1 Introduction

Mobile telecommunications services are becoming more and more prevalent globally. Individuals frequently use their phones next to their heads while doing so. Mobile phone makers are now required to consider the interactions between human bodies and mobile terminals due to the global expansion of wireless mobile services. In one sense, the electromagnetic wave that the antenna transmits is partially absorbed by the human skull. However, the presence of a human head alters a number of characteristics related to mobile phone antennas, including radiation efficiency, bandwidth, return loss, and radiation pattern. A number of academics have studied the relationship between the human head and the antenna.

Most research societies are focusing on achieving high throughput and high data rates at a low cost for fifth-generation (5G) connectivity [1,2,3,4,5]. The anticipated data rate for fifth-generation (5G) communication systems is 1000 times faster than that of fourth-generation (4G) communication systems [1]. In addition to supporting a range of frequencies, It is anticipated that the 5G Radio Access Networks (RANs) would manage several 5G bands simultaneously [6]. The ITU-R has created many parameters to evaluate the demand for the spectrum needed for international mobile telephony (IMT). Nearly all of the research needed to meet the 2020 criterion for this demand was recently completed by ITU-R [7].

The suggested antenna covers the 5G frequency bands ITU n77 (3.3–4.2 GHz) and n79 (4.4–5 GHz). It is predicted by extensive research that the lower frequency band will offer far better coverage for modern wireless communications. By utilizing frequency ranges lower than 6 GHz, 5G communication would be able to provide better data rates and wider coverage regions with outside-to-inside network coverage [8]. A large number of well-performing sub-6 GHz antenna designs that have been published in the literature heavily utilize printed antenna technology. Discrete wavelet transforms (DWTs) are employed in orthogonal frequency division multiplexing (OFDM) systems to improve spectral efficiency. Therefore, wavelet transforms can be used to design antenna diversity scheme to improve performance of system and spectrum efficiency [9,10,11,12,13,14].

Contemporary technologies have yielded incredibly small and highly effective antennas. Printed microstrip slot antennas are the most common type of antenna in this class [15,16,17,18,19]. Applications for slot antennas include WiMAX, WLAN, Bluetooth, 4G LTE, and many more. Apart from the applications mentioned, slot antennas are widely employed in wireless 5G applications, which now mostly involve mobile terminal devices. The sub-6 GHz 5G applications for mobile portable devices have been the main focus of the suggested rectangular slot antenna design.

The literature contains descriptions of a number of different slot antenna construction methods, including the following: the transformer triple band slot antenna [15], the cross-shaped slot coupler antenna [20], the circular patch antenna with asymmetric open slots [21], the octagonal slot antenna with U-shaped strips for UWB applications [22], the monopole radiator with square slot and L-shaped strips [23], the C-shaped coupled fed antenna with L-shaped monopole slot having orthogonal polarization [24], and so on and a broad slot antenna with hypothetical resonances [25], An F-shaped slotted MIMO antenna with users hand effect [26], two monopole antennas with two rectangular etched slots and a T-shaped stub [27], an elliptical patch antenna with an elliptical slot and dipole fed [28], an antenna with a radiating element made of CSRR slots and fed by a meandered CPW [29], and U-shaped slot antennas with wide band applications are among the antenna types that have been identified [30, 31].

The comparatively high gain, improved efficiencies, and small footprint of printed slot antenna designs, however, continue to be an issue. The literature lists a few drawbacks of slot antennas for 5G applications in the sub-6 GHz range, including their bigger slot size, narrow impedance bandwidth, low gain, low efficiency, etc. The antenna must be installed in conjunction with the mobile device's dielectric back cover because space is at a premium in 5G portable devices. Low-profile antennas are necessary for 5G applications because of this. According to this, the entire antenna thickness at 3.3 GHz should be around 1 mm [32].

A defined statistic called the Specific Absorption Rate (SAR) is used to calculate how much energy is absorbed by human tissue. The Federal Communication Commission (FCC) established SAR limitations as safety regulations to shield consumers from harmful radiofrequency radiation. The time derivative of the excess energy absorbed by (dissipated in) an additional mass contained in a volume element with a particular density (1/2) is what IEEE definitions state for SAR [33]. IEEE C95.1:2005 states that the SAR limit for any 10 g of tissue is 2 W/kg [33]. This limit and the recommendations of the International Committee on Non-Ionizing Radiation Protection are similar [34,35,36].

The design and execution of planar semicircular slot antennas with dual bands and wide bands for 5G wireless applications that operate at sub-6 GHz are presented in this paper. The feed for the U-shaped radiating patch comes from a transmission cable. A ground structure (DGS) deformed in a semicircle, which is etched beneath the radiating patch, allows the bandwidth to be changed while also reducing the antenna's size. The U-shaped radiating region has a T-shaped stub structure attached in the middle to allow for dual-band operation. When the transverse part of the T structure is connected to the two U's limbs, the antenna becomes a wideband antenna (WBA). A band-stop filter with a controllable resonant frequency can be additionally made by shortening and adjusting the length. LB Of the transverse sector of the T structure, which creates and controls the antenna's two operational bands. When LB = 10.3 mm, the antenna acts as WBA with frequency band from 1.8 to 6 GHz. When LB = 4.3 mm, the antenna generates dual bands from 1.8 to 3.7 GHz and from 4.05 to 5.5 GHz. The antenna design has a compact size of (0.35λ0 × 0.35λ0) where λ0 Denotes the free space wavelength, peak gain, and efficiency of about 8.5 dBi and 93%, respectively. When the dielectric back cover and human head-hand model were present during the simulation, the antenna worked properly. The study has determined a specific absorption rate (SAR) value of 0.121 W/kg at 3.3 GHz and 4.5 GHz that is safe for use while maintaining superior efficiency and radiation patterns. The simulations and measurements have very good agreements.

The key contributions of this study are summarized as follows:

  1. 1.

    The 0.35λ0 × 0.35λ0. The compact size of the suggested antenna construction

  2. 2.

    It operates throughout a wide frequency range of 1.8–6 GHz.

  3. 3.

    By adjusting the resonance frequency of the embedded band stop filter, which is carried out by the T-shaped structure in the design, it may offer a dual-band operation.

  4. 4.

    The suggested antenna has a realized peak gain of about 8.5 dBi and is intended for ITU sub-6 GHz 5G applications.

  5. 5.

    The suggested antenna's advantages of simple manufacturing, affordability, and low profile make it a strong contender for the design of multi-band antennas.

  6. 6.

    Two working bands are offered by the suggested dual band antenna: 1.8–3.7 GHz and 4.05–5.5 GHz.

  7. 7.

    At 3.3 GHz and 4.5 GHz, the compact dual-band slot antenna offers a low SAR value of roughly 0.121 W/kg.

  8. 8.

    Over the operational frequency bands, the suggested dual-band slot antenna produced directed radiation and good gain.

The paper is organized as follows: As explained in Sect. 2, the suggested slot antenna is built and simulated using the computer-simulated technology (CST) microwave package. Section 3 contains specifics about the experiment and manufacture. An understanding of how the dielectric back cover affects the antenna properties is provided in Sect. 4. An understanding of how the user's hand-head model affects the antenna characteristics is provided in Sect. 5. A comparison of the suggested antennas with the relevant works is shown in Sect. 6. Lastly, the conclusion is provided in Sect. 7.

2 Method and experiment

2.1 Proposed antenna structures

In this section, highly efficient and compact designs for wideband and dual-band antenna structures are introduced. The proposed semicircular slot antenna structures are designed on an FR4 substrate of thickness (h = 1.5 mm), dielectric constant/relative permittivity (εr = 4.5), and the loss tangent (δ = 0.025). The radiating patch is designed to have a U-shaped structure with a transmission line feed having a width (Wf). Beneath the radiating patch, a semicircle-defected ground structure (DGS) is etched to control the bandwidth and minimize the size of the antenna. A T-shaped stub construction is affixed in the center of the U-shaped radiating area to enable dual-band operation. The antenna functions as a wideband antenna (WBA) if the transverse portion of the T construction is joined to the two U's limbs, as shown in Fig. 1. Furthermore, as seen in Fig. 2, shortening and modifying the transverse sector of the T structure constitutes constructing a band-stop filter with an adjustable resonant frequency that controls the antenna's two operating bands. Table 1 lists the dimensions of the two suggested antenna structures.

Fig. 1
figure 1

Wideband antenna structure at LB = 10.3 mm: a top view b bottom view

Full size image
Fig. 2
figure 2

Dual-band antenna structure at LB = 4.3 mm: a top view b bottom view

Full size image
Table 1 The dimensions of the proposed antenna structures for both wideband and dual-band modes of operation
Full size table

The U-shaped antenna, often referred to as a U-shaped patch antenna, is a type of microstrip antenna that includes a U-shaped patch. The design and analysis of such antennas can be complex, as it involves understanding the distribution of the electromagnetic fields, the resonant frequencies, and the impedance characteristics. However, a basic outline of the equations involved in the design of a U-patch antenna can be given. The equations from 1 to 5 used for calculating dimensions of antenna.

For a simple rectangular microstrip patch antenna, the resonant frequency f0 is given by:

$$ f_{0} = \frac{1}{{2\sqrt {\epsilon_{{\text{r}}} L} }}\sqrt{\frac{c}{2L}} $$
(1)

where c is the speed of light in free space (3 × 108 m/s), \(\epsilon_{{\text{r}}}\) is the relative permittivity of the substrate, L is the length of the patch.

The effective dielectric constant (\(\epsilon_{{{\text{re}}}}\)) can be approximated as:

$$ \epsilon_{{{\text{re}}}} = \frac{{\epsilon_{{\text{r}}} + 1}}{2} + \frac{{\epsilon_{{\text{r}}} - 1}}{2}\left( {1 + 12\frac{h}{w}} \right)^{ - 0.5} $$
(2)

where h is the height of the substrate, W is the width of the patch.

The dimensions of the patch (width W and length L) are given by:

$$ W = \frac{c}{{2f_{0} }}\sqrt {\frac{2}{{\epsilon_{{\text{r}}} + 1}}} $$
(3)
$$ L = \frac{c}{{2f_{0} \sqrt {\epsilon_{{{\text{re}}}} } }} - 2\Delta L $$
(4)

where ΔL is the length extension due to fringing fields and is given by:

$$ \Delta L = 0.412h\frac{{\left( {\epsilon_{{{\text{re}}}} + 0.3} \right)\left( {\frac{w}{h} + 0.264} \right)}}{{\left( {\epsilon_{{{\text{re}}}} - 0.258} \right)\left( {\frac{w}{h} + 0.8} \right)}} $$
(5)

The U-shaped dimensions will vary based on the desired performance and tuning. The u shaped length and width will influence the impedance bandwidth and can be optimized based on experimental results or simulation.

Figure 3 shows the simulated scattering parameter. |S11| of the proposed antenna structure at various values of the parameter LB from 4.3 to 10.3 mm. When the length of T-shaped structure LB = 10.3 mm, the antenna functions as a WBA with a broad operating frequency band from 1.8 to 6 GHz considering that |S11|≤ − 10 dB.

Fig. 3
figure 3

The simulated scattering parameter |S11| of the proposed antenna structure at various values of the parameter LB from 4.3 to 10.3 mm

Full size image

Nonetheless, the antenna operates as a dual band antenna with two operational bands from 1.8 to 3.7 GHz and from 4.05 to 5.5 GHz when the length of the T-shaped structure. LB = 4.3 mm, which makes the antenna suitable for sub-6 GHz 5G applications. Resonant at 3.85 GHz, the T-shaped structure with length LB = 4.3 mm functions as a band-stop filter. As seen in Fig. 4, at this frequency, the antenna's return loss increases above |S11|> − 10 dB and suppresses the frequencies between 3.7 and 4.05 GHz.

Fig. 4
figure 4

Simulated return loss |S11| of the proposed antenna at LB = 4.3 mm

Full size image

Figure 5 shows the extracted surface current distributions, which provide a better understanding of the design's operation. Two distinct frequency bands are formed, ranging from 1.8 to 3.7 GHz and from 4.05 to 5.5 GHz, respectively, by the copious surface currents flowing along the double-folded edge of the feed patch at 3.3 GHz and the center T-shaped structure at 4.5 GHz. Both frequency bands were produced with the help of the surface currents.

Fig. 5
figure 5

Simulated current distributions for LB = 4.3 mm at a 3.3 GHz and b 4.5 GHz

Full size image

3 Results and discussion

Photoetching is used to make the suggested antenna prototypes, as shown in Fig. 6a, b. The antenna measurement setup, which is used to characterize the manufactured prototypes and compute the radiation parameters in the azimuth and elevation planes, comprises a test measurement horn antenna configuration in an anechoic chamber and a Vector Network Analyzer (VNA-E5071C).

Fig. 6
figure 6

Fabricated prototypes of the proposed antennas: a At LB = 10.3 mm, and b At LB = 4.3 mm

Full size image

The simulated and measured return losses of the proposed antenna prototypes for LB = 10.3 mm and LB = 4.3 mm, respectively, are compared in Fig. 7a, b. The results of the testing indicated that the antenna resonates in the frequency bands. 0.8–3.7 GHz and from 4.1 to 5.6 GHz, respectively, are remarkably similar to the simulations’ results at LB = 10.3 mm and. The sub-6 GHz frequency bands identified by ITU fITU define or next-generation 5G applications, n77 (3.3–4.2 GHz), n78 (3.3–3.8 GHz), and n79 (4.4–5 GHz), on the other hand, are included in the frequency range of 1.8–6 GHz, according to measurements at LB = 10.3 mm [28].

Fig. 7
figure 7

Measured and simulated return loss of the proposed antennas at: a LB = 10.3 mm, and b LB = 4.3 mm

Full size image

Together with the simulated and measured 2D radiation patterns for the intended frequencies of 3.3 GHz and 4.5 GHz. Figure 8 shows the computed 3D radiation patterns for the proposed antenna for LB = 4.3 mm. The radiation patterns in the H- and E-planes show plots of cross- and co-polarization, respectively. The excellent coincidence between the generated and measured patterns is evident. The little discrepancies between the simulated and measured findings could be the consequence of errors occurring in the connectors' soldering and manufacturing processes. Furthermore, variations in the S-parameters and radiation patterns that have been observed may be connected to the anechoic chamber's.

Fig. 8
figure 8

The 3D radiation patterns of the proposed antenna for LB = 4.3 mm at a 3.3 GHz, and b 4.5 GHz, and the measured and simulated 2D radiation patterns of the proposed antenna for LB = 4.3 mm at c 3.3 GHz, and d 4.5 GHz

Full size image

The suggested antenna design's gain has been assessed using the Gain Transfer/Gain Comparison Method in compliance with the IEEE standard test methodology. The anechoic chamber's antenna positioner holds a single reference antenna with a known gain, which is used in the gain computation. Once that is done, the constructed prototype is aligned parallel to the reference horn antenna and toward the direction of greatest radiation. During the VNA measurement process, we turned on the S21 parameter in order to determine the gain of the suggested antenna in comparison to the reference antenna. Figure 9 depicts the measuring apparatus, which consists of a VNA and an anechoic chamber. According to the observed data, the suggested antenna's realized peak gain for LB = 4.3 mm is 8 dBi. This result agrees with the simulation's estimated peak gain of 8.5 dBi, which is depicted in Fig. 10.

Fig. 9
figure 9

Measurement setup of the proposed antennas a VNA and b anechoic chamber

Full size image
Fig. 10
figure 10

Gain of the proposed antenna for LB = 4.3 mm

Full size image

The gain is calculated before the directivity and reflection coefficient because all antenna measurement equipment is automated. The antenna's efficiency is computed last. According to the observed data, the constructed prototype radiates at a total efficiency of about 90%, which is similar to the outcomes of the simulations displayed in Fig. 11.

Fig. 11
figure 11

Efficiency of the proposed antenna for LB = 4.3 mm

Full size image

4 Impact of the back cover on the antenna characteristics

Potential ramifications have been investigated for the rear cover of the suggested antenna, whose permittivity and loss tangent values are 3.32 and 0.002, respectively, for the dielectric substance that makes up the structure. The antenna schematic is shown in Fig. 12, together with the back cover, whose measurements are provided in the same figure. The simulated S-parameters and the antenna's ability to match the back cover are shown in Fig. 13. The antenna resonates in the two frequency bands of 0.8–3.7 GHz and 4.1–5.6 GHz, respectively, according to the simulated |S11| without a rear cover. On the other hand, when inserting the back cover, the simulated |S11|. Bands are slightly shifted to the frequency bands from 1.65 to 3.7 GHz and from 4.3 to 5.2 GHz, respectively, that is due to the dielectric loss of the material constituting the back cover.

Fig. 12
figure 12

Diagram for an antenna with a dielectric rear cover

Full size image
Fig. 13
figure 13

Comparison of the |S11| parameter of the antenna with and without a dielectric back cover

Full size image

The antenna with a rear cover at 3.3 GHz and 4.5 GHz is shown in Fig. 14's 3D and 2D radiation patterns. Without a back cover, the antenna has a gain of 8.5 dBi and an efficiency of 90%. Conversely, the antenna's efficiency and gain drop to 67% and 7.47 dBi, respectively, once the back cover is installed. The primary cause of the slight performance variation between an antenna with and without a back cover is the dielectric loss of the material that makes up the rear cover, according to the data. Despite this decline, the antenna radiation efficiency is still demonstrated to be roughly 67%, which is more than sufficient for 5G mobile applications.

Fig. 14
figure 14

3D radiation patterns of the antenna with a back cover only at a 3.3 GHz and b 4.5 GHz, 2D radiation patterns of the antenna with a back cover only at c 3.3 GHz and d 4.5 GHz

Full size image

5 Impact of the hand-head model of the users on the antenna characteristics

This section examines how the user's hand-head model affects the antenna's performance while taking into account the specific absorption rate (SAR) value, realized gain, radiation efficiency, pattern, and return loss, as shown in Fig. 15. The head model has an isotropic resolution of (1 mm × 1 mm × 1 mm). with (196 × 310 × 176) voxel components.

Fig. 15
figure 15

Antenna placement for user’s head-hand scenario

Full size image

Its twenty-five dielectric property values allow this model to be classified into almost forty different tissue types. To streamline the model and minimize the number of dielectric properties, tissues in the same category, such as the eye, sclera, cornea, and vitreous, were given the same electric conductivity and permittivity values. Reducing the number of mesh cells and speeding up the simulation time was possible by simply considering the model's head area. Table 2 lists the attributes of the human head model [37].

Table 2 Properties of human head model
Full size table

Figure 16 shows the 3D radiation patterns of the antenna with a back cover and human hand-head model combined, respectively.

Fig. 16
figure 16

3D Radiation patterns of the antenna with a combined back cover and human head-hand model at a 3.3 GHz and b 4.5 GHz. 2D radiation patterns at c 3.3 GHz and d 4.5 GHz

Full size image

The SAR value is a critical metric that governs how an antenna's radiation pattern varies when human head and hand tissues are present. The SAR calculates how much radiofrequency radiation is absorbed by human tissue while it is being transmitted. SAR depends on the electrical conductivity (σ, in Siemens/meter), the mass density of the tissue (ρ, in kg/cubic meter), and the E-field produced by the radiated energy (measured in Volts/meter). As mentioned, the SAR is calculated by integrating or averaging across a certain volume, often a region weighing one or ten grams [38].

$$ {\text{SAR}} = \int {\frac{{\sigma \left( r \right)\left| {E\left( r \right)} \right|^{2} }}{\rho \left( r \right)}{\text{d}}r} $$
(6)

SAR is measured in W/kg, or mW/g, as an equivalent. In the USA, the SAR limit for cell phones is 1.6 W/kg, averaging over 1 g of tissue. The SAR limit in Europe is 2.0 W/kg, averaging over 10 g of tissue. If the phone satisfies the US requirements, it will usually also fulfill the European specification because the US specification is generally more difficult to attain than the European specification [39]. Research has been done on the antenna's SAR effect, which determines how much energy the body absorbs when the antenna is broadcasting or receiving. When designing the antenna for portable mobile devices, that was an important factor to take into account.

According to FCC regulations, 5G devices can have their antenna input power adjusted to 15 dBm, 18 dBm, or 20 dBm as long as there is a 5 mm space between the antenna and head [40,41,42].

Results for SAR1g and SAR10g at 15 dBm at 3.3 GHz and 4.5 GHz are shown in Fig. 17. The figure indicates that SAR increases with increasing distance from the antennas.

Fig. 17
figure 17

The proposed antenna was SAR-researched for 15dBm at 3.3 GHz a 10 g, b 1 g, and 4.5 GHz c 10 g, d 1 g

Full size image

Table 3 displays the simulation results for SAR1g and SAR10g of the suggested structure at 3.3/4.5 GHz with 15 dBm power. It is clear that all of the simulated values meet the structural limitations established by the FCC and ICNIRP guidelines and that the SAR10g values at 3.3/4.5 GHz are equivalent to 0.121 W/kg, which is within allowable bounds.

Table 3 15 dBm power simulations of the proposed structure's SAR1g and SAR10g
Full size table

It is worth mentioning that the performance of an antenna can be influenced by environmental factors such as humidity and temperature. These factors can affect both the materials of the antenna and the surrounding environment, leading to variations in performance. To analyze the impact of humidity and temperature on antenna performance, simulations using CST Microwave Studio can be conducted. This tool allows for detailed modeling of environmental conditions [41, 42].

6 Comparison with related works

Despite attempts to produce a dual-band slot antenna with outstanding performance, design considerations like the low profile and improved gain make it challenging to sacrifice the compact size. In this paper, a compact and low SAR effective design for a dual-band slot antenna is introduced. As the most important factors for mobile terminal applications, the impact of human tissues and the dielectric back cover were studied when developing the proposed structure for 5G applications.

In comparison to the works presented in the papers from #1 to #9 listed in Table 4, the proposed dual-band slot antenna exhibits excellent features and performance in terms of size, fractional bandwidth (%), the operating band (GHz), and gain (dBi). Table 4 lists the competing variants of the available dual-band slot antenna designs that are compared. The findings unequivocally demonstrate that in terms of overall antenna size, including the ground plane, the recommended antenna—fit for 5G terminal devices—is smaller than the reference antennas.

Table 4 Detailed comparison of the proposed dual-band slot antenna with the most recent research literature
Full size table

Lower SAR values will result in less electromagnetic field penetration into human tissue. To compare the SAR results with other literature, Table 5 is included. Table 5 demonstrates that generally speaking, the SAR values for the recommended antenna are lower than those from the previous study.

Table 5 SAR comparison among the suggested design and preceding work
Full size table

7 Conclusion

The novel dual-band planar semicircular slot antenna for sub-6 GHz 5G wireless applications is the main focus of this work. CST Microwave Studio has been used to simulate the proposed dual-band antenna design. The performance of the antenna with respect to its parameters was analyzed considering the 10 dB return loss. |S11|, peak gain (dBi), total efficiency, radiation patterns, and SAR value calculations. The stand-alone antenna without a back cover has a small footprint of (0.35λ0 × 0.35λ0), peak gain, and efficiency of about 8.5 dBi and 93%, respectively. When inserting the back cover, the simulated |S11| bands were slightly shifted to the frequency bands from 1.65 to 3.7 GHz and from 4.3 to 5.2 GHz, respectively, that is due to the dielectric loss of the material constituting the back cover. Additionally, the antenna's efficiency and gain were decreased to 7.47 dBi and 67%, respectively; despite this, they are still more than sufficient for 5G mobile applications. Furthermore, the effect of the user's hand-head model and the dielectric rear cover on the antenna performance was investigated. According to the simulations, all of the simulated values adhere to the structural limitations established by the FCC and ICNIRP standards, and the SAR10g values at 3.3/4.5 GHz are extremely low, equal to 0.121 W/kg, which are within allowable bounds. This antenna's less complicated design, cheaper manufacturing cost, and all the associated factors make it a potential competitor for sub-6 GHz 5G applications.