Effects of an FSS Reflector on a Microstrip Line Fed Bow-Tie Slot Antenna

This paper describes the analysis of the effects of a frequency selective surface (FSS) on the characteristics of a microstrip line fed antenna with a bow-tie slot in the ground plane for operation in the frequency range 1 to 10 GHz. The use of the FSS is proposed to miniaturize the antenna structure and increase the number of resonance frequencies and the gain. Air gaps can be inserted between the antenna and the FSS to allow for the adjustment of the resonance frequencies and increase the bandwidth. To prove the concept, antenna prototypes printed on a 30×30 mm2 FR4 low-cost substrate are designed, fabricated and tested. Good agreement is obtained between numerical simulations and experimental results, thus validating the proposed design procedure. The obtained miniaturization and multiband effects can be most convenient for multiservice wireless system applications such as WLAN, WiMAX, 4G, and 5G mobile communication systems.


III.
BOW-TIE SLOT ANTENNA Different shapes of microstrip line fed slots in the ground plane have been proposed mostly to increase impedance bandwidth or provide miniaturization [13], [14], [17]. A very common shape is the bow-tie slot antenna shown in Fig. 3. It keeps the simplicity of the rectangular slot and provides a performance enhancement similar to more complex shapes. The geometrical parameters of this slot are the same of the rectangular one, plus W3, the width of the slot at the ends. To evaluate the size reduction and other changes in the antenna performance, the S11 and radiation pattern were simulated using HFSS and compared to those of the rectangular slot antenna designed in the previous section. The width of the bow-tie slot at the ends, W3, was varied from 6 mm to 15 mm in steps of 3 mm.
The other dimensions were the same of the rectangular slot. The S11 HFSS simulation results are shown in Fig. 4. It can be verified that the increase of the slot width leads to a decrease of the resonance frequency which corresponds to a miniaturization effect. As expected, a decrease of the impedance bandwidth is also obtained. Instead of miniaturization, increase of the bandwidth could have been obtained if the resonance frequency were kept constant by changing other antenna parameters, mainly L2.   Table I compares the resonance frequency, bandwidth, S11 level at resonance, and size reduction rates of   the proposed bow-tie slot antenna. The size reduction values are calculated from the decrease in resonance frequency. It is important to note that the resonance frequency of the proposed bow-tie slot antenna can be 1.6 GHz lower than that of the rectangular one. These results show that the proposed antenna design procedure successfully reduced the size. Figs. 5 and 6 present a 2D and the 3D radiation patterns, and the current distribution at 4.2 GHz for the antenna featured in Fig. 3 (W 3 = 6 mm). A gain of 3.66 dBi is observed.

IV. BOW-TIE SLOT ANTENNA WITH FSS
Many different techniques have been proposed to improve the parameters of antennas used in specific applications. The FSS is one of them, and has been widely used in modern antenna designs owing to its useful properties [14]- [16]. Periodic configurations such as a series of periodically perforated screens or conducting strips that are either printed on dielectric substrates or free-standing, exhibit FSS characteristics.
The FSS surface can be used with different purposes, such as minimize the negative effects of the surface waves. However, the presence of the FSS near the radiator changes its resonance frequency, which can be controlled in the design mostly by adjusting the dimensions of the element [17], [18].
The FSS structure shown in Fig. 7 is used to optimize the bandwidth and gain of the bow-tie slot antenna ( Fig. 3). The methodology used to calculate the slot dimensions and spacing between them is determined through a simulation in HFSS using the Finite-Difference Time-Domain (FDTD) method [19]. As the FSS structure acts as a filter that only allows certain frequencies to be transmitted, the optimal dimensions to obtain the envisaged goal can be found from the observation of the reflection coefficient diagram.
The FSS structure used in the design consists of a matrix of five rows and five columns of square slots, as depicted in Fig. 7. Both the antenna and the FSS have simple structures and are printed on the same substrate. Aiming at the best configuration, the width and length of the FSS square slots (WU=LU) were varied from 0 to 4.5 mm in steps of 1.5 mm. The FSS is used overlapping the antenna under analysis, as shown in Fig. 8. Initially, we have analyzed the input reflection coefficient of the configurations presented in Fig. 1 and Fig. 3. The corresponding results are presented in Fig. 4. It can be noted that, between 1 and 10 GHz, the bow-tie slot antenna without FSS has only one resonance (at 4.2 GHz for W3=6 mm). However, from the results shown in Fig. 9, for the bow-tie antenna plus FSS, it can be concluded that the use of the FSS provides miniaturization (the first resonance decreased to 2 GHz) and introduces additional resonances (at about 5.0, 7.2, 7.7, and 8.9 GHz). This is the best performance obtained and corresponds to WU = LU = 3 mm.      Fig. 15. The agreement between the simulated and measured results is very good for the entire frequency band, which goes up to 10 GHz.    Fig. 16). The presence of the structure did not       and Fig. 17 (at 5.00 GHz). Measurements were performed for both E-plane and H-plane and the higher gain values happen at angles between 30 and 120, mainly due to the slot in the ground plane. The maximum values found for these antennas are 2.59 dBi, in Fig. 19(a), and 5.66 dBi, in Fig. 19(b), confirming that the inserted FSS has increased the antenna gain.  Fig. 19. E-plane (YZ) and H-plane (XZ) measured radiation pattern results for the antennas shown in: (a) Fig. 14 and (b) Fig. 17. In order to increase the degrees of freedom for the frequency adjustment, air gaps were inserted between the antenna and the FSS structure, as can be seen in Fig. 20.        For the FSS structure presented in Fig. 20 when g = 3.14 mm (FSSN3), the 2D and 3D radiation patterns, and current distribution at 5.03 GHz are shown in Fig. 25, 26, and 27, respectively. The characteristics of this configuration are remarkably similar to those of the FSSN2 structure, with a small increase in the antenna gain, which is now 4.70 dBi.