1 Introduction

In the current era, the dielectric resonator antenna (DRA) is commonly used in wireless communication due to the more alluring property like low metallic loss, high gain, large bandwidth and high radiation efficiency compare with microstrip antenna [1]. S. A. Long and his research team had developed a dielectric resonator (DR) as a radiator and studied the field inside the DRA in 1983 [2]. The dielectric resonator has some basic shapes like cylindrical, spherical and rectangular with some modified shapes like conical, elliptical, triangular, ring, dish, half split and notch rectangular [1]. Some researchers have used a multi-segment DR technique for the improvement of the impedance bandwidth and radiation pattern [3]. In ring DR, the bandwidth can be improved by reducing the quality factor. The quality factor is decreases due to removing the central portion of dielectric material [4]. The slotted feed line is another technique for the improvement of impedance bandwidth [5].

In wireless communication, dual/multi-band DRA is more useful than single-band DRA because a single DRA can be used for more than one application. At starting, the DRA was used only in linear polarized (LP) antenna. However, linearly polarized (LP) DRA is restricted due to some disadvantages like misalignment between transmitting and receiving antenna and multipath between them [1]. The circular polarized (CP) antenna is used in the place linearly polarized (LP) antenna to overcome this disadvantage [6, 7]. The essential condition for the creation of CP is that two orthogonal modes have equal magnitude and 90°phase difference [8]. Sharma et al. [9] designed a CDRA consisting trimmed square shape slotted radiator with V-shape feed line of large size 50 × 50 mm offering AR bandwidth of 12.2 and 4.2% and impedance bandwidth of 30.82% and 6.57% in lower and upper band respectively. Kumar and Chaudhary [10] presented a CP single band CDRA of large size 44 × 56 mm using modified CPW feed consisting rectangular and slotted patch with impedance bandwidth of 20.91% (1010 MHz) and AR bandwidth of 15.25% (740 MHz) while Gupta and Gangwar [11] designed CP and dual band RDRA using triangular ring shape aperture with strip line feed operated with AR bandwidth of 2.29 and 3.04% and impedance bandwidth of 15.16% and 14.55% in lower and upper band respectively. The single band DRA with narrow axial ratio bandwidth (ARBW) of 3.91% (80 MHz) is achieved by using single feeding technique in Huang et al. [12] while dual feeding technique is used in single band DRA by Li et al. [13] operated with impedance bandwidth of 32.8% (1060 MHz). Another single band DRA having AR bandwidth and impedance bandwidth of 18.37% both excited by cross slot is presented in [14]. G. Varshney et al. [15] designed an inverted sigmoid shape DRA of large size 50 × 50 mm and achieved AR bandwidth of 19.98 and 3.07% in lower and upper band respectively. A. Sharma et al. [16] designed a ring DRA consisting two connected ring shape aperture using T-shape feed line of large size 50 × 50 mm providing AR bandwidth of 12.28% (320 MHz) and 5.29% (240 MHz) and impedance bandwidth of 24.95% (650 MHz) and 8.75% (390 MHz) in lower and upper band respectively. Zuo et al. [17] designed a dual band CP antenna of large size 75 × 75 mm using combination of cross slot aperture and rectangular DRA with 3% (60 MHz) and 3.5% (100 MHz) 3-dB AR bandwidth while 14% (270 MHz) and 12.8% (350 MHz) impedance bandwidth. A. K. Dwivedi et al. [18] designed CP antenna of dual band using four ports MIMO of large size 80 × 80 mm providing impedance bandwidth of 20.86% (750 MHz) and 10.71% (560 MHz) while AR bandwidth of 4.97% (200 MHz) and 2.35% (160 MHz) in lower and upper band respectively. A. Sharma et al. [19] presented a dual band CP antenna of size 40 × 40 mm using modified circular shape aperture with CDRA exhibiting AR bandwidth of 6.45% (170 MHz) and 14.74% (780 MHz) while impedance bandwidth of 18.04% (480 MHz) and 19.85% (1080 MHz) in lower and higher band respectively. Pathak et al. [20] designed CP antenna of large size 50 × 50 mm operated in dual band using reformed square shape slot with ring CDRA providing impedance bandwidth of 25.45% (840 MHz) and 6.69% (550 MHz) while AR bandwidth of 9.52% (400 MHz) and 5.85% (340 MHz) in lower and higher band respectively.

In this article, a dual-band circularly polarized hybrid ring CDRA of size 40 × 40 mm is proposed. The proposed ring CDRA is a combination of multiple inverted Z-shaped aperture, circular ring inclusive of a slotted cross-shaped microstrip feed line and ring CDRA. Slotted microstrip feed line and ring CDRA are used for the enhancement of bandwidth. The proposed antenna provides impedance bandwidth of 20.58% (780 MHz) and 29.31% (2490 MHz) along with 4% (160 MHz) and 8.23% (760 MHz) axial ratio bandwidth (ARBW) in lower and upper band respectively. Both resonating band of proposed ring CDRA is suitable for wireless application in C and X-band. The proposed radiator is excited by a circular ring inclusive of slotted cross-shaped microstrip feed line that generates HEM11δ and TEM01δ modes.

The geometry of proposed ring CDRA is shown in Sect. 2. The step by step design and working operation of the proposed ring CDRA are discussed in Sect. 3. In Sect. 4, generation of circular wave along with AR bandwidth is described. The experimental results and performances comparison of proposed ring CDRA with pervious work are discussed in Sect. 5.

2 Antenna Layout of Proposed Ring CDRA

The layout of the proposed ring CDRA is shown in Fig. 1a, b and c. The FR-4 epoxy substrate (length L = 40 mm, width W = 40 mm, height (H1) = 0.8 mm, ɛrsub = 4.4 and tanδ = 0.02) is used to fabricate the proposed ring CDRA. Multiple inverted Z-shaped apertures of horizontal arm length 7.75 mm and vertical arm length 3.5 mm with same width 1.25 mm are etched on the ground plane of the substrate. A circular ring of inner radius 6 mm and outer radius 7 mm inclusive of a slotted (length 4 mm and width 1 mm) cross-shaped microstrip line feed (50Ω) of width 2 mm is planted below the substrate. Ring CDRA is manufactured by alumina (ɛr = 9.8 and tanδ = 0.002) and pests above the substrate with adhesive material. Details of optimized parameters show in Table 1. The archetype structure of the proposed ring CDRA is shown in Fig. 2.

Fig. 1
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The illustrative diagram of proposed ring CDRA a Isometric view b Top view c Feeding structure

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Table 1 Optimized parameter of the proposed ring CDRA
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Fig. 2
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Archetype structure of proposed ring CDRA a Feeding structure b Top view without ring CDRA c Top view with ring CDRA

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3 Design Procedure and Working Operation of Proposed Ring CDRA

The step-by-step design procedure of the proposed ring CDRA is shown in Fig. 3 while return loss |S11| graph corresponding to each steps are shown in Fig. 4. The Ansys HFSS simulator is used to analyze the working operation of the proposed ring CDRA between frequency range 3 to 10 GHz. In step-1, an inverted Z-shaped aperture and simple microstrip line are used as shown in Fig. 3. It is clear from Fig. 4, that the antenna designed in step-1 is not resonating. In step-2, two inverted Z-shaped apertures are loaded in antenna designed in step-1 as shown in Fig. 1b and antenna is resonating at 3.56 GHz in single band between 3.45 and 3.88 GHz with impedance bandwidth of 11.73% (430 MHz). However antennas designed in step-3, 4, 5 and 6 are resonating in dual band. The antenna designed in step-3 with only three inverted Z-shaped apertures and excited by a simple microstrip feed line while antenna designed in step-4, 5 and 6 with three inverted Z-shaped apertures and excited by cross-shaped microstrip feed line, circular ring inclusive of a cross-shaped microstrip feed line and circular ring along with slotted cross-shaped microstrip feed line respectively. Thus, it is clear that dual-band characteristic is generated due to multiple (three) inverted Z-shaped apertures.

Fig. 3
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The step by step design procedure of proposed ring CDRA

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Fig. 4
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The stepwise return loss |S11| curve of proposed ring CDRA

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The antenna designed in step-3 with only three inverted Z-shaped apertures and excited by a simple microstrip feed line is resonating at 3.73 GHz between 3.38 and 4.01 GHz with impedance bandwidth of 17.05% (630 MHz) in lower band and 8.39 GHz between 7.24 and 8.96 GHz with impedance bandwidth of 12.07% (1020 MHz) in upper band however antenna designed in step-4, resonating at 7.16 GHz between 6.97 and 7.48 GHz with impedance bandwidth of 7.05% (510 MHz) in lower band and 9.05 GHz between 7.95 and 9.68 GHz with impedance bandwidth of 19.62% (1730 MHz) in upper band when excited by a cross-shaped microstrip feed line along with three inverted Z-shaped apertures. The antenna designed in step-5 excited by circular ring along with cross-shaped microstrip feed line is resonating at 3.75 GHz between 3.43 and 4.10 GHz with impedance bandwidth of 17.79% (670 MHz) in lower band and 8.28 GHz between 7.53 and 8.87 GHz with impedance bandwidth of 16.34% (1340 MHz) in upper band. Finally, antenna designed in step-6 excited by a circular ring along with slotted cross-shaped microstrip feed line is resonating at 3.73 GHz between 3.40 and 4.18 GHz with improved impedance bandwidth of 20.58% (780 MHz) in lower band and 7.79 GHz between 7.25 and 9.74 GHz with impedance bandwidth of 29.31% (2490 MHz) in upper band. It is confirmed from Fig. 4 that the bandwidth enhancement in step-5 is due to circular ring inclusive of a cross-shaped microstrip feed line however the ring CDRA designed in step-6 has wider impedance bandwidth with improved return loss |S11| compare to antenna designed in step-5 is due to the slotted microstrip feed line. The performance comparison of all antennas designed in step-1 to step-6 is shown in Table 2.

Table 2 Performance comparison of all antennas designed in step-1 to step-6
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3.1 Effect of Ring CDRA

The variation (without DRA, with CDRA and with ring CDRA) of return loss |S11| graph is shown in Fig. 5. The antenna designed without DRA is resonating in single band characteristic while antenna designed with CDRA and with ring CDRA is resonating in triple band and dual band characteristics respectively. It is clear from Fig. 5, that the lower frequency band is because of CDRA while the upper frequency band is because of the combining effect of with and without CDRA. The antenna designed without DRA is resonating at 7.62 GHz with impedance bandwidth of 15.56% (1220 MHz) between 7.23 and 8.45 GHz while antenna designed with CDRA is resonating at 3.28 GHz with impedance bandwidth of 10.07% (330 MHz) between 3.11 and 3.44 GHz in lower band, 3.94 GHz with impedance bandwidth of 3.55% (140 MHz) between 3.87 and 4.01 GHz in middle band and 7.82 GHz with impedance bandwidth of 9.24% (700 MHz) between 7.22 and 7.92 GHz in upper band. However antenna designed with Ring CDRA is resonating at 3.73 GHz with impedance bandwidth of 20.58% (780 MHz) between 3.40 and 4.18 GHz in lower band and 7.79 GHz with impedance bandwidth of 29.31% (2490 MHz) between 7.25 and 9.74 GHz in upper band. Ring CDRA provides large bandwidth compared to CDRA because of removing the central portion of ceramic material CDRA (due to decreasing the Q-factor and increasing impedance bandwidth) [16].

Fig. 5
figure 5

Return loss |S11| curve with and without CDRA

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3.2 Effect of Slotted Feed Line

The variation of return loss |S11| curve of with and without slotted microstrip feed line is shown in Fig. 6. It is clear from Fig. 6 that the impedance bandwidth both bands of slotted microstrip feed line is better than without the slotted microstrip feed line. The antenna designed without slotted microstrip feed line is resonating in triple band while antenna designed with slotted microstrip feed line is resonating in dual band. The antenna designed without slotted microstrip feed line is resonating at 3.21 GHz with impedance bandwidth of 11.83% (380 MHz) between 3.02 and 3.40 GHz in lower band, 7.62 GHz with impedance bandwidth of 2.49% (190 MHz) between 7.52 and 7.71 GHz in middle band and 8.60 GHz with impedance bandwidth of 19.39% (1680 MHz) between 7.82 and 9.50 GHz in upper band. However antenna designed with slotted microstrip feed (length 4 mm and width 1 mm) line is resonating at 3.73 GHz with impedance bandwidth of 20.58% (780 MHz) between 3.40 and 4.18 GHz in lower band and 7.79 GHz with impedance bandwidth of 29.31% (2490 MHz) between 7.25 and 9.74 GHz in upper band. The enhancement of bandwidth is because of provides good coupling between the antenna and feed line [5]. The performance comparisons of all antennas designed without DRA, with CRDA, with ring CDRA, without slotted microstrip feed line and with slotted microstrip feed line are shown in Table 3.

Fig. 6
figure 6

Return loss |S11| curve of the proposed ring CDRA with slotted and without slotted feed line

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Table 3 Performance comparison of all antennas designed without DRA, with CRDA, with ring CDRA, without slotted and with slotted microstrip feed line
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The distribution of E-field within the ring CDRA at frequencies 3.73 and 7.79 GHz are shown in Fig. 7. It is clearly observed from Fig. 7a and b, that hybrid HEM11δ mode is generated at frequency 3.73 GHz which is a fundamental mode whereas Fig. 7c represents that TEM01δ mode is generated at frequency 7.79 GHz [21]. A dielectric waveguide field line cannot spin perfectly by half wavelength. The spin of field line is slightly less than half wavelength. It is due to the boundary condition of the dielectric waveguide are not absolute as compare with the metallic waveguide. Thus, the field line variation in the direction of propagation are neither exactly one nor zero [21].

Fig. 7
figure 7

The distribution of E-field of the proposed ring CDRA a Top view at 3.73 GHz b Side view at 3.73 GHz c Top view at 7.79 GHz

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The generated modes in simulated antenna can also verify by using the following mathematical formulas [16][16].

$$f_{r} HEM_{11\delta } = \frac{6.324c}{{4\pi r_{1} \sqrt {\varepsilon_{dra } + 2} }}\left\{ {0.27 + 0.36\frac{{r_{1} }}{2H} + 0.02\mathop {\left( {\frac{{r_{1} }}{2H}} \right)}\nolimits^{2} } \right\}$$
(1)
$$f_{r} TEM_{01\delta } = \frac{2.327c}{{4\pi r_{1} \sqrt {\varepsilon_{dra} } + 1}}\left\{ {1 + 0.2123\left( {\frac{{r_{1} }}{H}} \right) - 0.00898\left( {\frac{{r_{1} }}{H}} \right)^{2} } \right\}$$
(2)

where r1 = internal radius of ring DR (dielectric resonator), H = Height of ring DR (dielectric resonator)

$$\varepsilon_{dra} = \left[ {\frac{{9.8\left( {V_{Alumina} } \right) + V_{Ring} }}{{V_{Alumina} + V_{Ring} }}} \right]$$
(3)
$$V_{Alumina} = V_{cylindrical} - \, V_{ring}$$
(4)

Using Eqs. (1) to (4) and related parameters given in Table 1, the resonant frequency of HEM11δ and TEM01δ mode is achieved as 3.70 and 7.93 GHz respectively which is lies very closed to simulated resonant frequency 3.73 GHz in lower band and 7.97 GHz in upper band.

4 Circular Wave (CW) Generation

The variation of the axial ratio (AR) graph with the step-by-step design procedure of the proposed ring CDRA is shown in Fig. 8a and b in the lower and upper band respectively. It is clear from Fig. 8a that only ring CDRA of step-6 generate circular polarization characteristics in lower band while it is clear from Fig. 8b that step-4, step-5 and ring CDRA of step-6 generates circular polarization characteristics in upper band out of six steps. The proposed ring CDRA designed in step-6 generates circular polarization characteristics between frequency range 3.92 GHz to 4.08 GHz with 4% (160 MHz) axial ratio (AR) bandwidth in lower band while frequency range 8.85 to 9.61 GHz with 8.23% (760 MHz) axial ratio (AR) bandwidth in upper band while axial ratio (AR) bandwidth in upper band only 50 MHz between 9.45 and 9.50 GHz and 60 MHz between 9.27 and 9.33 GHz is obtained in step-5 and step-6 respectively. The essential condition for the creation of CP characteristics that two orthogonal modes have equal magnitude and 90° phase difference [8]. In the proposed ring CDRA, multiple inverted Z-shaped apertures and circular ring along with slotted cross-shaped microstrip feed line are responsible for CP wave generation.

Fig. 8
figure 8

Stepwise axial ratio curve in a Lower band b Upper band

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The magnitude (EX / EY) and phase difference (between EX and EY) plot of proposed ring CDRA against frequency in lower band and upper band is shown in Fig. 9a and b respectively. It is clearly observed from Fig. 9 that the value of magnitude of (EX / EY) is near one while the phase difference between EX and EY is around 90° in lower band (3.92 to 4.08 GHz) and upper band (8.85 to 9.61 GHz). Thus, the proposed ring CDRA is satisfied the necessary condition of circularly polarization in both resonating bands [23].

Fig. 9
figure 9

Magnitude and phase graph vs frequency in a Lower band b Upper band

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5 Experimental Results

In this section, a detail discussion of measured return loss |S11|, axial ratio bandwidth, radiation pattern and gain plot against frequency is performed. VNA (Agilent technologies, model: E5071C) is used to measure the return loss |S11| of the proposed radiator between 3 and 10 GHz range. The measured and simulated return loss |S11| graph is shown in Fig. 10. The measured antenna is resonating at 3.70 GHz in lower band between 3.36 and 4.14 GHz with impedance bandwidth 20.8% (780 MHz) whereas the upper band is resonating between 7.50 and 9.63 GHz at 8.56 GHz with impedance bandwidth 24.86% (2130 MHz). A good return loss |S11| of − 23.75 and − 46.87 dB is observed in both bands respectively. In both bands, a good similarity is shown in both simulated and measured performances. Small changes in upper band is found due to glue for fixing ceramic material above the substrate [19]. Thus, the measured results validate the geometry of proposed ring CDRA for dual-band operation.

Fig. 10
figure 10

Measured and simulated return loss |S11| graph

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The proposed ring CDRA’s field pattern is measured in the anechoic chamber. A CP horn antenna is used as a reference antenna for pattern measurement. The dual pattern measurement method is used to measure the proposed radiator’s axial ratio graph [16]. The comparison of simulated and measured axial ratio (AR) bandwidth in lower band and upper band is shown in Fig. 11a and b respectively. A good similarity is seen between simulated and measured AR graph. The measured antenna is resonating between 3.95 and 4.10 GHz with AR bandwidth of 4.48% (180 MHz) in lower band whereas it is resonating between 8.87 and 9.52 GHz with AR bandwidth of 7.06% (650 MHz) in upper band. Thus, the measured results of AR bandwidth is justified the proposed radiator as dual CP characteristics in both bands. The simulated and measured performance comparison of proposed ring CDRA is shown in Table 4.

Fig. 11
figure 11

Simulated and measured AR bandwidth graph in a Lower band b Upper band

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Table 4 Simulated and measured performance comparison
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Normalized measured and simulated LHCP (Left hand circularly polarized) and RHCP (right hand circularly polarized) pattern in broadside (θ = 0, ϕ = 0) and (θ = 0, ϕ = 90) direction are shown in Figs. 12 and 13 respectively. It is clearly seen from both Figs. 12 and 13, a good similarity is seen between measured and simulated outcomes. It is also observed that the proposed ring CDRA has more than 13 dB difference between LHCP and RHCP pattern at both frequencies. The LHCP and RHCP pattern can be calculated with the help of vertical plane (Ev) and horizontal plane (EH) [9] by using Eqs. (5) and (6).

$$E_{LHCP} = \left[ {\frac{1}{\sqrt 2 }\left( {E_{H} - jE_{V} } \right)} \right]$$
(5)
$$E_{RHCP} = \left[ {\frac{1}{\sqrt 2 }\left( {E_{H} + jE_{V} } \right)} \right]$$
(6)
Fig. 12
figure 12

Normalized LHCP/RHCP pattern of proposed Ring CDRA a (θ = 0, ϕ = 0) at lower frequency 3.73 GHz b (θ = 0, ϕ = 0) at upper frequency 7.79 GHz

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Fig. 13
figure 13

LHCP/RHCP pattern of proposed Ring CDRA a (θ = 0, ϕ = 90) at lower frequency 3.73 GHz, b (θ = 0, ϕ = 90) at upper frequency 7.79 GHz

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The measured and simulated gain in the broadside direction (θ = 0, ϕ = 0) of the proposed ring CDRA is shown in Fig. 14. Two antenna method is used to measure the absolute gain of proposed ring CDRA in the anechoic chamber [24]. The peak value of gain from 5 to 5.8 dB (simulated) and 4.5 to 5.8 dB (measured) in lower band while peak value of gain from 5.9 to 7.7 dB (simulated) and 5.2 to 8.2 dB (measured) in upper band is observed. It is clear from Fig. 14 that a good similarity is observed between measured and simulated outcomes in both bands except the value of gain is slightly high in upper band because of low metallic loss (because the upper band is created due to multiple inverted Z-shaped aperture) [1].

Fig. 14
figure 14

Gain vs. frequency curve of proposed ring CDRA in a Lower band b Upper band

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A comparison of proposed ring CDRA with previous work on DRA in terms of size, type of DRA, feeding method, impedance bandwidth, AR bandwidth and gain is shown in Table 5. It is clear from Table 5 that the proposed ring CDRA is better in terms of size, impedance bandwidth and peak gain. The proposed ring CDRA has compact size compared to all reported antenna in Table 5 except antenna [11] and [19] which has same size. However, impedance bandwidth of upper band is wider compared to all reported antennas but impedance bandwidth of lower band is smaller compared to antennas [9, 10, 15] and [20]. The lower band AR bandwidth of proposed ring CDRA is larger than antennas [11] and [17] while upper band AR bandwidth is larger than all antennas except [19] but its impedance bandwidth is smaller in both bands. The lower band peak gain of proposed ring CDRA is larger than all antennas except [9, 19] and [20] while upper band peak gain is larger than all antennas reported in Table 5. The performance comparison of proposed ring CDRA with previous work on DRA in terms of size, impedance bandwidth and axial ratio bandwidth by bar graph is shown in Fig. 15a, b and c respectively.

Table 5 Comparison between proposed ring CDRA and previous work on DRA
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Fig. 15
figure 15

Performance comparison of proposed ring CDRA with previous work on DRA a Size b Impedance bandwidth c Axial ratio bandwidth

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6 Conclusion

This article demonstrates a dual-band circularly polarized hybrid ring CDRA using multiple Z-shaped apertures and circular ring along with slotted cross-shaped microstrip feed line. The proposed hybrid ring CDRA is resonating between 3.40 and 4.18 GHz (AR: 3.92 to 4.08 GHz) in lower band while 7.25 to 9.74 GHz (AR: 8.85 to 9.61 GHz) in upper band for wireless applications in C and X-band. The proposed hybrid ring CDRA produces dual band operation offering impedance bandwidth of 20.58% (780 MHz) and 29.31% (2490 MHz) along with AR bandwidth of 4% (160 MHz) and 8.23% (760 MHz) in both bands respectively. The stable gain of peak value 5.8 dB (simulated) and 5.8 dB (measured) in lower band while 7.7 dB (simulated) and 8.2 dB (measured) in upper band is observed.