1 Introduction

A tapered slot antenna (TSA) is a special class of antennas which can be used in different applications such as Ultra-Wideband (UWB) [1], cognitive radio [2], medical imaging [3, 4], satellite communications, large ships [5] and GPR system [6] etc. TSA antenna was firstly introduced by Gibson [7]. It possessed the numerous advantages of low profile, compact, planar structure, ease of fabrication, compatibility with microwave integrated circuits, high efficiency, directional radiation pattern, broadband impedance bandwidth and high gain realization [8, 9]. All these properties, specially the low-profile, planar structure and compactness are essential to have when it comes to underwater wireless communication. Since past years researchers have published more papers on antenna design layout, to improve the array performance parameters of each element such as impedance bandwidth, gain, radiation efficiency, radiation pattern in the operating frequency. Increasing the bandwidth of the antenna elements can influence the radiation pattern [10]. The design of the TSA array elements depends on its antenna length, width and the ground extension parameters.

Tapered slot line is flared to provide an aperture for microwave radiation in free space, a flared slot line can be provided with the aid of a transition line, an elliptical curve, and exponential curve equation. The combination of two curves has some specific users to get the desired characteristics of the antenna. TSA produce the end-fire travelling wave hence the phase velocity and guide wavelength which are liable to the substrate thickness, taper rate and dielectric constant. The width, length and the taper profile of TSA involve the radiation characteristics of an antenna and its gain is proportional to the \(L/\lambda _g\) [11, 12]. It is etched on the thin metal layer placed on the substrate. It can be fed through different feedlines i.e., stripline, coplanar waveguide, a microstrip line or coaxial feedline. The microstrip and strip lines feeding of TSA can work over the wide bandwidths and high gain. It should have a perfect impedance matching when it achieves the broad bandwidth by adjusting the feedline location connected with the cavity stub and the slotline [13].

There has been growing demand to the underwater communication technology from the industry and the scientific community, which is due to the broad range of applications like coastline protection, surveillance, off-shore oil and gas field monitoring, underwater environmental observation for exploration, oceanographic data collection. It involves enough bandwidth for high data rates, which may be required for real-time video exchange among the underwater bodies that is not possible with acoustic waves.

In underwater communications, the three widely used communication technology for underwater applications are acoustics signals, optical signals and Electromagnetic (EM) signals [14]. In the EM wave, the Ultra-wideband (UWB) and broadband antennas are radio transmission innovation which involves a wide bandwidth, i.e. \(> 500\,\hbox {MHz}\) or possibly 20% of the Centre frequency [15], is additionally a progressive methodology for short-range high-bandwidth remote communication.

Xu et al. [16], have presented a work on a wideband monopole antenna for Bluetooth and UWB application, utilized lower pass Band-U formed parasitic strips reciprocally close to bolster line on a FR4 substrate with measurements of (\(18 \times 32 \times 0.8)\,\hbox {mm}^3\), examined the reflection coefficient by changing length and feed crevice. The peak gain at Bluetooth band of 1.6 dB. Other studies have been proposed by Chang et al. [17], in which the inverted F antenna for a 313 GHz short range \((<10\,\hbox {m})\) UWB indoor, remote communication has been proposed. A planar monopole is top stacked with a rectangular patch connected to two rectangular plate, one shorted to ground and other suspended on an FR4 substrate with a measurement of \((20 \times 10 \times 7)\,\hbox {mm}^3\). Another work on a differential bolstered magneto-dielectric dipole has been proposed in [18], which mainly focuses on unidirectional radiation design and an increase of \(8.25\pm 1.05\) dBi on a Doored 5870 substrate with a measurement of \((65 \times 65 \times 9.8)\,\hbox {mm}^3\). However, impedance transmission capacity of 114% for frequencies from 2.95 to 10.73 GHz range. Besides, the radiation pattern in E and H planes are all around carried on up to 9.4 and 8.9 GHz, individually, after which side flaps show up because of the high request modes radiation. Moreover, Arash et al. [19], a couple line nourished planar, (patch antenna) which has a double band score with two coordinated monopoles that endeavors to incorporate the UWB innovation with Bluetooth and Global System for Mobile Communications (GSM) at 900 MHz has been proposed. Another author Kwai et al. [20, 21], has discussed a magneto electric dipole for UWB application that can be effortlessly imprinted on Duroid 5880 substrate for 60 GHz frequency. In this a level tie electric dipole with an impedance transmission capacity of 110%, with SWR \(\le 2\) was broke down from 3.08 to 10.6 GHz. Li et al. [22], have presented UWB antennae in light of time domain or frequency domain on one side low gain and high gain on the other side. Li Dissected ringing, bunch delay, signal loyalty and separation parameters. A coordinated Bow tie antenna outlined by Abdou et al. [23], have demonstrated, a RL of − 16 dB at 433 MHz which implies that more than 95% of the force is transmitted in air and the reenactment introduces a sharp valley at low frequencies of 154 MHz with a high esteem RL of − 43 dB and data transfer capacity of 90MHz in undersea, this antenna is completely waterproofed in paste. Moreover, our work presented in this paper, have achieved the return loss (RL) about − 61 dB at 6.1 GHz resonant frequency, in order to meet the application for microwave band, a simulation design of higher order antenna array with maximum directivity and wide bandwidth has been presented which is essential to obtain for a long distance communication with high data rates.

Few other authors have presented work in [24], on the EM wave propagation through seawater at MHz frequencies, Shaw conducted diverse class tests in a fiber tank with dipole, circle, two-fold circle and collapsed circle antennas. Another study conducted by Waheed et al. [25], on very low frequency (VLF) antenna for undersea interchanges. The authors have utilized copper wires which were rewound like a transformer center in bearing. Besides a low power regulated and speaker circuit was intended for short separation interchanges between two submarines. Hector et al. [26], outlined a cradle for decreasing the transmission misfortune in submerged interchanges. Consequently, the reflection coefficient observed was − 25.98 dB in 2.38 GHz (without spread) and − 34.25 dB in 2.58 GHz (with glass cover). When glass spread is utilized the antenna transmission capacity diminished from 100 MHz to 70 MHz, because of the permittivity of glass.

In this manuscript, design and development of \(2\times 4\) antenna array for underwater microwave band application have been studied. We have worked on total antenna array performance by designing and implementing the Wilkinson power divider and its characteristics of TSA with different antenna array arrangement resulted in impedance bandwidth and high gain. The strip line is etched on the top surface of the glass epoxy FR4 substrate and a ground plane. Moreover, the radiation characteristics and impedance bandwidth of the proposed antenna array have been improved as compared to a single element. The proposed antenna array (single element, \(1\times 2\), \(1\times 4\) and \(2\times 4\)) has obtained the impedance bandwidth of more than \(>50\%\) with the peak realized gain of 4.82 dBi, 6.85 dBi, 9.65 dBi and 10.75 dBi and radiation efficiency of 90% at the operating frequency of 6 GHz. This antenna possesses the compact dimension of over all four elements arrays are \((1.334\lambda \times 1.612\lambda \times 0.016\lambda )\,\hbox {mm}^3,\) the wavelength is the minimum frequency of the desired microwave band and are satisfying the better radiation efficiency, impedance bandwidth and peak realized gain. Hence, we conclude that this proposed design of an antenna array is well suited for under water communication microwave band applications.

This paper consists of five sections: Sect. 1 is introduction of a proposed antenna, Sect. 2 discusses the design of simulation and fabrication of single element and impedance matching, Sect. 3 gives simulated results and analysis, Sect. 4 discusses the performance analysis of \((1\times 2,1\times 4\) and \(2\times 4)\) element array finally the Sect. 5 gives the conclusion.

2 Design of the Antenna Element Array and Description

2.1 Designing and Fabrication of Single Unit

Designing and fabrication of TSA are shown in Fig. 1a, b. The proposed antenna consists of a substrate, feedline, and the ground plane and cavity circle. The feed line is placed on the top of the dielectric substrate and cavity circle is used with slot line and the linear taper profile. Moreover, tapered profile structure can be classified into two categories: substrate parameters and antenna element parameters, which can be subdivided into the stripline/slotline transition, the tapered slot, and radius of the circular slotline cavity [27, 28].

The stripline/Slotline transition is specified by strip line width \((w_{1})\) and slotline width \((w_{sl})\). The exponential taper profile is defined by the opening rate R and two points \(P_1(x_1,y_1)\) and \(P_2(x_2,y_2)\) [29]. TSA taper length has been selected as \(0.666\lambda _\circ\) and the opening rate aperture width of proposed antenna is chosen as \(0.333\lambda _\circ\) at the lowest operating band, where \(\lambda _\circ\) is the free space wavelength calculated at 4 GHz to work as a travelling wave antenna [30, 31]. The proposed antenna dimensions are providing efficient radiation from the TSA in (4–8 GHz). The linear exponentially tapered can be determined by:

$$\begin{aligned} y = c_1e^{Rx}+c_2 \end{aligned}$$
(1)

where

$$\begin{aligned} c_1= & {} \frac{y_2-y_1}{e^{Rx_2}-e^{Rx_1}} \\ c_2= & {} \frac{y_1e^{Rx_2}-y_2e^{Rx_1}}{e^{Rx_2}-e^{Rx_1}} \end{aligned}$$

The tapered \(T_{sl}\) is \((x_2-x_1)\) and aperture height H is \(2(y_2-y_1)+ w_{sl}\). In the limiting case where opening rate R approaches zero, the exponential taper results in a linearly tapered slot antenna (LTSA) for which the taper slope is given by \(s_\circ = (y_2-y_1)/(x_2-x_1)\). For the exponential taper defined by (1), the taper slope s changes continuously from \(s_1\) to \(s_2\), where \(s_1\) and \(s_2\) are the taper slope at \(x=x_1\) and \(x=x_2\) respectively and \(s_1< s <s_2\) for \(R > 0\). The taper flare angle is defined by \(\alpha = tan^{-1}s\). The flare angles, however, are interrelated with and defined parameters, i.e., \(H, T_{sl} , R\) and \(w_{sl}\). The parameters related to the stripline feeding and circular slotline cavity shown in Fig. 1a are as follows in Table 1.

The following figures show the side and top view geometry of the tapered slot antenna. However, the array design and impedance matching of proposed antenna structure is described in the below section.

Fig. 1
figure 1

a Side view and b top view geometry of proposed antenna

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Figure 1 shows the optimized values of Vivaldi antenna geometry are provided in the Table 1. The parameters: Width (W), length (L) & thickness (h) of substrate and patch remains same for the designed antenna (Fig. 2).

Table 1 Geometric parameters
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Fig. 2
figure 2

a, b Printed Feedline connection with SMA connector and measured at Anechoic chamber c fabrication process of proposed antenna

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2.1.1 Vivaldi Antenna Array

For operating frequency within the underwater communication microwave band spectrum the part of the planar Vivaldi antenna structure relevant to the frequency works. However, the width (w) of this part is very near to the corresponding wavelength, hence electromagnetic wave is radiated out of the antenna. Whereas, the operating frequency changes, the radiation pattern region of proposed antenna also changes accordingly. consequently, the electrical size of operational region in the Vivaldi antenna remains constant across the operational frequency band. Besides, input impedance and radiation pattern may also maintain approximately constant across the entire operational frequency band of spectrum. As a result, the Vivaldi antenna possessed the wideband characteristic [32].

Moreover, the TSA has some advantages as a radiator for phased arrays, imaging arrays, underwater communication microwave spectrum and integrated active antennas because of the broad impedance bandwidth, symmetrical radiation pattern, and planar structure. In general, mutual coupling produces several effects including impedance mismatch, scanning blindness, and distortion of radiation patterns. The horizontal and vertical mutual coupling between two adjacent elements was investigated by calculating the transmission coefficient (\(S_{21}\) and \(S_{31}\)).

2.2 Impedance Matching

In order to get a transition that has low \(S_{11}\) over a broad frequency band and the impedance of the slot line and strip line must be matched to each other to reduce the reflection. To achieve an impedance values up to \(50\varOmega\), the characteristics impedance of slotline increases with the increase of the slot width, therefore, suitable width of slotline should be chosen to match with \(50\varOmega\) input. The strip line feed used in a TSA is connected directly to the transmitter or receiver or fed by a coaxial attached to an SMA connector, the slotline width, guided wavelength, and strip width is calculated by using formulas mentioned in [33]. Distance between the antennas, dimensions of the feedline and guided wavelength plays a major role on the performance of each antenna element.

2.3 Design of Broadband Wilkinson Power Divider

The power divider is needed to feed the array design of \(1\times 2\) elements. The power divider signals are used to balance the phases and amplitudes from the other two ports [34, 35].

Firstly, the two output ports and the input port must be matched with the impedance characteristic, only then the power divider can be directly connected with the proposed antenna. Figure 3a, b represent the power divider of insertion loss (\(S_{21}\) and \(S_{31}\)) and the simulated return loss \((S_{11})\) at the operating band from 4 to 8 GHz, the Fig. 3 indicates that the return loss is below 10 dB have good bandwidth which has reached 57% at the resonant frequency of 6 GHz and the two output ports have better power divider level with insertion loss of 3.8 dB.

\(2\times 4\) Vivaldi antenna array has been designed to operate in the 4–8 GHz C band frequency spectrum. Moreover, spacing between two adjacent antenna elements is set as \(0.5\lambda _\circ.\)

Fig. 3
figure 3

a, b Simulated return loss \((S_{11})\) and insertion loss (\(S_{21}\) and \(S_{31}\)) of Wilkinson power divider

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3 Simulation Results and Analysis

3.1 Return Loss, \(S_{11}\)

The simulated result of a unified single element is shown in the following Fig. 4. It has been observed, that the relative bandwidth at minimum return loss of 10 dB, and 15 dB is obtained as 47.4% and 28.7%. In addition, maximum return loss is observed as 61 dB at the resonant frequency of 6.1 GHz.

Fig. 4
figure 4

Variation of return loss with the frequency of the proposed slot antenna

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All these results have been observed and tested using Agilent PNA-X-N5224A network analyzer (VNA). However, the peak realized gain of unit element is 4.82 dBi at the resonant frequency of 6.1 GHz.

3.2 Voltage Standing Wave Ratio (VSWR)

Voltage standing wave ratio results of a single element are presented in Fig. 5. It clearly shows that the value of VSWR for this antenna is 1.0 at the resonant frequency of 6.1 GHz. It is also observed from the plot that, for the frequency value from 4.7 to 7.9 GHz the VSWR value remains less than 2. Figure 6 shows the radiation efficiency of the proposed antenna is 84%.

Fig. 5
figure 5

Variation of VSWR vs. frequency of tapered slot antenna

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Fig. 6
figure 6

Radiation efficiency of tapered slot antenna

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3.3 Radiation Pattern

The 2-dimensional radiation pattern plots in both azimuth and elevation plane are shown in Fig. 7a.

Fig. 7
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a, b An isotropic radiation pattern and peak realized gain

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It is observed from the Fig. 7b that the proposed antenna gain is 4.82 dBi at the resonant frequency of 6.1 GHz. The antenna beam points towards the 90-degree direction which is expected for an endfire type of antenna. Almost all antenna radiates equally in the other plane. Figure 8 shows the surface current distribution of single element tapered slot, the electric field at 3.02 GHz with no phase shift is observed at the surface.

Fig. 8
figure 8

J Surface current distribution of the proposed antenna

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4 Array Elements Analysis

4.1 Performance Analysis of Dual Element

Designing of TSA array is etched on a glass epoxy FR4 substrate (\(\varepsilon _r= 4.4\)) with a same configuration of the single element omit that the impedance matching transformer is utilized for broadband impedance bandwidth as depicted in Fig. 9a and it covers the desired microwave band. The impedance of feedline characteristics width and the adjustment of the distance between elements of the antenna have the half of the wavelength \(0.5\lambda _\circ\) of the operating frequency. The distance between element shouldn’t overlay with each other because they will interface together and degrade the performance of transition. In order to observe the array spacing elements, the distance of the two elements design and the feeding power divider is very important for proper matched the impedance techniques and achieve the better results and radiation efficiency.

The dual element of the proposed antenna has been observed from the simulated results that the relative bandwidth of minimum return loss of 10 dB, 15 dB and 20 dB is obtained as 57.33%, 46.88% and 28.2% at the resonant frequency of 6 GHz as shown in the Fig. 9a. From the figure it is observed that the impedance bandwidth of 57.3% or has been achieved at 10 dB return loss.

VSWR results are presented in Fig. 9b. It shows that the value of voltage standing wave for this antenna is 1.0 at the resonant frequency of 6 GHz. It is also observed from the figure that, for the frequency value from 4.25 to 7.9 GHz the VSWR value remains less than 2.

Fig. 9
figure 9

a, b Variation of return loss and VSWR with the frequency of the proposed slot antenna

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Moreover, the peak realized gain of the dual element has been observed as 6.85 dBi at the resonant frequency of 6 GHz as shown in the Fig. 10a. However, the efficiency is improved than the single element which is 94%, it means the performance of the radiation efficiency is better than the unit element design as shown in the plot Fig. 10b.

Fig. 10
figure 10

a, b Variation of realized gain and the efficiency with the frequency of the proposed slot antenna

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From the Fig. 11a, shows the antenna directivity with the variation of angle (Phi), it can clearly observe the wide beam of an antenna at above 6 dBi and the right side Fig. 11b represented that the radiation pattern of elevation and azimuth plane radiated at (\(0^{\circ }\)–\(90^{\circ }\)).

Fig. 11
figure 11

a, b The radiation pattern of E & H plane and directivity of the proposed antenna

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4.2 Performance Analysis of \(1\times 4\) Array Elements

At the feed port of four element array, the two power dividers were shunted together to make the array matched at \(50\varOmega\). The spacing between the two adjacent single elements is \(0.5\lambda _\circ\) with an operating frequency of 6 GHz. The performance of two antenna elements has been analyzed and connected with the aid of four-way power divider. First, we designed the structure of power divider with four-way ports of \(50\varOmega\) with single output etched to each antenna element and achieve the proper impedance matching of the antenna.

To achieve the proper matching, the glass epoxy FR4 substrate material and thickness of the single element has been chosen. As illustrated in Fig. 12a of the minimum return loss of the \(1\times 4\) array element design, the fractional bandwidth of 55% the proposed antenna has been achieved the desired microwave band.

Fig. 12
figure 12

a, b Variation of return loss and VSWR with the frequency of the 1\(\times\)4 array element

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Moreover, fractional bandwidth is calculated at the lower and upper frequency (4.5–7.8) GHz as measured at maximum return loss of 10 dB at the resonant frequency of 7.2 GHz. The observed impedance bandwidth at 3.4 GHz shows better performance than a single and dual element. The bandwidth of array performance covers the wide bandwidth at C band (4–8) GHz and hence this antenna makes it appropriate to work for the underwater communication. Considering the Fig. 12b, the value of VSWR is observed less than 2 from the 4.45 to 7.90 GHz frequency range. It’s clearly observed form the plot that the antenna is perfectly matched.

As depicted in the Fig. 13a, b, observed the gain and radiation efficiency of the array. The plot Fig. 13a clearly shows the peak realized gain of an antenna array design is 9.65 dBi at the desired frequency. The performance of the radiation efficiency of the proposed antenna array is shown in the Fig. 13b, which is same as unit element achieved at 84%. As shown in Fig. 14a, b the directivity and radiation pattern of the array design elements.

Fig. 13
figure 13

a, b Variation of the realized gain and radiation efficiency with a frequency of the array antenna

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Fig. 14
figure 14

a, b Directivity and radiation pattern of proposed array \(1\times 4\) element antenna

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4.3 Performance Analysis of \(2\times 4\) Array Elements

The broadband array antennas have different array pattern in the frequency of operation. The broadband antennas will effect on the total array performance. Larger the width will impact on the larger feeding line space among the elements. It will lead higher side lobe level, even grating lobe, especially for higher frequency. Grating lobe can be occurred when the distance of an antenna elements is more than one wavelength of its working frequency.

The design performance of \(2\times 4\) elements has been observed from the \(1\times 4\) element arrays. We used the same dimensions of shunted series \(1\times 4\) elements and have same substrate FR4 epoxy material with thickness. The optimized power dividing feeding network and spacing among elements array have been achieved the proper results under the desired band. The space between the elements has been occurred \(0.5\lambda _\circ\) of the operating frequency of 6 GHz.

As illustrated in the Fig. 15, the variation of return loss with the desired frequency band, the final \(2\times 4\) array antenna is achieved at the multiple bands in the operating C band, it has been resonated at three different frequency bands and simulated at the maximum return loss of 10 dB. The array antenna has been resonated three different values of 4.5 GHz (4.3–4.75) GHz, 5.4 GHz (5.3–6.4) GHz and 7.35 GHz (6.8–7.7) GHz as the maximum return loss of 29.3 dB, 40 dB and 35.4 dB. The radiation efficiency has been observed upto 61% of the proposed antenna element array.

Fig. 15
figure 15

Variation of \(S_{11}\) parameter with frequency of the array antenna

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The peak realized gain of proposed array antenna design has been calculated from each resonated values of 4.5 GHz at 7.58 dBi, 5.4 GHz at 8.56 dBi and 7.35 GHz at 10.75 dBi of the gain as shown in the Fig. 16. The maximum peak realized gain of proposed antenna array has been reached at 10.75 dBi.

Fig. 16
figure 16

Realized gain of the proposed antenna array

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5 Conclusion

In this paper, the authors designed a \(2\times 4\) TSA array for under water communication. The proposed structure of an antenna consists of the substrate, patch, ground plane and feeding network. Simple and an effective feeding technique i.e. stripline feed has been used which resulted in an enhanced impedance bandwidth of more than 50% at a resonant frequency of 6 GHz, peak realized gain of 10.75 dBi and the radiation efficiency is more than 90%. The simulation and measured results have been analyzed and validated by using simulation software Ansoft HFSS and anechoic chamber. Based on the analysis and discussion presented in the paper, the optimum results of return loss, VSWR, gain and radiation efficiency proved that this designed of an antenna is suitable for underwater communication microwave band applications.