Novel Angular and Polarization Independent Band-Stop Frequency Selective Surface for Ultra-Wide Band Applications

A novel compact band-stop frequency selective surface (FSS) with angular and polarization stability performance for ultra-wide band (UWB) applications is presented in this paper. The designed unit cell consists of a square loop element and a crossed dipole with a ring slot element. The novel unit cell size is miniaturized to 0.047λ × 0.047λ, where λ is free-space wavelength corresponding to the lowest frequency of the UWB band. The –3dB bandwidth of the proposed FSS is between 3.05 GHz and 10.73 GHz frequencies which covers the whole UWB band as defined by FCC. Due to compact size of unit cell, the proposed FSS has good angular stability up to 60° incident angles both perpendicular (TE) and parallel (TM) polarization. The maximum resonance frequency deviation is 1.75% for TE polarization. In addition, the proposed FSS has excellent stable resonant frequency. The designed FSS is fabricated and experimental measurements are performed. There is a good consistency between numerical simulations and measurement results.


Introduction
Frequency selective surfaces are essentially two-dimensional infinite periodic structures [1].In recent years, frequency selective surfaces have become an interesting subject for many researchers.Frequency selective surfaces can be utilized to filter some specific frequencies in civil or military applications aiming to improve the front-to-back ratio of the antennas, reduce the out-of-band RCS of the antennas, shield the electromagnetic wave and suppress the interference from other communication systems [3][4][5][6][7][8][9][10][11].It is difficult to design band-stop FSS with ultra-wideband angularly stable and sharp roll-off transition filtering response.The response of FSS is not only dependent on frequency, but also, it is affected by the electromagnetic wave polarization and incidence of angle.
Since the US Federal Communications Commission (FCC) released the frequency range of 3.1 GHz to 10.6 GHz for unlicensed usage in 2002, UWB communication technology has become very popular among researchers.In UWB communication systems, ultra-short pulses are used for transmission and reception, and the spectrum spreads to a very wide frequency range [2].Thus, a very large bandwidth is required for UWB applications.UWB systems with low power spectral density can use the frequency spectrum reserved for other communication systems without interference.For this reason, various UWB microwave components have been studied especially UWB filters like FSSs.
Many UWB-FSS studies are proposed in the literature.Multilayer dielectric substrates are used to obtain a broad bandwidth and a sharper filtering response.This causes a bulky structure and an increase of fabrication cost.In [12], dual layered FSS with low profile is designed for UWB antenna reflector.Measurements exhibit that proposed FSS transmission curve is under -10 dB between 3.5 GHz and 11.45 GHz frequency range.In [13], a fourlayered UWB band stop FSS is presented.At each layer, the unit cell contains different length of square loop to obtain broad bandwidth from 10 GHz to 16.1 GHz.UWB-FSS design exhibiting band stop filter from 2.5 GHz to 14 GHz is presented in [14].The UWB-FSS unit cell is placed between two dielectric substrates.It is angularly stable up to 40° incident angle for TE and TM polarization.There are many studies about single layer UWB band stop FSS.Most of them are broadband, but they are not suitable for bandwidth defined as FCC UWB standards exactly.In [15], the unit cell consisting of cross dipole and ring metallic shapes is designed for shielding over UWB.According to the transmission response of FSS, the shielding bandwidth is between 6.5 GHz to 14 GHz for 20 dB attenuation.In [16], the unit cell consisting of a square loop and a cross dipole printed on one side of a single layer substrate is proposed for UWB applications.This FSS has band-stop filter response over frequency range from 7 GHz to 14 GHz.In many researches, the single layer UWB FSS techniques with unit cell printed on two sides of the substrate are investigated.The unit cell printed on both sides of a substrate is used, the sharp roll-off transition filter response can be obtained.However, it is difficult to achieve flat top filtering response at the out of band.In [17], a novel band stop FSS is presented and its unit cell consists of two metallic spirals with different lengths printed on both sides of a dielectric substrate.It has a stable transmission response with respect to various incident angles.In [18], a miniaturized FSS with wide stop band characteristics is presented and its unit cell consists of garland shape etched on both two sides of a dielectric substrate for UWB applications.For different angle of incidences, angularly stable frequency response is obtained for both TE and TM polarization modes.Therefore, it is aimed for designing a novel band stop FSS that has angularly stable frequency response through the whole UWB range.
In this paper, a novel single layer band-stop UWB-FSS is presented.Compared to other studies in the literature, it has the smallest unit cell with single layer design so far.To operate over ultra-wide band, a combination of a square loop, a cross dipole and ring slot shapes has been used.The designed FSS is printed on a single side of Arlon AD600 substrate.The FSS is designed to exhibit the stable angular transmission response for both TE and TM polarizations of incident angle up to 60° under -3-dB transmission.

Designed FSS Configuration
There are four main types of geometrical forms of the FSS unit cell exhibiting their own frequency response characteristics as shown in Fig. 1 [1].The combination of these unit cells can be used to generate new elements with a range of specific properties, such as elements for multiband FSSs, polarization independent FSSs and miniaturized element, and so on.
In this study, a new compact single layer band stop FSS with angular and polarization stability performance in the entire UWB is designed.The designed single layer FSS with broad bandwidth is the most challenging subject at the design process.A multi-layer FSS structure can be useful to obtain a broadband response.However, using of a multilayer structure results in an increase of the size and fabrication costs of the FSS.To change geometrical form of the unit cell, to reduce the inter-element spacing and to increase the thickness of dielectric layer can be used to construct a broadband single layer FSS filter.A new unit cell can be constructed by using the resonant elements and/or non-resonant elements to form a broadband FSS.To obtain an angularly stable and broadband response, FSS consisting of two resonant elements such as a square loop and a crossed dipole with a ring slot is proposed (Fig. 2).
The proposed design is set to cover the ultra-wideband (UWB) frequency range of 3.1-10.6GHz approved by the Federal Communications Commission (FCC).When a square loop element is used alone, a bandwidth larger than the range of 3.1-10.6GHz is obtained.In other words, when this type unit cell is used, the waves outside this frequency range are also blocked.In this study, it is aimed   to design a FSS that will block only the UWB extending from 3.1 GHz to 10.6 GHz.Two resonant elements have been suitably integrated to form a new element in order to set the bandwidth to the range of 3.1−10.6GHz.
The outer square loop dimension determines the lower cut-off frequency of the FSS.The cross dipole with ring slot element added to the square loop causes the resonant frequency shift towards lower frequency side.According to transmission line theory, band-stop FSS is modeled as a serial LC circuit when it is illuminated by electromagnetic wave.The resonant frequency of the FSS is f r = 1/[2π(LC) ½ ].L represents the inductance of the metallic strip, and its value is proportional to the length of the metal strips.C represents the capacitance between the slot of metallic strips.When a square loop FSS is illuminated by the electromagnetic wave, it acts as an inductive component in series with a capacitive component.Adding cross dipole element to unit cell geometry means adding an in-ductor to the equivalent circuit.The resonant frequency shifts toward higher frequency region.Forming of a slot on the cross dipole means adding of a capacitor to the FSS equivalent circuit.So forming of a slot on the cross dipole causes resonant frequency shifting towards lower frequency region.So that it is possible to design the FSS with the desired frequency response by suitably combining the different geometric shapes.
Physical dimensions of the unit cell used in this study are illustrated in Fig. 4(a).Figure 4(b) shows the whole structure of the FSS array.While the conductive surface is shown with a dark colored region, the other region shows a dielectric substrate.The geometrical symmetric nature will provide polarization independent operation.Table 1 gives dimensions of the proposed geometry.
The proposed structure is a simple single layer FSS printed on a substrate Arlon AD600 with a thickness h = 0.508 mm and a relative permittivity  r = 6.15 without a ground plane.Additionally, the dimension of the overall unit cell is 4.6 × 4.6 mm 2 .The designed unit cell is very compact in size.The size of the unit cell is 0.047λ × 0.047λ, where λ is free-space wavelength corresponding to the lowest frequency of the UWB band.The FSS should exhibit stable responses for larger incidence of angles.In this study, the purpose of using the miniaturization method is to eliminate the angular dependencies.Miniaturization of the unit cell removes the sensitivity of incidence angle variations.Based on Munk's FSS designing theory, the angular stability of the FSS is affected both the inter-element spacing and shape of the unit cell element.In general, large inter-element spacing leads to occurrence of grating lobes.Although small inter element spacing can improve the angular stability of FSSs, this effect is limited.Naturally, some types of elements have better angularly stable response like as cross dipole element.To increase the angular stability of the FSS with respect to the incident angle, the dimension of the unit cell is further reduced.Usually, the unit cell dimension should be a quarter of the wavelength at the resonance frequency for square loop geometry.In practice, FSS has a finite size and in order to achieve a desired frequency response, the finite FSS surface must contain sufficient numbers of element to act like an infinite FSS.Thus, the overall size of the FSS surface is large.To overcome this problem, miniaturized element FSSs are suggested [20], [21].
Since D is the unit cell length, to prevent grating lobes, the following equation [1] should be satisfied.
where  0 is the wavelength in the free space and  0 is the incident angle.As the incident angle increases the unit cell length D should be smaller.For 60° incident angle, D should be smaller than 21.15 mm at resonant frequency.
Utilizing MATLAB code given in study [19], the optimal transfer coefficient and phase graphs by using S21 data of FSS (amplitude, phase, real and imaginary parts) via CST MWS is generated.The comparison of transmission coefficient, phase simulation graphs with the optimal transfer function and locations of zeros and poles of the transfer function for the proposed FSS design are shown in   Fig. 5.The optimal transfer function for the proposed FSS structure is obtained when the number of poles-zeros is equal to 6 as shown in Fig. 5.The estimated transfer function given in (2) has almost the same characteristic with CST result.

Oblique Incidence Simulation Results for TE&TM Polarization
The simulation of the transmission responses is performed using the CST Microwave studio under the unit cell boundary conditions and Floquet modes.Conducting surfaces of the unit cell geometry are modelled as PEC (Perfect electric conductor) with zero thickness for simulation simplicity.Many parametric studies were simulated to achieve final unit cell geometry.Firstly, the numerical simulations have been implemented to investigate transmission response with different polarization modes respect to normal of the surface.The simulation of the transmission response is shown in Fig. 6 for TE and TM polarization modes.As it can be seen, the designed FSS provides stop band characteristics for a wide-band ranging 3.05 GHz and 10.73 GHz and has 7.5 GHz resonant frequency.Also, the design FSS demonstrates identical transmission response for both TE and TM polarization because of its symmetrical geometry.
The simulations have been implemented for oblique incident angles up to 60° both TE and TM polarization mode shown in Fig. 7(a) and Fig. 7(b).It is clearly understood that the proposed FSS behaves like as a stable band stop filter for oblique incidence.The transmission coefficients of designed FSS have a stable performance over the entire UWB frequency region for TE polarization.For TE polarization, the resonant frequency is remained almost the same under different oblique incidence and the bandwidth has increased.The fractional bandwidth is more than 100% with respect to resonant frequency.The increase in the angle of incidence wave causes the bandwidth to decrease for TM polarization.In all angles of incidence, the narrowest bandwidth is obtained for 60° incident angles.Detailed data that contain the entire bandwidth analysis are given in Tab. 2. The designed FSS exhibits a stable resonant frequency at about 7.6 GHz as the angle of incidence ranging from 0 to 60 degrees.In this table f L , f R and f H represent respectively lower cut-off frequency, resonant frequency and higher cut-off frequency.The designed FSS provides stable resonance frequency with different polarization and angles.Resonant frequency deviation values are presented in Tab. 3.
When the deviation values are compared to frequency for normal incidence both TE and TM polarization, it is observed that maximum amount of relative deviation is where f r is the resonance frequency, f ang is the angular frequency, and Δf is the relative frequency deviation.
Surface current distribution diagram at 7.6 GHz is shown in Fig. 8.As seen from the figure, the current distributions are stronger on the horizontal arm of the cross dipole element in TE mode and the vertical arm of the cross dipole is active in TM mode.

Fabrication and Measurement Results
To validate the simulation results, designed FSS structure was fabricated and tested as shown in Fig. 9.The prototype of FSS was printed on single side of Arlon D600 substrate with 0.508 mm thickness.The FSS array consists of 43 × 61 element that its are approximately 297 mm × 217 mm.Experimental measurements are performed using two identical broad band waveguide horn antennas in a free space measurement setup at 3-meter anechoic chamber with 900 MHz-20 GHz frequency range.The broad band waveguide horn antennas operate from 1 GHz to 18 GHz with a nominal 11 dB gain.They have 244 mm × 164 mm slot and 204 mm taper length.UWB horn   Simulation data and measurement results were compared at each oblique angle incidence for both polarizations.With 20° and 60° incidence angle, comparison of the transmission coefficients simulation data and measurement results are as illustrated at Fig. 11(a) and (b) There is a good agreement between simulation data and measurement results.
There is a comparison table which compares the presented work with the earlier published similarly works (Tab.4).As seen from Tab. 4, this FSS provides an angular stability up to 60° with only one side layer.

Conclusion
In this study, a novel miniaturized frequency selective surface with ultra-wide stop band characteristics is presented.The -3dB bandwidth of the proposed FSS is between 3.05 GHz and 10.73 GHz frequencies and covers the whole UWB band as defined by FCC.Utilizing miniaturization, the stable resonant frequency and angularly stable transmission response is obtained under oblique incidence up to 60°.The presented FSS is a good candidate for UWB filtering applications.

Fig. 2 .
Fig. 2. Design steps: (a) Square loop element of the FSS, (b) crossed dipole with ring slot element of the FSS, (c) geometry of the proposed FSS unit cell.

Fig. 3 .
Fig. 3. Transmission behaviors of the square loop and the designed FSS at normal incidence.

Fig. 4 .
Fig. 4. Transmission behaviors of the square loop and the designed FSS at normal incidence.

Fig. 5 .
Fig. 5. CST analysis and estimated transfer functions comparison: a) transmission coefficients, (b) phase graphs, (c) locations of zeros and poles of the transfer function for the proposed FSS design.

Fig. 8 .
Fig. 8. Surface current distribution diagram at 7.6 GHz.1.75%, for 60° TE-polarized waves.Relative deviation is calculated as r a n g r 100 f f f f
Oblique incidence bandwidth analysis for TE and TM polarization.
Results of comparıson to other FSSs.