1 Introduction

The Ultra-Wideband (UWB) systems have gained enormous importance in the past two decades. It is implemented for both academic and industrial communities of the telecommunication applications. The Federal Communications Commission (FCC) has declared an impedance bandwidth of 7.5 GHz, i.e., from 3.1 GHz to 10.6 GHz in 2002 [1, 2]. There are several advantages of UWB communication systems as compared to others, which makes them, an excellent candidate by giving an effective solution for wireless applications.

On the other hand, dielectric resonators (DR) have been applied in the microwave circuits design such as oscillators and filters due to their high Q-factor characteristic. Moreover, it is found that the DR with low dielectric constant can be used in antenna design as a radiating element because of their low radiating Q factor. Since the last two decades, dielectric resonator antennas (DRA) have received agreeable consideration for UWB antennas due to remarkable characteristics such as simple excitation mechanisms, high radiation efficiency, nearly constant gain, compact antenna size, wider bandwidth, and ease of excitation [3, 4]. It also has the advantages of no conductive and surface wave losses. Among various shapes of the reported DRA, the rectangular DRA has received much attention due to its mechanical simplicity [5]. Also, it provides more degree of freedom in the parametric selection. The solid DRA was exclusively studied in the early years until the hollow DRA was proposed in 1993 [6,7,8]. By introducing the hollow region, the effective dielectric constant and Q-factor of the DRA can be decreased, which further enhances the impedance bandwidth and gain of the antenna.

In this article, a hollow DR exited by transformer type microstrip feedline covering 3.28 to 10.4 GHz of the UWB band is proposed. It is used Rogers RT 5880 as substrate material, and partial ground having a Defective ground structure (DGS) printed on the bottom side of the substrate. Further, the two metallic rectangular strips on the bottom layer to provide stable gain and return loss of the designed antenna. Further, the antenna near field is defined by existing modes TE11δ at 3.8 GHz, TE22δ at 6.4 GHz, TE z231 at 8.8 GHz, and TE33δ at 10 GHz. The first mode is dominant, and other threes are higher-order modes to support the wide impedance bandwidth.

2 Antenna configuration

Figures 1A, B show the simulated and fabricated view of the proposed antenna. Inspired by the hollow-shaped DRA in [7, 8] is used for wideband application. A cylindrical shaped DR having a radius of D1 = 6 mm is cut out from rectangular DR for enhancement of both gain and impedance bandwidth of the antenna, VSWR and gain of the antenna are shown by figure 2. The DR having Al2O3 with εdr = 9.8, height hdr = 4 mm, excited by a quarter-wave transformer type fed printed on top of the substrate (Rogers RT 5880, εs = 2.2, height hs = 0.8 mm) and bottom layer of the substrate has a partial ground plane with a rectangular-shaped DGS and two metallic strip M2×M1 are provided stable S11 of the antenna within the UWB range. All optimized dimensions of the DRA are presented in table 1.

Figure 1
figure 1

Antenna simulated and a fabricated photograph of proposed hollow antenna. A. Top view and B. back view.

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Figure 2
figure 2

Effect of Hollow DR antenna A. VSWR, B. Gain.

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Table 1 Optimized parameters of the proposed DRA.
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3 Simulated and experimental results

The concept of hollow DRA for the UWB range is validated in the proposed design. The proposed antenna has a dimension of 50×40×4.87 mm3 simulated and fabricated. Figure 2 shows the simulated parametric results of hollow DR and the optimized value of D1 = 6 mm. If hollow diameter increases than antenna gain are improved. The hollow DR directly affects the impedance bandwidth and gain of the antenna, shown in figures 2A, B.

3.1 DGS of antenna evolution steps with parametric sweep

The three structure of DRA is presented in figure 3A as Ant.1, Ant.2, and Ant.3 respectively. All antennas are used for wideband applications, Ant.1 have without DR and show the parametric sweep of D6, shown in figure 3B Ant.1. It has optimized the value of D6 = 9.2 (−5.8 to 3.4). Ant.2 has a DR with DGS slot D6, shown in figure 3B. This antenna gives a frequency range from 3.6 to 9.6 GHz that is wider than Ant.2. Due to the effect of DR placed on top of the substrate layer. So, it can be observed that DR is given widen impedance bandwidth. Ant.3 has optimized dimension G2 to achieve the UWB region. The parametric sweep applies on D2 and achieves the best value 23.4 (−20 to 3.4), at this value DRA is given UWB range from 3.28 to 10.4 GHz, shown in figure 3B Ant.3.

Figure 3
figure 3

A. Antenna DGS evolution steps. B. S11 of Ant.1, Ant.2, and Ant.3.

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3.2 Surface current distribution

The current distribution of the metallic part of the antenna is presented in figures 4A-D. The rotation of the current is observed from the surface current distribution on the ground plane as shown in figure and also, it can be observed that the decoupling structure provided the maximum surface current on the edges of the decoupling rectangular slot with different frequencies. At frequency 3.8 GHz, current directions on the x-axis shown in figure 4A. Figure 4B shows the current cancelled with a single resonance path and figure 4C-D show multiple places in ground current canceled and provide multiple resonance path with supporting of higher-order modes.

Figure 4
figure 4

Surface current distribution at A 3.8 GHz, B 6.4 GHz, C 8.8 GHz, D 10 GHz.

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3.3 Modes and E-field distribution

Figure 5 shows the real and imaginary part of the impedance of the proposed UWB DRA. It is observed from figure that four maximum peaks with real values of impedance and touching the same at zero values of imaginary impedance at the frequencies of 3.8, 6.4, 8.8, and 10 GHz in the desired frequency band. To gain insight into the resonant modes at these four frequencies [9], the electric field distribution [10,11,12,13,14,15,16] model is depicted in figures 6A-L.

Figure 5
figure 5

Simulated Antenna impedance plot.

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Figure 6
figure 6

Near field distribution on hollow shaped DR at 3.8 GHz TE11δ A. X-Y plane B. X-Z plane C. Y-Z plane, at 6.4 GHz TE22δ D. X-Y plane E. X-Z plane F. Y-Z plane, at 8.8 GHz TE Z231 G. X-Y plane H. X-Z plane I. Y-Z plane, at 10 GHz TE Z33δ J. X-Y plane K. X-Z plane L. Y-Z plane.

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The near field depicted on the surface of DR is given in figure 6 with lower side existing fundamental mode TE11δ at 3.8 GHz in figures 6A, B, C with XY, XZ, and YZ plane, respectively. In the higher frequency side, it supports higher-order modes of TE22δ at 6.4 GHz in figures 6D, E, F with XY, XZ, and YZ plane, respectively, TE Z231 at 8.8 GHz in figures 6G, H, I with XY, XZ, and YZ plane, respectively, and TE33δ at 10 GHz in figures 6J, K, L with XY, XZ, and YZ plane, respectively. These modes support widen impedance bandwidth.

3.4 Simulated and measured results

The simulated and measured impedance bandwidth is 104.09% (3.28-10.4 GHz), 103.86% (3.23-10.21 GHz) with peak gain 7.42 dBi, and simulated radiation efficiency is 97.6% at 4.9 GHz, as shown in figures 7 and 8. In figures 9A, B, C the simulated and measured co and cross-polarization plots for the E plane of the antenna, at three different frequencies, i.e., 4.1, 6.2, and 8.1 GHz have been described. The radiation pattern has shown that the DRA radiates properly in all directions within the desired UWB band, further the cross-polarization level is observed to be lesser than the co-polarization level.

Figure 7
figure 7

Simulated and measured S-parameter of the proposed hollow DRA.

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Figure 8
figure 8

Measured and simulated gain, and simulated radiation efficiency.

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Figure 9
figure 9

Simulated and measured co and X polarization pattern of antenna A. 4.1 GHz, B. 6.2 GHz, C. 8.1 GHz.

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4 Conclusion

In this article, a new UWB hollow dielectric resonator antenna has been designed and fabricated. Gain enhancement has been achieved by using a hollow concept, in which a cylinder with the same height of DR is extracted from the rectangular DR. The simulated results are obtained by using CST microwave studio with good agreement between both simulated and measured results and peak gain is reported as 7.2 dB. Further, it gives stable and good performance across the entire UWB range, which makes it a powerful candidate for various UWB applications.