFSS. As the integrated structure is four-fold symmetric, the EM response of the proposed active AFSS is polarization insensitive at normal incidence. In our design, the substrate of the bottom active FSS is F4B material with lossy, and the substrate of the top-layer absorption structure is FR4 material with lossy, so the insertion loss is existing in the two states of the diode.
3. Experimental verification
In order to verify the performance of the active AFSS proposed in this paper, a sample was manufactured and tested, and the upper absorption structure and the lower active FSS are glued together with epoxy resin adhesive. Using standard PCB technology to fabricate the bottom active FSS on a 1 mm thick F4B substrate, NXP’s BAP 70-03 [33] PIN diodes are all mounted on the upper surface of the bottom active FSS using surface mounting technology. The lateral size of the bottom active FSS sample is 300 mm×300 mm, and a total of 400 PIN diodes were soldered in the structure. As shown in Fig. 7(a), the inside of the circular aperture is connected to the cathode of the diode, and the outside is connected to the anode of the diode. The cross-shaped symmetrical negative DC bias line is on the back of the dielectric substrate, while the positive DC bias is realized from the positive printing surface of the active FSS structure. Through this design, the feeding can be made simpler. The bipolar power supply IT6433 is used for power supply, which can control the bias voltage through a computer program during the experiment. The processed metallic blade array size is 300 mm × 300 mm × 20 mm, and the integrated structure is shown in Fig. 7(b).
Fig. 7. Fabricated sample of the active AFSS: (a) front view of active FSS, (b) the integrated structure. The measurement environment of S parameters: (c) S11, (d) S21.
The free space [34] technique is used to measure the fabricated sample, as shown in Figs. 7(c) and 7(d). A pair of broadband horn antennas are connected to a Keysight network analyzer for S11 and S21 measurements. Under normal incidence of the y-polarized wave, the reflectance and transmittance measurement results of the integrated structure are shown in Fig. 8. When all the diodes are turned on simultaneously by providing the required power supply voltage, a passband is observed at f2=4.78 GHz, the reflectivity at f1=2.88 GHz is close to 0 dB, and good absorption characteristic is found between 6.31 GHz and 8.34 GHz. After turning off the voltage, a passband is observed at f1=2.88 GHz, the reflectivity at f2=4.78 GHz is close to 0 dB, and good absorption characteristic is found between 6.33 GHz and 8.37 GHz. The simulation results and the measurement results are reasonably consistent, except for some small deviations and unwanted ripples, which can be attributed to the difference between the actual diode and the diode equivalent circuit model, the limited sample size, the uncertainty of the parasitic component value, and manufacturing tolerances.
4. Conclusion
In conclusion, an active AFSS to offer two switchable transmission or reflection (T/R) bands along with an absorption band at high frequency is proposed. The bottom layer is the active FSS structure, which realizes the switchable characteristic of the diode state by changing the DC voltage applied in the bias network, thus controlling the T/R bands in low frequency. The upper layer uses the plasmonic structure array to directly achieve continuous and efficient absorption at a higher frequency band. Due to the symmetry of the structure, the active AFSS shows stable broadband absorption and two switchable T/R bands at two polarizations. The measurement results are in good agreement with the simulation results, which verifies the design concept proposed in this paper. The design is expected to find applications in radome stealth and diversified active AFSS strategies.
Funding
National Natural Science Foundation of China (61671466, 61671467, 61801509, 61971341, 61971435, 61971437).
Disclosures
The authors declare no conflicts of interest.
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