Design and analysis of optically transparent high gain grid array antenna for vehicular communications

Abstract

The demand for transparent antennas in vehicular communication has increased due to their aesthetic integration, aerodynamics, and electromagnetic compatibility. Most researchers have employed multiple-input-multiple-output (MIMO) antennas for vehicular communication, but grid array antennas (GAAs) offer several advantages over conventional MIMO configurations, such as higher gain, improved directivity, and simpler feed structures. The presented work integrates the benefits of both GAAs and transparent antenna technology to achieve an efficient solution for modern vehicular communication systems. A transparent high gain GAA that operates in the X-band (8–12 GHz) for vehicular communication is proposed and developed. The transparent GAA is made up of 29 elements of varying sizes, arranged in two rows. Each element has two L-shaped arms that are connected to the top and bottom of the circular rings, along with a vertical strip that connects the horizontal long strips. The antenna measures 4.81λ₀ × 0.66λ₀ × 0.026λ₀, with a peak gain of 17 dBi at 12 GHz. The amplitude tapering of the Taylor synthesis approach results in a side lobe level of −19.2 dB. In the E-plane, the half-power beamwidth is 7.9˚, while in the H-plane, it is 173.6˚. Also, a virtual automobile model is used to investigate the housing effects of the proposed antenna. The findings show that GAA is ideal for advanced driver assistance systems, automotive radar, and vehicle to everything communication. The presented antenna offers aesthetic integration, space savings by using glass surfaces, and better signal reception from unobstructed locations.

Introduction

The use of automotive technology to create secure and effective communication systems is expanding quickly day after day¹. Vehicular communication is vital because it improves road safety, traffic efficiency, and enables future intelligent transportation systems (ITS)². There are various types of vehicular communication, such as vehicle to vehicle (V2V), vehicle to infrastructure (V2I), vehicle to network (V2N), vehicle to pedestrian (V2P), and vehicle to everything (V2X)^3,4. In V2V, vehicles communicate directly with each other to share speed, location, and provides warnings like collision, emergency braking alerts, and cooperative lane changes, thereby preventing road accidents and improving safety⁵. In V2I⁶, vehicles communicate with roadside units (RSUs), traffic signals, and toll booths and provides traffic congestion alerts, thus improves traffic efficiency and reduces waiting time in toll booths⁷. In V2N, vehicles connect to cellular or cloud networks for real-time data sharing like navigation updates and optimized route planning. V2P allows communication between vehicles and pedestrians (via smartphones or wearable devices) and provides pedestrian crossing alerts and bicycle collision warnings. V2X, is the combination of all communication modes (V2V, V2I, V2N, V2P) and provides seamless connectivity and supports autonomous driving^8–10.

Transparent antennas are becoming increasingly important in vehicular communication because of their aesthetic and hidden integration, better line of sight, multiband operation, and provides more space by integrating into windshields, rear windows, or panoramic sunroofs, leaving more room for radar sensors and LiDAR units^11–14. Recently, microstrip grid array antennas (GAAs) have gained popularity due to their high gain, high efficiency, sectoral beam, low side lobe levels (SLLs), and ease of manufacturing^15–18. GAAs are suitable for vehicular communications because they have a high gain and can overcome path loss. Grid arrays inherently produce narrow beams in the elevation plane, reducing interference and multipath. This is advantageous in vehicular scenarios, where communication is often point-to-point (e.g., radar, V2I links). The GAAs broad beam in the azimuth plane allows it to cover multiple directions without requiring mechanical rotation. It can be easily integrated into vehicle roofs or transparent surfaces.

Numerous antenna designs for vehicular communication have been reported in the literature. In¹⁹, the constructed transparent antenna worked from 4 to 7 GHz, with a maximum gain of 6.23 dBi for V2X communication. In²⁰, the developed transparent antenna was used in an ultra-wideband (UWB) automotive application, but it only provides a 2 dBi gain. In²¹, a transparent antenna for vehicular communication was developed that operates between 5.2 and 6.86 GHz with a peak gain of 2.94 dB. In²², a transparent antenna was reported for vehicular communication that operated from 1.82 GHz to 2.5 GHz and 4.66 to 11.84 GHz, with a gain of −0.145 dB. In²³, antenna operated from 8 to 12 GHz was developed and it had a maximum gain of 4.9 dB with SLL of 11.378 dB. In²⁴, a transparent multiple-input-multiple-output (MIMO) antenna was developed for vehicular communication and the obtained gain was 1 dBi. In²⁵, a transparent antenna with directional radiation was developed, with a gain of −6 dBi. A transparent antenna with a wheel-shaped radiator was developed for automotive applications in²⁶, with a maximum gain of 5.9 dBi. The article²⁷ describes a transparent wideband antenna for vehicular communication with a beamwidth of 34˚ in the E-plane and 107˚ in the H-plane. In²⁸, a non-transparent GAA was developed for vehicular communication, with a beamwidth of 95.7˚ at azimuth angle. In²⁹, an opaque GAA was developed with a side lobe level of −14 dB, achieving beamwidths of 18.5˚ in elevation and 34.5˚ in azimuthal plane. The antennas in literature^44–47 were developed for vehicular communications, which operates at 8 to 12 GHz. The literature suggests that the proposed antennas are suitable for a wide range of intelligent transportation and vehicular communication applications, including V2I, vehicle-to-satellite links, vehicle data transmission, V2V, ITS, V2E, and advanced driver assistance systems (ADAS). According to the literature, while transparent antennas have many advantages, they have very low gain, thus the signal cannot travel far enough to maintain reliable V2V or V2I links, resulting in increased interference. GAAs can be used to maximise gain, but there have been very few transparent GAAs developed in the literature, and those antennas are not intended for vehicular communication. In vehicular communication, a narrow beam in the elevation plane is required to focus energy on the road and minimise sky and ground reflections, while a broader beam in the azimuth plane is required to cover multiple lanes. However, the majority of antennas designed for vehicular communication are omnidirectional or have a wider elevation plane and a narrow azimuth plane. Also, SLLs should be kept to a minimum, but many antennas have levels close to or greater than −15 dB.

Considering the aforementioned problems, this research develops an optically transparent GAA that operates in the 8 to 12 GHz range for vehicular communication. To the best of the authors’ knowledge, no transparent antenna based on a grid array structure has been reported in the literature for vehicular communication applications. In this work, by combining the advantages of transparent antennas and GAA, an optically transparent GAA has been developed specifically for vehicular communication systems. It includes applications such as automotive radar (collision avoidance, blind spot detection), V2V, V2I (smart traffic control, toll collection systems), synthetic aperture radar (SAR) on vehicles (ground mapping from autonomous vehicles, military grade vehicle mounted systems), and satellite communication (in vehicular SATCOM) (vehicle mounted satellite dishes for emergency vehicles, news vans). The antenna is designed to provide a maximum gain of 17 dBi while remaining transparent. The beamwidth is 7.9˚ in elevation plane and 173.6˚ in azimuth plane. The amplitude tapering method is used to reduce SLLs by up to −19.7 dB. Furthermore, the developed antenna has a fractional bandwidth of 40%, indicating wideband performance. Antenna housing effects and on-car analysis are performed while antenna performance remains unchanged. Also, path loss and link budget analysis are performed to better understand the data transmission range.

Investigation of optical properties of transparent antenna

Transparent antennas are becoming increasingly popular in the automotive communication industry because of their aesthetic and hidden integration, better line of sight, multiband operation, and provides more space by integrating into windshields, rear windows, or panoramic sunroofs. In this research work, an optically transparent antenna is chosen instead of traditional opaque substrates such as FR4 and Rogers to support vehicular communication. This approach enhances antenna performance while preserving the aesthetics of the vehicle, making it highly suitable for emerging connected and autonomous vehicle platforms. Transparent antennas are commonly made from a glass substrate and a conductive layer made of transparent conductive oxides, allowing the antenna to transmit and receive electromagnetic signals while remaining visually unobtrusive. The glass substrate provides a smooth, sturdy, and durable base, assuring mechanical robustness and environmental resilience, making it ideal for vehicle applications that are subjected to vibration, temperature changes, and weather conditions. In the presented antenna design, indium tin oxide (ITO) is selected as the conductive layer due to its unique combination of high optical transparency (often exceeding 80% in the visible spectrum) and good electrical conductivity, which is essential for efficient RF performance.

For efficient performance of the transparent antenna, the conductivity as well as optical transparency has to be balanced. Here, thickness of the transparent material, sheet resistance, efficiency, skin depth are important parameters that determine the performance of the transparent ITO material. If the thickness of the material is high, conductivity of the material is high but it affects the optical transmittance of the material. If the thickness of the ITO is less, optical transmittance is high but results in higher sheet resistance. Higher sheet resistance leads to less conductivity of the material, which in turn affects the performance of the antenna. If the sheet resistance is less, conductivity will be more and the efficiency of the antenna is high. Skin depth of the material should be less than the thickness of the material, so that the current penetrates the entire film and it will have uniform conductivity, thus increasing the performance of the antenna. Therefore, it is essential to compute Eq. (1) to (4) to choose optimal thickness, sheet resistance, skin depth and optical transmittance of the ITO material to obtain better performance of the proposed optically transparent GAA. The sheet resistance can be computed using Eqs. (1),

1

where R_s represents the sheet resistance, while L, W, and t represent the length, width, and thickness of the patch, respectively. Skin depth and film conductivity are denoted as and  , respectively. The efficiency is calculated using the Eqs. (2),

2

The optical transmittance of the ITO film is investigated using Drude’s model, as shown in Eq. (3).

3

The skin depth of the material can be calculated using Eqs. (4),

4

where µ denotes the permeability of the material and the electrical conductivity is given as σ. Figure 1(a) and 1(b) show the sheet resistance and the efficiency in relation to ITO thickness. Lower sheet resistance improves conductivity, which is desirable for better RF performance in applications such as antennas and shielding, as shown in Fig. 1(a). However, extremely low sheet resistance frequently comes at the expense of optical transparency. Figure 1(b) demonstrates that greater thicknesses result in higher efficiency. However, an optimal trade-off between this and transparency is required in transparent antenna designs. Thus, optical transparency, conductivity, and efficiency must all be balanced. Transparent antennas would benefit from a thickness of 200 nm or less.

Fig. 1.

Properties of ITO at various film thicknesses: (a) Sheet resistance, (b) Efficiency.

Figure 2(a) depicts the optical transmittance of the ITO film with respect to frequency at various material thicknesses, while Fig. 2(b) depicts the skin depth of the ITO in relationship to frequency. Transmittance > 95% is achievable up to 600 nm thickness, as shown in Fig. 2(a), with a film thickness of 200 nm producing transmittance > 98%. In Fig. 2(b), the film thickness (100–900 nm) is much less than the skin depth (6.5–8 μm), thus the current penetrates the entire film. Thus, the film behaves as a uniform conductor and not a limiting factor in the proposed frequency range.

Fig. 2.

Properties of thin ITO film: (a) Variation of ITO film transmittance with frequency and film thickness, (b) Skin depth of the ITO in relation to different frequencies.

Development of the optically transparent GAA

The developed transparent grid array is made of a glass substrate with a dielectric constant of 4.6, loss tangent of 0.002, and thickness of 1.1 mm. The glass substrate has a transparent ITO film with a conductivity of 5 × 10⁵ S/m and a thickness of 200 nm. The ITO film serves as the conductive layer, and its optical transmittance exceeds or equals 90%. The top conductive layer of the proposed GAA is made up of 29 elements, arranged in two rows.

Each element consists of concentric rings attached to a vertical strip (column) that connects to two long horizontal strips (rows) at either end. The top right side of the concentric ring is connected to a rotated L-shaped structure, while the bottom left side is connected to another inverted L-shaped structure. The top and bottom horizontal strips are tapered because the vertical strips are of varying lengths and the concentric elements are varying sizes. The size of the centre concentric ring is large, but it decreases significantly for the other concentric rings. Similarly, the centre vertical strip (column) is longer than the other vertical strips. The middle concentric ring and vertical strip are the largest in size, while the adjacent elements gradually decrease in size, and the final element on both sides of the constructed antenna is the smallest in size. An elliptical slot is embedded in the bottom layer (ground) of the developed antenna. Figure 3(a) shows the top layer, Fig. 3(b) shows the bottom layer, and Fig. 3(c) shows the structure of the unit cell of the proposed transparent GAA. Table 1 lists the structural parameters of the constructed antenna.

Fig. 3.

Design of the developed GAA: (a) Top layer, (b) Bottom layer, (c) Structure of the element.

Table 1.

Structural parameters of the constructed GAA.

Parameter	Description	Dimensions (mm)
L	Length of the GAA	180.51
L 1	Length of the top horizontal strip	156.71
L 2	Length of the middle horizontal strip	176.35
L 3	Length of the bottom horizontal strip	168.71
L 4	Length of the middle element	10.41
L 5	Length of the left L-shaped structure	2.66
L 6	Length of the elliptical slot	7
W	Width of the GAA	24.9
W 1	Width of the middle horizontal strip	1.86
W 2	Width of the top L-shaped structure	4.92
W 3	Width of the vertical strip	0.71
W 4	Width of the L-shaped structure	0.71
W 5	Width of the elliptical slot	2.5
D 1	Inner diameter of the middle concentric ring	1.5
D 2	Outer diameter of the middle concentric ring	2.5

Evolution stages of the GAA

This work focuses on developing an antenna with an impedance bandwidth below −10 dB within the 8–12 GHz range. Any frequency components exhibiting an impedance bandwidth below −10 dB outside this range (i.e., below 8 GHz or above 12 GHz) must be suppressed. Additionally, the design prioritizes achieving low SLLs, with a target of maintaining SLL values below −15 dB to minimize radiation in undesired directions. The evolution stages of the proposed antenna are shown in Fig. 4. In stage I, the GAA started as two rows and 29 columns. The first row consists of 14 elements and the second row contains 15 columns. This type of arrangement along with optimal aperture size and element spacing helps to achieve narrow beamwidth in elevation plane and broad beamwidth in azimuthal plane. The narrow beam in the elevation ensures that the majority of the energy is focused along the road level, reducing unnecessary radiation upwards (to the sky) and downwards (to the ground). This maximises power towards other vehicles or RSU of comparable heights and the azimuthal angle helps the antenna vehicle to communicate with objects or other vehicles from a wide range of angles. Accordingly, a smaller number of elements are arranged in top row and a greater number of elements are placed in the bottom row with necessary element spacing and appropriate aperture size. The current distribution in vertical strip is in-phase and interfere constructively, while the current distribution is out-of-phase and interfere destructively in horizontal strips at far field, thus the arrangement produces horizontal coverage. Thus, arranging 14 elements on the top row and 15 elements in bottom row, provides narrow beam in elevation plane and wide beam in azimuth plane. Hence, non-uniform number of elements are placed in two rows rather than uniform number of elements. This is critical because vehicles can approach from multiple directions, and infrastructure (such as RSUs) may not always be directly in front of the vehicle. By arranging the antennas in two rows and 29 columns the obtained reflection coefficient is found to be less than −10 dB from 9.82 to 12 GHz, as well as in the unwanted bands of 7 to 7.9 GHz and 12.1 to 13 GHz. It is necessary to make the antenna operate in the required band (8 to 12 GHz) while also eliminating unwanted bands. The minimal reflection coefficient obtained is −42.19 dB at 9.92 GHz. The obtained reflection coefficients are shown in Fig. 5(a), and the SLL is demonstrated in Fig. 5(b). The minimal SLL is found to be −10.5 dB in stage I.

Fig. 4.

Evolution stages of the constructed GAA: (a) Stage I, (b) Stage II, (c) Stage III, (d) Stage IV, (e) Stage V.

Fig. 5.

Evolution stages: (a) Reflection coefficients, (b) SLL.

In stage II, concentric rings are added to the columns. The concentric rings generate multiple closely spaced resonances due to mutual electromagnetic coupling, which provides a wide impedance bandwidth of 7.51–13 GHz and an SLL of −12.2 dB is obtained. The minimum reflection coefficient obtained is −32.55 dB at 8.84 GHz. In Stage III, adding L-shaped stubs at the opposite corners introduces reactive loading that suppresses unwanted higher order modes and shifts the resonances to the 7.75–13 GHz range. This modification also improves current uniformity across the aperture, enabling the antenna to achieve an SLL of −15 dB. The lowest reflection coefficient is found is −36.83 dB at 10.09 GHz. In stage IV, to further eliminate the unwanted band and reduce the SLL, vertical stripes on the two rows are tapered and the sizes of the concentric rings are varied. The length of the vertical strips in the two rows is tapered, and the size of the 29 concentric rings in the two rows is gradually reduced beginning at the zeroth position. The size of the concentric ring is large in the zeroth position and gradually decreases by a factor of 0.03 mm on both sides. Tapering the length of the vertical stripes on either side of the array by a factor of 0.15 mm reduces the amplitude of the current flowing from the centre of the proposed antenna to the ends on either side. This significantly reduces the SLL of the constructed antenna to −19.2 dB and the antenna operated from 7.9 to 12.5 GHz. The minimal reflection coefficient attained is −35.43 dB at 10.07 GHz.

In stage V, an elliptical slot is introduced beneath the grid to eliminate the remaining unwanted band. Its broadband slot-dipole behaviour and ability to interrupt surface wave modes ensure smooth feed coupling and stabilize the reflection coefficient within the desired band. The designed antenna operates from 8 to 12 GHz with an SLL of −19.7 dB. In stage IV, the SLL obtained is below −17.2 dB throughout the frequency range and the minimal SLL attained is 19.2 dB. After introducing elliptical in stage V, the SLL is found lesser than −18.2 dB in the entire band and the minimal SLL obtained is 19.7 dB. Thus, elliptical slot enhances the performance of the antenna. The lowest reflection coefficient achieved is −50.98 dB at 11.53 GHz.

Discussion on tapering and reducing the size of the radiating elements

The SLL is important in GAAs for vehicular communications because high SLLs emit energy in an unintended direction, increasing the risk of interfering with nearby systems, other vehicles, or infrastructure nodes. In congested traffic areas, this can result in vehicle crosstalk and communication failure due to interference. In vehicular radar, the probability of false target detection is higher, and the link budget is less efficient. In this work, Taylor synthesis amplitude tapering method is used to reduce SLL. Taylor synthesis is an amplitude distribution technique that specifies the current (or excitation amplitude) assigned to each element in an antenna array. In conventional method, all elements in an array are equally excited, which results in high gain but also will have high SLL. In amplitude tapering method, instead of exciting all elements equally, higher amplitude is given to the centre elements and progressively lower amplitude to the elements on the edges. This smooth tapering reduces the sudden discontinuities that normally generate high side lobes. To gradually reduce the amplitude of the current flowing in the centre of array element to both the side of the array edges, the length of the vertical strips are tapered and size of the concentric rings is also reduced. The length of the vertical strip will be the longest in the centre and the length of the vertical strips gradually decreases for the nearby elements and the elements in the edge will be smallest in size. Similarly, the size of the concentric rings is large in the centre, gradually the size of the concentric rings is reduced and the size of the concentric rings on the edges will be smallest in size. The length of the vertical strip element in the zeroth position is 10.56 mm, the nearby elements are reduced by a factor of 0.15 mm and the length of the elements in the edge is 9.51 mm.

Figure 6(a) illustrates the gradual reduction in the length of the vertical strips, while Fig. 6(b) shows the resulting tapered strip configuration. To achieve amplitude tapering, the dimensions of both the inner and outer concentric rings must also be progressively reduced. The size of the outer ring size in the zeroth position is 2.5 mm, the nearby elements are gradually reduced by a factor of 0.03 mm and the outer rings on the either side of the edges is of size 2.26 mm. The inner ring size at the centre is 1.5 mm, the size of the nearby inner rings is reduced by a factor of 0.03 mm and the size of the inner element at the either side of the edges is 1.26 mm. Figure 6(c) represents the size of the outer ring and inner ring from the centre to the edge. Figure 6(d) demonstrates the varied size concentric rings. By using the amplitude tapering method the minimal SLL achieved is − 19.2 dB and the SLL is less than − 17.2 dB throughout the entire frequency.

Fig. 6.

Variation in the size of radiating elements: (a) Lengths of the vertical strips, (b) Tapered vertical strips, (c) Sizes of the concentric rings, (d) Concentric rings with different sizes.

In this work, the amplitude tapering method is used to reduce the SLL. Table 2 shows the obtained minimal SLL during the various evolution phases of the designed antenna. The obtained SLL for all the frequencies for all evolution stages is plotted in Fig. 5(b). In phase I, the obtained SLL is below −5.5 dB and the minimal SLL achieved is −10.5 dB. In phase II, the SLL is found to be below −8.5 dB and a minimal SLL of −12.2 dB is attained. The SLL is lesser than −10.3 dB and the minimal obtained SLL is found to be −15 dB in phase III. To achieve SLL below −15 dB throughout the entire frequency, amplitude tapering method is used in phase IV and the achieved SLL is below −17.2 dB and the minimal SLL attained is −19.2 dB. In phase V, the SLL is found lesser than −18.2 dB and the minimal SLL obtained is −19.7 dB. Thus, by using Taylor synthesis amplitude tapering method, well minimized SLL is achieved throughout the entire frequency range.

Table 2.

SLL at different evolution phases of the antenna.

Phases	Phase I	Phase II	Phase III	Phase IV	Phase V
SLL (dB)	−10.5	−12.2	−15	−19.2	−19.7

Fabrication process of the optically transparent GAA

The designed antenna is fabricated using wet chemical etching. Figure 7 depicts the steps involved in fabricating the constructed antenna. Figure 8 displays the fabricated protype of the GAA. The fabrication process begins with cleaning the double-sided ITO coated glass (shown in Fig. 8(a)) substrate with deionised water. The thickness of the glass substrate is 1.1 mm, and the thickness of the ITO film coated on it is 200 nm. Here, the designed grid array structure should be made of ITO film, with the remaining areas etched. Similarly, the elliptical slot in the ground structure should be etched, while the rest of the ground should be ITO film as shown in Fig. 8(b). Commercially available glass substrates coated with ITO on both sides are used in this work. The most common method used to coat glass substrate with ITO is by sputtering. The other methods include electron beam evaporation and chemical vapour deposition. The etching is done with a solution of HCL and water, shown in Fig. 8(c). Masking is used to prevent the ITO film from etching in the desired areas. The grid array and ground plane structures have been patterned with a polyamide sticker. Since the polyamide does not react with HCL and water solutions, the patch and ground plane will remain unaffected. The patterned masking is placed on both the top and bottom sides of the double-sided coated ITO glass substrate. Then, for a few minutes, it is immersed in a solution of HCL and water. This arrangement is left alone for several minutes. The time it takes to etch the unwanted ITO film is proportional to the concentration of HCL solution used. The HCL molecules react with the masked ITO-coated glass, removing unwanted ITO from the unmasked region. The patterned ITO glass is then exposed to sunlight for one minute before being cleaned with deionised water. Thus, the antenna is constructed using a wet chemical etching process. The connector is soldered to the manufactured antenna with conductive silver epoxy adhesive because the heat from conventional soldering could melt the ITO coating. Minor performance variation may have occurred due to the epoxy. Since the epoxy has high resistance, it increases the feed losses and the gain is affected. The uneven epoxy layer introduces parasitic reactance at the feed and causes slight frequency shift.

Fig. 7.

Procedure for fabricating ITO coated glass substrate using wet chemical etching.

Fig. 8.

Fabrication of the optically transparent GAA using wet chemical etching: (a) Double side coated ITO glass, (b) Masking the antenna pattern on ITO-coated glass, (c) Masked antenna dipped in HCL-water solution, (d) Fabricated prototype.

Results and discussion

The GAA is manufactured and a vector network analyser is used to study the reflection coefficient characteristics of the manufactured antenna, while the radiation pattern is analysed using an anechoic chamber, as shown in Fig. 9. This section discusses in detail the simulated and measured results of various characteristic parameters of the proposed optically transparent GAA.

Fig. 9.

(a) Prototype antenna with connector, (b) S₁₁ measurements using VNA, (c) Antenna measurements in anechoic chamber setup.

Impedance characteristics

The reflection coefficient describes the amount of an incoming signal that is reflected back due to impedance mismatch. Figure 10 depicts the simulated and measured impedance characteristics of the developed GAA. It demonstrates that the proposed antenna achieves reflection coefficients of −10 dB impedance or lower from 8 to 12 GHz in both scenarios, as demonstrated by the simulated and measured data. Hence, the constructed optically transparent GAA operates in the X-band. The slight frequency shift observed between the simulated and measured results can be attributed to fabrication tolerances, connector and soldering effects, measurement inaccuracies, and surface losses.

Fig. 10.

Reflection coefficients of the developed optically transparent GAA.

The maximum cut-off frequency in the presented work is 12 GHz, the minimum frequency is 8 GHz, and the centre frequency is 10 GHz. If the fractional bandwidth (FBW) value exceeds 20%, it is classified as wideband. For a narrow band, the FBW should be less than 1%, while for a moderate band, it should be between 1% and 20%. Wideband FBW should be greater than 20%, while ultra-wideband FBW should be greater than 50%. Thus, the developed antenna operates in a wideband mode due to its 40% FBW.

Radiation characteristics

Higher gain means a more focused beam, which improves signal strength in the desired direction, and higher gain allows signals to travel farther. High gain improves reliability in noisy environments and reduces the power required to transmit a given distance. A GAA is a high gain antenna designed for vehicular communication, such as automotive radar and V2E applications. It has a high gain towards the RSU, reduces interference from other directions, and supports high speed data or radar imaging.

Varying the number of elements has a significant impact on the antenna’s gain. Reducing the number of elements lowers the gain, whereas increasing the number of elements enhances the gain but also requires more physical space. Therefore, a trade-off must be maintained between the antenna size and the achievable gain. The design target is to achieve a peak gain greater than 15 dBi. Simulation results obtained for different numbers of elements show a steady increase in gain as the array size is increased. Initially, with 2 rows and 23 columns (23 elements), the peak gain achieved is 9.37 dBi. When the number of elements is increased to 25, 27, and 29, the corresponding peak gains improve to 11.98 dBi, 14.3 dBi, and 17 dBi, respectively. Since the target gain is met with 29 elements, this configuration is selected as the optimal array size. The simulated gain variations for different element counts are shown in Fig. 11(a). The proposed GAA achieve a peak gain of 17 dBi at 12 GHz. The gain ranges from 6.44 to 17 dBi between 8 and 12 GHz. The gain obtained at 8 GHz is 6.44 dBi, while at 8.5 GHz it is 9.69 dBi. The gain values at 9 GHz, 9.5 GHz, 10 GHz, 10.5 GHz, 11 GHz, and 11.5 GHz are 10.7 dBi, 13.1 dBi, 13.4 dBi, 15 dBi, 15.9 dBi, and 16.2 dBi, respectively. The simulated and measured gains are shown in Fig. 11(b), and the results show that the achieved simulated and measured gains and values are satisfactory. The measured gain may be reduced by practical issues such as SMA connector loss, imperfect feeding, increased surface-wave leakage, edge diffraction, and inaccuracies in chamber measurements. The efficiency of the developed GAA is also shown in Fig. 11(b). The maximum efficiency achieved is 84%, and it is greater than 80.5% throughout the entire X-band.

Fig. 11.

(a) Variation of gain with number of elements, (b) Gain and efficiency of the developed transparent GAA.

Beamwidth is important for determining directionality, range and coverage, antenna alignment, and interference reduction. Narrow beamwidth indicates that the antenna is more focused in one direction. A narrow beam provides a longer range, whereas a wide beam provides broader coverage. Beamwidth aids point-to-point communication when precise alignment is required. A narrower beamwidth increases the gain, while a wider beamwidth decreases the gain. In vehicular communication, narrow beamwidth is useful for point-to-point links (e.g., car to roadside unit), as it promotes focused communication and reduces interference. Wide beamwidth in vehicular communication is ideal for short-range, all-direction V2V or V2I communication. In vehicular communication, it is critical to provide narrow beam divergence in the elevation angle and wide angular width in the azimuth angle to ensure that the beam focusses on the desired direction while also covering a large area. To determine how narrow the beam is in the E-plane and how wide it is in the H-plane, the beamwidths of the proposed antenna are measured in both planes. At 12 GHz, the proposed GAA achieves a beamwidth of 7.9˚ in the E-plane and beamwidth of 173.6˚ in the H-plane. Figure 12(a) and 12(b) depict the simulated and measured angular widths for the electric and magnetic field planes, respectively. Since the proposed antenna provides narrow beam divergence in the electric field plane and broad beam divergence in the magnetic field plane, the developed antenna covers a larger area while having a focused beam and thus suitable for vehicular communications.

Fig. 12.

Beamwidth of the developed GAA: (a) Electric field plane, (b) Magnetic field plane.

The radiation patterns of GAA represent a graphical representation of how an antenna radiates energy in space. The E-plane pattern depicts the electric field vector and direction of extreme radiation (typically φ = 0˚), while the H-plane pattern depicts the magnetic field vector and direction of extreme radiation (typically φ = 90˚). Figure 13 shows the simulated and measured E-plane radiation patterns, while Fig. 14 shows the simulated and measured H-plane radiation patterns at 8 GHz, 9 GHz, 10 GHz, 11 GHz, and 12 GHz. The developed antenna has a narrow angular width in the electric field plane and a wide angular width in the magnetic field plane, making it suitable for vehicular communications.

Fig. 13.

Radiation patterns in the E- plane: (a) 8 GHz, (b) 9 GHz, (c) 10 GHz, (d) 11 GHz, (e) 12 GHz.

Fig. 14.

Radiation patterns in the H-plane: (a) 8 GHz, (b) 9 GHz, (c) 10 GHz, (d) 11 GHz, (e) 12 GHz.

Analysis of the path loss

Path loss is the unavoidable weakening of an RF signal between transmit and receive antennas caused by distance and the environment. Path loss occurs when the transmitted signal spreads over distance and is absorbed, reflected, refracted, or scattered by obstacles. Path loss behaves differently in vehicular communication than in simple free-space scenarios because vehicles operate in dynamic, cluttered, and rapidly changing environments. Vehicles move through environments full of obstacles such as other vehicles, buildings, trees, and road signs. Vehicles move at high speeds, causing rapid channel changes. If large trucks or buses suddenly block line of sight (LoS), shadows will fade. The log-distance path loss model is used in this case as it is adaptable to a variety of environments, including highways (LoS), cities (LoS), and suburban areas (non LoS). It also aids in the analysis of shadowing effects caused by frequent blockages in the vehicle environment. The log-distance path loss is given as (5),

5

where PL(d) is the path loss at distance d, and PL(d₀) is the path loss at reference distance d₀, n is the path loss exponent and it varies depending on the environment, such as highways or urban areas, d is the distance between the transmitter and receiver antenna, and X_σ is the shadow fading. The free space path loss (FSPL) Eq. (6) is used to calculate PL(d₀),

6

where the frequency is denoted as f, and the speed of light is represented as c. The path loss is calculated using Eqs. (5) and (6) for frequencies such as 8 GHz, 9 GHz, 10 GHz, 11 GHz, and 12 GHz. Under ideal conditions, the distance between transmitter and receiver ranges from 1 to 11 km. The path loss exponent is set at 2.2³⁸, and no shadow fading is considered. In a real-time scenario for vehicular communication, the transmitter antenna is the developed transparent GAA that can be mounted on the windscreen of the vehicle, and the receiver antenna is the infrastructure antenna (RSU) or another vehicle antenna. Figure 15 depicts the obtained path loss for the proposed GAA at different frequencies. The FSPL increases with distance, and path loss increases with frequency.

Fig. 15.

Path loss analysis at different frequencies and distances.

Analysis of link budget

A link budget analysis determines whether the receiver will receive enough signal power to ensure reliable communication. To investigate signal reliability, the separation between the transmitter and receiver antennas is varied. The link budget can be calculated using Eqs. (7)−(10).

7

8

9

10

where, P_Tx and G_Tx represent the power and gain of the transmitter antenna, respectively. G_Rx is denoted as receiver gain, free space loss is denoted as L_F, and polarisation loss is represented by P_L. According to phase shift keying, the value of E_b/N₀ is 9.6. The Boltzmann constant is represented as K, and T₀ is the absolute zero temperature. The bit rate, B_r, is considered as 12 Mbps, which is more than sufficient to transfer a high-quality image. Distance and wavelenth are denoted as d and λ, respectively. The transmitter antenna power is 10 dBm, and the transmitter gain is 6.44 dBi, 10.7 dBi, 13.4 dBi, 15.9 dBi, and 17 dBi at 8 GHz, 9 GHz, 10 GHz, 11 GHz, and 12 GHz, respectively, whereas the receiver gain is 20 dBi at all frequencies.

Figure 16 shows the obtained link budget after substituting all of the values in the above equations from (7) to (10). The data transmission range at 8 GHz is 4.95 km, 9 GHz is 7.2 km, and 10 GHz is 8.8 km. The data transmission range at 11 GHz and 12 GHz is 10.7 km and 11.1 km, respectively. This can be interpreted as the higher the gain, the greater the data transmission range.

Fig. 16.

Link budget analysis at different frequencies.

Antenna housing effects

Integrating an antenna on car is challenging as modern cars have curved body surfaces, small mounting areas, strict aesthetics requirements limits antenna size, shape, and placement. The metal body in the car, causes interference, and affects radiation pattern leading to reduction in gain and changes in the S-parameter. These may shift the operating frequencies of the antenna. The antenna must withstand harsh environmental conditions like temperature, rain, dust, vibrations which may detune or damage the antenna. The aesthetics constrains such as invisible or minimally visible antennas limits the design to shark fin antennas. Many car surfaces are metal, plastics which affects bandwidth and gain of the antenna. An optically transparent antenna can be installed in both the front and rear windscreens of a vehicle. To test the performance of the antenna when placed on the windscreen, a large glass material is designed to mimic the windscreen of a vehicle. The antenna is tested on glass materials of various sizes (200 mm × 200 mm × 5 mm, 400 mm × 400 mm × 5 mm, and 800 mm × 800 mm × 5 mm) at a distance of 10 mm. Figure 17 depicts the antenna mounted on a glass surface measuring 800 mm × 800 mm × 5 mm.

Fig. 17.

Proposed GAA mounted on the glass material.

Figures 18 and 19 show the reflection coefficients and radiation patterns of the designed antenna when placed on a glass material, respectively. From 8 to 12 GHz, the obtained reflection coefficients are less than −10 dB impedance, and the radiation pattern is narrow in the electric field plane and broad in the magnetic field plane, even when placed above the glass material. A 3D car model is imported to analyse the radiation characteristics of the antenna when installed in the windscreen of a vehicle, and the developed antenna is placed in the front and back windshield of the car. The on-car performance for 8 GHz, 10 GHz, and 12 GHz is studied, and even after mounting the constructed antenna on the windscreen of the car, as seen in Fig. 20, the results indicate that the antenna’s performance is not significantly impacted. The proposed antenna demonstrates excellent on-car performance, with minimal degradation from surrounding metallic structures. Its wide azimuth beamwidth ensures uninterrupted V2V and V2I connectivity during turns and lane changes, while the narrow elevation beamwidth focuses energy toward other vehicles and RSUs for improved range.

Fig. 18.

Reflection coefficients of the GAA when placed on the large glass material.

Fig. 19.

Radiation patterns of the GAA: (a) 8 GHz (E-plane), (b) 10 GHz (E-plane), (c) 12 GHz (E-plane), (d) 8 GHz (H-plane), (e) 10 GHz (H-plane), (f) 12 GHz (H-plane).

Fig. 20.

On-car performance of the GAA: (a) 8 GHz (Front windshield), (b) 8 GHz (Back windshield), (c) 10 GHz (Front windshield), (d) 10 GHz (Back windshield), (e) 12 GHz (Front windshield), (f) 12 GHz (Back windshield).

Table 3 compares the characteristics of the proposed antenna, such as antenna size, fractional bandwidth, gain, efficiency, beamwidth, and transparency, to antennas in the literature. The developed antenna is suitable for vehicular communication such as 10.5–10.6 GHz, automotive radar (collision avoidance, blind spot detection), V2V, 9–11 GHz, V2I (smart traffic control, toll collection systems), 9.6–10.4 GHz, SAR on vehicles (ground mapping from autonomous vehicles, military-grade vehicle-mounted systems), 7.9–8.4 GHz (X-band uplink, military), 8.0–8.4 GHz (downlink in vehicular SATCOM).

Table 3.

Comparison of the developed transparent GAA with existing antennas in the literature.

Refs.	Dimensions (λ0 × λ0)	Frequency (GHz)	Fractional bandwidth (%)	Peak Gain (dBi)	Efficiency (%)	Angular width in E-plane (˚)	Angular width in H-plane (˚)	SLL (dB)	Transparent/Opaque
29	---	75.75 − 90	17.19	13.52	---	12	62	−14	Opaque
30	2.36 × 1.67	2.38 − 2.51	5.32	15.4	98.2	18.5	34.5	---	Opaque
31	6.81 × 0.84	14 − 26	60	14.2	80	7.8	101.6	−16.3	Opaque
32	5.4 × 5.1	25 − 26	3.92	18.18	59.6	---	---	---	Transparent
33	10.22 × 1.26	21 − 26	21.28	13.87	---	7	90	−16	Opaque
34	7.2 × 2	24	---	10	80	20	80	---	Opaque
35	12.3 × 2.25	22.5 − 25.25	11.52	12.11	---	11	136	---	Opaque
36	2.71 × 5.27	22 − 26	16.67	11.1	40.3	10	150	---	Opaque
37	3 × 3	90 − 95.12	5.53	17.1	93.21	25	22	---	Opaque
Prop.	4.81 × 0.66	8 − 12	40	17	84	7.9	173.6	−19.7	Transparent

The primary characteristics of the proposed GAA are as follows:

• The antenna footprint is substantially reduced, being 19.45%, 44.5%, 88.47%, 75.35%, 77.95%, 88.52%, 77.77%, and 64.73% smaller compared to the designs in^30–36, and³⁷, respectively. Such miniaturization enables easy integration into constrained vehicular spaces like windshields or body panels without compromising aesthetics or aerodynamics.

• The achieved fractional bandwidth is broader than that reported in^{29,30,32,33,35,36}, and³⁷, specifically 22.81%, 34.68%, 36.08%, 18.72%, 28.48%, 23.33%, and 34.47% wider, respectively. This wider bandwidth ensures compatibility with multiband vehicular communication standards such as DSRC, C-V2X, and emerging 5G/6G vehicular links.

• The proposed antenna design achieves a peak gain that is 25.74% greater than²⁹, 10.39% higher than³⁰, 19.72% greater than³¹, 22.57% higher than³³, 70% higher than³⁴, 40.38% more than³⁵, and 53.15% greater than³⁶. Higher gain directly improves communication range and link reliability, which are critical in dynamic vehicular environments.

• The achieved radiation efficiency exceeds that of^31,32,34, and³⁶. Specifically, it is 4% higher than^31,34, 63.6% greater than³², and 43.7% higher than³⁶. Improved efficiency ensures better utilization of input power, reducing energy loss and enhancing system performance.

• For vehicular communications, a narrow elevation plane beamwidth helps focus energy toward the horizon for long distance coverage, while a wide azimuth plane beamwidth ensures 360˚ horizontal coverage for nearby vehicles and RSUs. The proposed GAA meets these requirements effectively.

• The SLLs are lower than those in^29,31,33, being 28.93% lower than²⁹, 17.26% lower than³¹, and 18.78% lower than³³. Lower sidelobes minimize interference and improve the antenna’s ability to focus energy in the intended direction.

• Despite being transparent, the proposed antenna outperformed other opaque antennas in the literature.

From the literature, it can be seen that most researchers develop MIMO antennas for vehicular communication. GAAs are more suitable than MIMO antennas for vehicular environments because they produce highly directional, high-gain beams that support long-distance and provide stable links in fast-moving V2V and V2I scenarios. They work as a single compact structure, saving space, cost, and power, unlike MIMO systems that require multiple antenna elements. Their directionality also reduces multipath fading, and they maintain better performance than MIMO when channels change rapidly at high speeds. In modern vehicles, transparent antennas are important because they can be integrated on windshields, windows, and other surfaces without affecting design, visibility, or aerodynamics, while still providing reliable communication^39–43. To the best of the authors’ knowledge, no transparent antenna based on a grid array structure has been reported in the literature for vehicular communication applications.

In this work, the advantages of transparent antenna technology with the high-gain characteristics of GAAs are combined to develop an optically transparent GAA suitable for vehicular communication systems. The proposed design exhibits a narrow beam in the E-plane and a wide beam in the H-plane, enabling it to support long-distance communication, while simultaneously covering a broad angular region. The data transmission range of the antenna is calculated theoretically and it shows the proposed antenna covers long distance because of its high gain. The antenna housing effects of the antenna is also studied and it is found that the proposed antenna can be easily integrated in the windshield of the vehicle without affecting the aesthetics or visibility of the vehicle.

In Table 3, size of the antenna, fractional bandwidth, peak gain, efficiency, beamwidth in E-plane, beamwidth in H-plane, SLL, transparency of the proposed antenna is compared with the literature to find out how efficiently the developed antenna works when compared to the other antennas in the literature. From the literature, it can be seen that most of the antennas are opaque in nature. The proposed antenna even being transparent provides better performance than opaque antennas. The developed antenna provides better performance than most of the other antennas in the literature in terms of size, fractional bandwidth, peak gain, efficiency, beamwidth, and SLL.

Conclusion

An optically transparent GAA has been developed for vehicular communications. The antenna operates between 8 and 12 GHz, covering applications such as automotive radar, V2V, V2I, SAR on vehicles (ground mapping from autonomous vehicles, military grade vehicle mounted systems), and satellite communication in vehicular SATCOM. The developed antenna is made up of 29 elements of varying sizes arranged in two rows, and the vertical strips of the antenna are tapered to provide SLLs of less than −19.7 dB. The maximum gain of the antenna is 17 dBi, and the antenna produces a narrow beam spread vertically and a wide angular width horizontally. The proposed antenna also provides 40% fractional bandwidth, demonstrating wideband characteristics. The constructed antenna is developed using the wet chemical etching method, and the measured results are consistent with the simulated results. The effects of antenna housing on antenna performance are investigated, and they have no significant impact. Path loss and link budget analysis are also performed to better understand the data transmission range of the proposed GAA. The proposed GAA outperforms reported designs in terms of compactness, bandwidth, gain, efficiency, beamwidth, and sidelobe performance. It achieves up to 89.11% size reduction, 36.08% broader bandwidth, 70% higher peak gain, and 63.6% greater efficiency compared to literature. Its narrower elevation beamwidth ensures long range focus, while a wider azimuth beamwidth provides near omnidirectional coverage, and reduced SLLs improve link quality and minimize interference in vehicular communication environments.

Author contributions

A.J.J., S.K.P. and S.K. conceived and performed simulations, experiment, and drafted the manuscript. S.K.P. and M.K. conducted the experiment. A.J.J and M.I.H. analyzed the results. M.K., S.K. and M.I.H. supervised the overall work and provided funding for the experiments. All authors reviewed the manuscript.

Data availability

The datasets used and/or analysed during the current study available from the corresponding author on reasonable request.

Declarations

Competing interests

The authors declare no competing interests.