The design of the proposed work is carried out in two steps; first a UWB monopole antenna is designed with simple half circular metallic patch and then a compact FSS reflector 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 reflection coefficient 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 reflection coefficient is also maximum. Thus, it is well suited as a reflector for the gain enhancement of the antenna.
C) FSS Reflector with Antenna
The FSS reflector 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 reflected wave from the FSS are in phase at the antenna interface. Therefore, the following formulae must be satisfied [11].
∅_fss-2βh = 2nπ, n=-∞ to +∞ (1)
Where, ∅_fss is the FSS reflection 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 reflection coefficient bandwidth for the ref. and proposed design are92.80 % and 84.74 %, respectively. The measured reflection coefficient 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 reflector when placed below the antenna at a distance of 12 mm and given in Fig. 7.
The parametric studies on height (h) on the reflection 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 reflected signal inversely proportional with the distance between the antenna and reflector.
Figure 9 illustrates the radiation patterns of the antenna (ref.) and antenna with FSS (prop.) structures on the two orthogonal planes (E and H). The results clearly demonstrate the improvement with loading of FSS reflector to the antenna. The improvement in gain in the main lobe with narrow bandwidth and reduction of back-lobe levels is due to the addition of FSS reflector.
The main parameters of the proposed structure are compared with existing reported designs and tabulated in Table 1 below. In [12], a UWB FSS reflector with unit cell dimension of 13.5× 13.5 mm2 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 mm2. 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 mm2 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.
Table 1
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Ref.
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Structure dimensions (mm2)
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FSS unit cell size (mm2)
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Bandwidth (GHz)
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Max gain enhanced (dBi)
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[12]
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115 × 115
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13.5× 13.5
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3.0–11
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4.5
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[13]
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84 × 84
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14 × 14
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4.7–14.9
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3.5
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[14]
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33 × 33
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11 × 11
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3.8–10.6
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3.5
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[15]
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82.5 × 82.5
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8.25× 8.25
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2.5–11.0
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3.0
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|
This work
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36 × 36
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6 × 6
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2.5–10.8
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4.25
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