диафрагмированные волноводные фильтры / 86bf3b87-e888-43d9-ab7a-680f4af80d05

Abstract—In this paper, frequency selective surface (FSS) properties of a uni-planar electromagnetic bandgap (EBG) unit cell are studied. The unit cell consists of meander line inductor and interdigital capacitors on one side of a substrate. Simulation results indicate that the unit cell exhibits passband characteristics centered at 10.04 GHz. FSS property of the structure is verified by measurement using x-band waveguides. The measured results show passband characteristics at 9.45 GHz. A 13 × 13 array of these unit cells (FSS screen) is used as a superstrate at a distance0.5 0 over a patch antenna operating at 10.8 GHz, offset from centre frequency of the FSS passband. Directivity improvement of 6.95 dB is observed along 0 in the measurements of patch antenna with FSS superstrate as compared to the patch antenna without superstrate.

Index Terms—Frequency selective surface (FSS), Directivity enhancement, Patch antenna, Superstrate.

I. INTRODUCTION

Frequency selective surfaces (FSS) are resonant structures having either stopband [1]–[5] or passband [6]–[11] performance, due to which they are widely used as radomes, spatial filters, electromagnetic absorbers and shielding structures. FSS are also used in cavity resonant antenna to enhance the gain of the antenna. Fresnel zone plates are also used to improve focusing thereby achieving high directivity antennas [12], [13]. Many configurations of FSS structures are proposed to enhance the gain. In [1]-[2], FSS made with patch array is used as one reflecting surface to form cavity resonant antenna (CRA) to improve the gain of the patch antenna. In [3], gain bandwidth product is enhanced by using FSS in fabry-perot antenna. In [4], three metallic ring-resonators are used in achieving triple band reflective FSS which is further used to improve gain of the broad band triangular slot antenna. Though in [6]–[11], transmission (passband) type FSS are proposed, they are not used for improving gain of an antenna except in [8], where the gain of the printed dipole antenna is improved by 3 dB.

In this paper, FSS properties of the uni-planar EBG structure (unit cell is depicted in Fig. 1) are investigated. In

TThe authors are with Centre for Applied Research in Electronics (CARE), Indian Institute of Technology (IIT) Delhi, Hauz Khas, New Delhi, INDIA. (email: lkurra@gmail.com, mpjosh@care.iitd.ac.in, ananjan@care.iitd.ac.in, s.k.koul@ieee.org).

our previous works, this structure is used, to realize a bandstop filter [14], [15] and as a coupling structure in bandpass filter and diplexer [16]. With ground plane removed, the same structure exhibits a transmission type FSS. A FSS screen formed with array of these unit cells is used as a superstrate over a patch antenna giving 6.5 dB directivity enhancement which is better than that reported in [8].

II. FSS PROPERTIES OF UNI-PLANAR EBG

The unit cell shown in Fig. 1 is 3.51×3.51 mm2 (0.117λo 0.117 0), which is scaled down version of the unit cell proposed in [14]–[16]. The black colored pattern in Fig. 1 indicates metal on top of a GML 1000 substrate (εr =3.2, loss tangent is 0.004 and thickness 0.762 mm). The FSS characteristics of the unit cell are studied using frequency domain solver in CST Microwave Studio software. Simulation set up is made in CST with unit cell boundary conditions on four sides of the unit cell. Floquet ports are placed at arbitrary distance below and above the unit cell to excite plane waves of transverse electric (TE) and transverse magnetic (TM) modes of polarization. The transmission characteristics |S21| between the two ports for both modes of polarization in Fig. 2, shows that the proposed FSS is polarization independent and transparent to frequency band centered at 10.04 GHz with insertion loss of 0.25 dB and half power (3-dB) bandwidth of 3.02 GHz (30%).

Fig. 1. Schematic of the unit cell (All dimensions are in mm).

Frequency (GHz)

Fig. 2. FSS characteristics of the unit cell for TE and TM mode of polarization.

(a)

(b)

Fig. 3. (a) Structure 'B' (b) Structure 'C' (All dimensions are in mm).

Frequency (GHz)

Fig. 4. Comparison of FSS properties for different unit cell structures.

The selection of transmission frequency band depends on the size of the unit cell and the associated inductors and capacitors. We further studied the effect of inductors and

capacitors of the unit cell. This is done by using three different structures viz. structure 'A' (Fig. 1), structure 'B' and structure 'C' (Fig. 3). Structure ‘B’ is obtained from structure ‘A’ by changing the inductance of the unit cell through meander lines keeping the capacitance same. Structure ‘C’ is obtained from structure ‘A’ by changing the capacitance of the unit cell by adding more fingers to IDC and changing the gap ‘g’ between them. The transmission properties of the three structures are compared in Fig. 4. Structure ‘B’ has less inductance than structure ‘A’ and hence the passband shifts towards higher frequency. Structure ‘C’ has more capacitance than structure ‘A’ and hence the passband shifts towards lower frequency.

Structure ‘A’ is used further to enhance directivity of a microstrip antenna. The angular stability of the structure is also verified in simulations by illuminating structure with TE and TM modes of polarizations for different angle of incidence, they show good stability at the center frequency.

The transmission characteristics are verified by placing the fabricated FSS in between two X-band waveguides which are connected to Agilent PNA E8364C. The measured and simulated results are compared in Fig. 5. Measured results show a 0.6 GHz downward shift as compared to the simulated results. The shift in the experimental results is due to the fact that measurement are done on 13 × 13 array, whereas in simulation, infinite array is considered by using unit cell boundary condition. In addition, over etching during fabrication process makes meander lines in the unit cell to be narrower due to which inductance increases and the frequency shifts down.

III. APPLICATION OF FSS FOR ANTENNA DIRECTIVITY

IMPROVEMENT

FSS resonator antennas are implemented by forming a resonant cavity with a ground plane and FSS superstrate which is acting like partial reflecting surface (PRS) [1], [2]. By placing a radiating element in this cavity, the directivity of the antenna is enhanced. Each time the wave from radiating element strikes the FSS, some part is transmitted and other part is reflected back. The reflected wave bounces back towards the FSS after reflection from the ground plane. If all the waves passing through FSS are coherent, they result in enhanced directivity. The directivity enhancement can be increased if the reflection coefficient of the FSS is more [1],

Frequency (GHz)

Fig. 5. Measured and simulated results of FSS characteristics.

Fig. 6. Photograph of the patch antenna with FSS superstrate. Zoomed view of single cell is shown in the inset in left lower corner. The patch antenna is shown in the top right corner.

therefore a patch antenna operating at 10.8 GHz which is offset from the center frequency of FSS is chosen for directivity enhancement application. A patch antenna is designed and fabricated to operate at 10.8 GHz. 13 × 13 array of FSS unit cell (Fig. 1) is used as a FSS screen, fabricated on a 55 mm×55 mm GML substrate. This FSS screen is used as a superstrate over the antenna supported by spacers as shown in Fig. 6. The distance ‘d’ between the antenna and superstrate is kept at 14.5 mm ( 0.5 λo), which is the optimized in simulation for maximum directivity. Distance 'd' =14.5 mm is also verified with phase calculation. The FSS is simulated with setup given in section II with ports de-embedding to the surface of the structure. The simulated S11 with port deembedding is shown in the Fig. 7. The reflection phase of FSS at 10.8 GHz is 236 . FSS screen is (d) 14.5 mm above patch antenna, phase due to twice the distance at 10.8 GHz is -375 . The substrate thickness of the patch antenna is 0.762 mm with εr=3.2, phase due to twice the thickness at 10.8 GHz is -35 . The reflection phase of ground of patch antenna is -180 . Thus the total phase is -354 close to 2π. Thus a coherent beam emerges out of FSS screen resulting in improved directivity.

The reflection coefficients of the patch antenna with and without FSS superstrate are shown in Fig. 8. The resonant

Frequency (GHz)

Fig. 7. Simulated reflection phase ( S11) of FSS.

Frequency (GHz)

Fig. 8. Measured S11 of patch antenna with and without FSS superstrate.

(b)

Fig. 9. Simulated radiation pattern of the patch antenna, patch antenna with dielectric as superstrate and patch antenna with FSS superstrate operating at 10.8 GHz. (a) E - plane (y-z ) (b) H - plane (x-z plane).

frequency of the patch antenna with superstrate is slightly lower due to loading with superstrate. To identify the exact role of the FSS structure, simulations are carried out with only dielectric as a superstrate over the antenna at same distance from the antenna. The simulated radiation patterns of the reference patch antenna, antenna with dielectric superstrate and antenna with FSS superstrate are shown in Fig. 9. Patterns are normalized with respect to the maximum of antenna with FSS structure. It is seen that the antenna with dielectric superstrate shows hardly any improvement in the directivity as compared with the reference antenna without any superstrate. However, with the proposed FSS there is significant increase in power along broadside. This proves that substrate material itself does not affect the antenna performance.

The measured radiation patterns are shown in Fig. 10. The E- plane and H-plane patterns of antenna with superstrate are normalized with their maximums. The patch antenna patterns are normalized with corresponding maximums of the antenna with superstrate for comparison. The substrate size of the antenna is intentionally taken larger in order to account for

larger ground plane size as is the case in most transceiver systems. As a result, the E-plane beam of the patch antenna is almost flat around the broadside direction with a maximum at 25 , whereas in patch antenna with FSS the maximum is along broadside 0 . The simulated directivity from CST simulation software of the antenna without and with FSS is 7.44 dBi and 14.39 dBi respectively. The directivity is improved by 6.95 dB. Measured directivity calculated numerically from the two measured principle planes patterns of the antenna without and with FSS is 5.6 dB and 13.6 dB. An improvement of 8 dB is seen in the measurement. Because of only 4 samples in azimuthal plane in measured calculations, 1 dB difference is seen in the measured and simulated. From Fig. 10, front to back ratios (FBR's) of the patch antenna are 22 dB and 22.8 dB in H-plane and E-plane respectively. FBR's for patch antenna with FSS are 19.8 dB and 16.2 dB in H-plane and E- plane respectively. FBR's of antenna with FSS has slight more difference, because of the slight alignment error of 3 . With FSS directivity is improved sacrificing the FBR slightly.

A comparison of this work with other reported works is given in table I. Many researchers have used reflective type FSS for directivity enhancement except in [8]. In this work we achieved directivity improvement of 6.95 dB by using transmission type FSS.

TABLE I

COMPARISON TABLE

IV. CONCLUSION

The FSS characteristics of the proposed unit cell are studied using simulations and verified with measurement. It is found that it exhibits passband behavior. This screen is then used as a superstrate over a simple patch antenna forming a resonant antenna to enhance the antenna directivity. It is observed that with the proposed FSS superstrate, antenna directivity is enhanced by about 6.95 dB along broadside direction.

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