Interdigital Coupled Compact FSS Reflector for UWB Antenna Gain Enhancement


 A compact UWB FSS reflector is presented based on interdigital structure for gain enhancement of an UWB antenna. An equivalent circuit approach is proposed for the analysis of the FSS reflector. The reflector comprises a 6 × 6 array of unit cell dimension 6 mm × 6 mm and which is very compact. The reflector gives a linear phase response over UWB band. A UWB monopole antenna is designed with a half circular disc structure based on microstrip technology. A maximum of 5 dBi gain enhancement is achieved with this compact FSS reflector when placed at a distance below the antenna. The measured results closely follow the simulated ones which proves feasibility of this design.


I. Introduction
Over the last few years, Ultra-wideband (UWB) technology is found to have potential in impulse radio, communication systems, ground-penetrating radar (GPR) and microwave imaging [1]. The commonly used radiator for UWB antenna system is monopole antenna due to their omnidirectional radiation pattern. However, it suffers from low gain and higher back side radiation. In order to provide better signal to noise ratio (SNR), the main lobe gain is to be much higher as compared to side lobes. The use of FSS re ector is found to be prominent solution for gain enhancement of monopole antennas. FSS having band stop response with linear decaying phase over the UWB frequencies is widely used as re ector of antenna system to minimize back lobe radiation. FSS has found wide space application in radar [2], antenna array [3], and RFID [4].
Multi-layer structures are popular for obtaining UWB frequency response. The use of stack FSS layers with different frequency responses are commonly used technology for UWB re ector design [5][6]. In [6], a UWB FSS re ector based on four-layer structure improves the gain of a slot antenna over the wide band.
The use of multiple layers is also enhanced the design cost and complexity and not suitable for low pro le compact antenna system. Therefore, the double layer and single layer structure is gaining attention by the researcher over the years [7]. In [8], a gain enhancement of maximum 7 dB over UWB frequency is obtained with a dual layer FSS comprises 5 × 13 array of unit cell with dimension of 22.4 mm × 6.5 mm. Recently, metamaterial is used for the design of miniaturized FSS [9]. FSS re ectors based on single layer structure are found to have potential for gain enhancement in a low-pro le compact antenna system [7,[10][11]. In [12], a stop band characteristic of FSS re ector is obtained with ring-based structure. Miniaturized FSS is designed with higher dielectric constant material but it also increases the design cost. This paper presents a compact FSS re ector based on interdigital capacitor for gain enhancement of a UWB monopole antenna. The FSS re ector exhibits a linear phase response with band stop characteristic over UWB frequency.
The unit cell size of FSS is 6 mm × 6 mm, which is 0.06λ_L (corresponding to lowest resonant frequency) and hence is compact. A 6 × 6 array of the unit cell FSS is loaded to a UWB monopole antenna to achieve a gain enhancement of maximum 4.25 dBi as compared to antenna without loading over the UWB range.
The design and theoretical analysis of the proposed work is carried out with CST Microwave Studio. The design is also fabricated and the measured results are compared with the simulated one to validate the work.
This work is represented as follows. Section 2 discusses the design of unit cell, UWB monopole antenna, the FSS re ector loaded UWB antenna and their simulation. In Sect. 3, the prototypes of the proposed design are fabricated and measured, and the test results are compared with the simulated ones. Finally, in Sect. 4, the conclusions of the proposed work are carried out.

Ii. Design And Results
The design of the proposed work is carried out in two steps; rst a UWB monopole antenna is designed with simple half circular metallic patch and then a compact FSS re ector is realized using interdigital coupled resonator to enhance the antenna performance over the wide band.

A) Antenna Design & Results
The UWB antenna can be simply realized by monopole circular disc patch. The motivation of the proposed antenna comes from this but with half circular disc for compact design. The proposed antenna in Fig. 1 is fabricated with a substrate of permittivity 3.2, loss tangent 0.003 and height of 1.6 mm. The ground layer at the back plane is truncated around the feed for impedance matching purpose. Figure 2 depicts the re ection coe cient below − 10 dB is from 2.37 GHz to 10.80 GHz with fractional bandwidth of 129% and hence a good impedance matching over the wideband is achieved.

B) FSS Design & Results
The proposed FSS comprises periodic array of unit cell elements having periodicity of 6 mm×6mm in the vertical and horizontal direction. The top metallic layer is a compact interdigital coupled resonator printed on a dielectric substrate of permittivity 4.4, loss tangent 0.02 and height of 0.5 mm as given in Fig. 3 (a).
The magnitude and phase plot of the FSS (in Fig. 3(b)) depicts that it has linear phase response over the antenna operating frequencies and the re ection coe cient is also maximum. Thus, it is well suited as a re ector for the gain enhancement of the antenna.

C) FSS Re ector with Antenna
The FSS re ector with size equal to the antenna is obtained by constituting 6×6 array of unit cell elements and placed beneath the antenna at a distance h from the substrate and given in Fig. 4. Antenna gain can be enhanced when the EM wave radiated by the antenna and the re ected wave from the FSS are in phase at the antenna interface. Therefore, the following formulae must be satis ed [11].
∅_fss-2βh = 2nπ, n=-∞ to +∞ (1) Where, ∅_fss is the FSS re ection phase; β is the free space propagation constant. The airgap h is calculated as 12 mm. Fig.5 shows the fabricated prototype of the proposed design and its experimental validation is done by comparing with simulated one (in Fig.6). The measured -10 dB re ection coe cient bandwidth for the ref. and proposed design are92.80 % and 84.74 %, respectively. The measured re ection coe cient patterns are closely following the simulated one except extra losses are added due to fabrication. A maximum gain enhancement of 4.25 dBi is obtained with the FSS re ector when placed below the antenna at a distance of 12 mm and given in Fig. 7.
The parametric studies on height (h) on the re ection and gain of the proposed antenna is given in Fig. 8 and Fig. 9, respectively. The result shows that with an increase in h, the return loss performance is improved due to loading effect. Whereas the gain performance decreases with increase in h due to magnitude of re ected signal inversely proportional with the distance between the antenna and re ector. The main parameters of the proposed structure are compared with existing reported designs and tabulated in Table 1 below. In [12], a UWB FSS re ector with unit cell dimension of 13.5× 13.5 mm 2 is proposed to obtain a maximum gain enhancement of 4.5 dBi over the band. But the structure suffers from larger size of 115 × 115 mm 2 . A relatively lesser structure dimension with 3.5 dBi gain enhancement is achieved in [13]. In [14], a compact FSS with unit cell dimension of 11 × 11 mm 2 is able enhance the antenna gain over UWB range. A more compact FSS structure is reported in [15] but with larger structure and lesser gain. The proposed work is much compact as compared to others and also have good performances.

Iii. Conclusion
A compact FSS re ector is proposed for gain enhancement of UWB antennas. The design topology is based on interdigital coupled resonator for miniaturized structure. The structure is able to give maximum gain enhancement of 4.25 dBi over the UWB frequency when placed below a half circular disc radiator. The radiation results clearly demonstrate improvement in gain in the main lobe with narrow bandwidth and reduction of back-lobe levels is due to the addition of FSS re ector. The experimental results validate the design performance over the UWB frequency. This FSS re ector can be applied to any other antennas with some optimization in the design parameters and hence proves novelty of the structure.

Declarations
Funding: None

Con icts of interest/Competing interests: None
Availability of data and material (data transparency): Not Applicable Code availability (software application or custom code): Not Applicable Proposed antenna re ection response Proposed antenna re ection response  Simulated and measured results Figure 7