Switchable Wideband Metamaterial Absorber and AMC reflector for X-band Applications and Operations

A single layered metamaterial structure with capabilities of switching from a wideband metamaterial absorber to an AMC reflector and vice versa is presented in this paper. A flame retardant 4 substrate with physical thickness of 1.60mm was used. The absorption rate, reflection rate, reflection phase and surface current distribution were studied and discussed. The operational incidental wave angles were varied from 0 o to 65 o . A peak reflection of about 90% was achieved at 11.20 GHz with a usable bandwidth (-90 to +90) of 3.01 GHz by the AMC reflector. The metamaterial absorber demonstrated a wideband performance (from 8.10 GHz to 14.30 GHz). It achieved 100% absorption at 11.20 GHz and not less than 65% absorption across the entire X-band frequencies except for incidental wave angles above 55 o .


Introduction
Metamaterials (MTM) are said to be materials or structures that do not exist in nature. They are purposely or artificially engineered to exhibit properties not found in nature. These properties are obtained by manipulating the substances of structures to yield desired outcome [1]. A lot has been said with the emergence MTM and theories and concept behind bizarre properties of metamaterials MTM have long been established [2]. It is evidently proven that manipulating the substances yields unusual properties which are desired for certain applications such as EM filters, cloaking, low profile ground plane, sensing, reflectors, focus antenna beam, phase shifting [3] etc.
In general, Electromagnetic waves (EMW) absorbers are structures that absorbs incidental electromagnetic waves. They are designed to minimize reflection and transmission by maximizing energy loss within the structure [4]. Electromagnetic wave reflectors or Artificial magnetic conductor (AMC)s are structure purposely designed with unusual boundary conditions to be selective in supporting surface wave currents. [5].
Conventional electromagnetic waves absorbers such as Salisbury [6] and Dallenbach absorbers [7] in existence have limitations of being electrically thick, narrowband and can only operate in quarter wavelength [8,9]. similarly, conventional metallic conductors and perfect electric conductor (PEC) used for antenna ground planes have their drawbacks of reversal or out of phase image currents and propagation of surface current which is radiation caused by infinite ground plane. AMCs counter these drawbacks and even exhibit the ability to reduce back-radiation as well as increase gain. [3].
These new MTM based EMW absorbers and reflectors have high potentials for applications such as cloaking [7,8], solar cells [9] thermal emission [10,11], photo detector [12], radar imaging [13], antennas and tagged antenna for RFID [15], etc. AMC or electromagnetic bandgap (EBG) is a structure designed with unusual boundary conditions so as to be selective in supporting surface wave currents. In modern antennas, AMC's are used to replace the perfect electric conductor (PEC) ground plane, this is simply because of the AMC's ability to reduce back-radiation as well as increase gain. [3]. One of the most importance things about AMC is its reflection angle, the structure is only considered AMC when its reflection phase is -180 0 to 180 0 and resonates at 0 0 . It operates as an AMC at the point where the resonance frequency crosses 0 0 . The useful bandwidth of an AMC is at -90 to 90 on either side of the resonance or central frequency.

Proposed design and simulation
The proposed MTM structure is patched on an FR4 substrate with a dielectric constant of 4.6, loss tangent of 0.019 and height of 1.60mm as shown in Figure 1. The structure consists of four square patches (P1-P4), two switches (S1 & S2), a long bar patch at the center and is full copper ground plane. The switches are used in selecting operation mode i.e either wideband MTM absorber mode or AMC reflector mode. The unit cell of the proposed structure measures 6.70mm by 5.90mm. The sizes of square shapes are given as P1=P3 measures 2.88mm by 2.60mm and P2=P4 measures 2.68mm by 2.70mm.
When both switches (S1 & S2) are closed, the structure operates as an AMC reflector while when switch S1 is open and S2 closed, it functions as a wideband Absorber. The structure is designed to operate at resonance frequency of 11.20 GHz. . The performance based on reflection and absorption magnitude, reflection phase and surface current were studied and reported.

Absorbance Magnitude for Different IWAs (TE and TM Mode)
In this section, the structure was simulated in "wideband absorber" mode. In order to switch to this mode, switch S1 is turned off while switch S2 is turned on. The TE mode simulation demonstrated three jointed resonances at 9.02 GHz, 11.20 GHz and 12.70GHz thus making it a wideband. At these three resonances, it achieved 100% absorbance. It also achieved above 92% absorbance for all frequencies bands within the X-band frequency range (8GHz -12GHz) except for IWA greater than 54 o . It is noted that at larger angles (55  o & 65   o   ), the absorbance at resonance 11.20 GHz dropped to as low as 73.30% and 55.20% respectively.
On the other side, with respect to the TM mode simulations, similar results were obtained. At the TM mode, the three jointed resonances achieved slightly less than 100% , the absorbance at resonance 11.20 GHz dropped to as low as 66.70% and 53.60% respectively. The absorber has a total bandwidth of 6.20 GHz. This is shown in Figures 2a and 2b.

AMCs reflection phase for Different IWAs (TE and TM Mode)
As stated earlier, for a structure to qualify or perform as an AMC reflector, it must be able to operate zero degrees phase wise. This is because for at zero phase, a constructive interference is established while at other phases a destructive interference is established. Thus, the phase is really important as well as critical. At zero-degree phase it has a periodic repetition of -180 o to 180 o but the usable bandwidth is only take from -90 o to +90. Putting that into consideration, it is safe to state that the reflector has a usable bandwidth of 3.01 GHz.
Here, the structure was simulated for different IWA just like in the absorber's case. For TE mode, it was noted that, for the first four angles

AMCs reflection Magnitude for Different IWAs (TE and TM Mode)
Similar simulation was carried out here but this time, it is for the magnitude of the reflection at various IWA. It was observed that for both TE and TM mode there is shift in the resonance frequency as well as deterioration in reflection magnitude. With respect to TE mode, the shift witnessed in angles (0  With respect TM mode simulation, the scenario and results looks similar except that, for the shift in resonance frequency is to the left. This is to say as the IWA is increased, there is reduction in resonance frequency as well as reduction in reflection magnitude. This is shown in Figures 4a and 4b. , it is evidently shown that the current distribution is more widely spread. This can be validated by the shift in resonance frequency and reduction is absorption rate. This is shown in Figure 5a to 5f. In order to have better understanding of the entire structure, the AMC structure was simulated and the current activities/ behavior was observed. The same IWAs were chosen as that of the absorbers surface current. For IWA 0 o and IWA 25 o and it was noted that for the TE mode, the current is concentrated around the two Switches and the center bar. Also, there is concentration in the gap between P1 & P2 and P3 & P4 and very little concentration at the edges.
The scenario is similar for the TM mode also, but this time, there is no current activity or less concentration in between P1 & P2. This is shown in Figures 6a-6d. For IWA 65 o , the surface current distribution is widely spread across the structure but this is expected as there is considerably large shift in the resonance frequency. This is show in Figures 6e and 6f.

Conclusion
In this report a structure capable of switching between wideband MTM absorber and an AMC reflector was designed and presented. It was simulated for various IWAs (0 ) and its performance was tested. Additionally, the surface current and phase were observed. It resonated at 11.20GHz and demonstrated 100% absorption with a bandwidth of 6.20GHz as an absorber. Similarly, the structure demonstrated up to 90% reflection with a usable bandwidth of 3.01GHz as an AMC reflector. Furthermore, as the IWAs are increased to above 54 o , the performance dropped to around 65% and there was significant shift in the resonance frequency.