Introduction

The growing demand for reliable, long-range, and spectrum-efficient vehicular communication systems has intensified the need for compact, multiband antennas capable of operating across both sub-1 GHz and 5.9 GHz bands. The sub-1 GHz frequency range (700–900 MHz) offers inherent advantages such as reduced path loss, greater signal penetration, and extended coverage, making it ideal for vehicle-to-infrastructure (V2I), vehicle-to-network (V2N), and internet of things (IoT)-based vehicle monitoring systems1,2. Meanwhile, the 5.9 GHz band is reserved globally for intelligent transport systems (ITS), particularly dedicated short range communication (DSRC) and cellular vehicle-to-everything (C-V2X) standards3,4. To meet these requirements, there is a strong need for a compact, structurally simple antenna solution that supports multiband operation, maintains high radiation efficiency, and offers robust performance in complex vehicular environments5,6,7.

Traditional antenna designs for automotive applications—such as transparent soda lime glass antennas, cubical monopoles, or metamaterial-integrated structures—often suffer from complex geometry, high profile8,9, or materials that are difficult to integrate with standard vehicle platforms10,11. Furthermore, many reported antennas operate predominantly at higher frequencies (above 2 GHz), which while suitable for high data rates, are limited in coverage and prone to degradation under harsh conditions such as fog, rain, or multipath reflections12,13,14. This has created a pressing need for antenna solutions that combine the extended range of sub-1 GHz bands with the latency-critical high-band operation of 5.9 GHz, all within a compact, rugged design suited for vehicle mounting15,16,17.

In response to these challenges, this paper presents a new square patch-based multiband antenna with a full ground plane, engineered to support both sub-1 GHz (700, 800, 850, and 900 MHz) and 5.9 GHz ITS communication. The antenna incorporates strategically placed slots within the radiating patch, which selectively excite multiple resonant modes while maintaining impedance stability and radiation efficiency. Unlike existing designs that rely on glass substrates or metamaterial layers, this antenna utilizes standard printed circuit board (PCB) fabrication techniques, enhancing manufacturability and integration with existing vehicular systems.

The proposed antenna design is validated through simulation and on-vehicle measurements, with results showing a return loss better than −10 dB, stable gain characteristics (up to 4.1 dB at 5.9 GHz), and well-defined radiation patterns across all bands. The full ground plane not only suppresses back radiation but also contributes to mechanical robustness and consistent performance regardless of mounting surface, which is critical in real-world vehicular environments. The key contributions of the presented work are as follows:

  • Development of a dual-port square patch antenna with slot-based multiband operation, achieving resonance at four sub-1 GHz bands and the 5.9 GHz ITS band.

  • Use of a full ground plane configuration to improve radiation directionality, impedance matching, and structural stability for vehicular deployment.

  • Demonstration of compactness and ease of fabrication using conventional PCB technology, without the need for complex metamaterial structures.

  • Validation of the antenna on-vehicle, proving its suitability for V2X communication, fleet monitoring, and connected car systems.

This approach bridges the gap between compact multiband antenna design and the stringent practical requirements of modern vehicular platforms, providing a future-ready solution for global intelligent transport applications.

Antenna design

Figures 1a and b depict the front and side views of the proposed automotive antenna, respectively. The antenna is constructed on Rogers RT Duroid 5880 dielectric material, which has a dielectric constant of 2.2 and a loss tangent of 0.0009. The proposed multiband antenna is made up of a square patch with multiple slots and a full ground plane. Multiple slots introduce additional resonant modes, which combine with the fundamental mode to achieve multiband performance. A full ground plane reduces back radiation, directing more energy in the intended direction and improving antenna performance. The antenna simulations are performed in CST Microwave Studio Suite, and the size of the presented vehicular antenna is 40 mm × 40 mm, with its dimensions listed in Table 1. The antenna covers a range of sub-1 GHz frequencies, including 700 MHz, 800 MHz, 850 MHz, 900 MHz, and 5.9 GHz.

The antenna structure employs multiple strategically placed slots etched into the square radiating patch to achieve multiband operation and enhance performance characteristics such as impedance matching and gain. The breakdown of the functional role of each slot region in the antenna design is as follows:

  • Concentric square ring slots (regions defined by W2, W3, W4 and L2, L3, L4): These nested square slots act as resonators, each tuned to specific sub-1 GHz frequencies. The outermost ring resonates at the lowest frequency (around 700 MHz), while the inner rings contribute to higher frequencies (800–900 MHz). Their combination enables multiband operation by supporting multiple current paths and surface wave interactions.

  • Vertical and horizontal connecting slots (W5, W6, L8): These narrow slots serve two primary purposes: (i) coupling energy between different resonant sections, and (ii) introducing additional path lengths for surface currents, which effectively lowers the resonant frequency and enhances impedance matching across multiple bands.

  • Diagonal/angular slot (L10): This uniquely shaped slot in the lower right quadrant introduces asymmetry into the current distribution, aiding in the excitation of higher-order modes and contributing to wideband or dual-band behaviour around 5.9 GHz (C-V2X or DSRC band). It also helps control the polarization and radiation pattern characteristics.

  • Cut and notched regions (L6, L7, W7, W8): These are introduced to modify the current flow across the patch, helping in the miniaturization and fine-tuning of the resonant frequencies. The notched sections assist in impedance matching and reduce the antenna’s profile without compromising performance.

  • Full ground plane: The ground plane plays a crucial role in reducing back lobe radiation and improving the front-to-back ratio (FBR), which is essential for automotive communication systems to direct energy efficiently.

Also, a parametric sweep is performed across various feed positions. The reflection coefficient values are analysed for different placements of the feed to identify the optimal point over the desired frequency bands. Figure 1(a) illustrates the location of the feed ports in the proposed antenna design, and the parametric sweep of the S11 for different feed port placements is displayed in Fig. 1(c).

Table 1 Dimensions of the proposed automotive antenna.
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Fig. 1
Fig. 1
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Schematic of the proposed automotive antenna: (a) front view, (b) side view, (c) parametric sweep for different feed port positions.

The physical interpretation of all resonant frequencies is represented by the following equations:

  • For 700 MHz,

    $$L_{{{\text{7}}00}} = {\text{ 4}}\left( {L{\text{2 }} + L{\text{4}}} \right){\text{ }} + {\text{ 3}}\left( {W{\text{2 }} + W{\text{3 }} + W{\text{4 }} + L{\text{3}}} \right){\text{ }} = {\text{ 157}}.{\text{65}} \;{\text{ mm}}$$
    (1)

    L700 = 157.65 mm corresponds to the λ/2 resonant as in Eq. (2),

    $$\:f1=\frac{c}{2{L}_{700}\sqrt{{\epsilon\:}_{reff}}\:}$$
    (2)

    where f1 is close to the simulated frequency of 0.7 GHz.

  • For 800 MHz,

    $$L_{{{\text{8}}00}} = {\text{ 2}}\left( {L{\text{2 }} + L{\text{3 }} + L{\text{4 }} + L{\text{5 }} + W{\text{2 }} + W{\text{3 }} + W{\text{4 }} + W{\text{5 }} + W{\text{6 }} + W{\text{8}}} \right){\text{ }} + L{\text{7 }} + L{\text{8 }} = {\text{ 137}}.{\text{7 }} \;{\text{ mm}}$$
    (3)

    L800 = 137.7 mm corresponds to the λ/2 resonant as in Eq. (4),

    $$\:f2=\frac{c}{2{L}_{800}\sqrt{{\epsilon\:}_{reff}}\:}$$
    (4)

    where f2 is near to the simulated frequency of 0.8 GHz.

  • For 850 MHz,

    $$L_{{{\text{85}}0}} = {\text{ 2}}\left( {L{\text{2 }} + L{\text{3 }} + L{\text{4 }} + L{\text{6 }} + W{\text{2 }} + W{\text{3 }} + W{\text{4}}} \right){\text{ }} + L{\text{7 }} + L{\text{8 }} + L{\text{9 }} = {\text{ 131}}.{\text{5}}\;{\text{ mm}}$$
    (5)

    L850 = 131.5 mm corresponds to the λ/2 resonant as in Eq. (6),

    $$\:f3=\frac{c}{2{L}_{850}\sqrt{{\epsilon\:}_{reff}}\:}$$
    (6)

    where f3 is close to the simulated frequency of 0.85 GHz.

  • For 900 MHz,

    $$L_{{{\text{9}}00}} = {\text{ 2}}\left( {L{\text{2 }} + L{\text{3 }} + L{\text{4 }} + L{\text{6 }} + W{\text{2 }} + W{\text{3 }} + W{\text{4}}} \right){\text{ }} + L{\text{6 }} + L{\text{7 }} + L{\text{8 }} + L{\text{9 }} = {\text{ 123}}.{\text{55}}\;{\text{ mm}}$$
    (7)

    L900 = 123.55 mm corresponds to the λ/2 resonant as in Eq. (8),

    $$\:f4=\frac{c}{2{L}_{900}\sqrt{{\epsilon\:}_{reff}}\:}$$
    (8)

    where f4 is near to the simulated frequency of 0.9 GHz.

  • For 5.9 GHz,

    $$L_{{{\text{5}}.{\text{9}}}} = L{\text{1}}0{\text{ }} + L{\text{8 }} + W{\text{5 }} + L{\text{6 }} = {\text{ 18}}.{\text{44 }}\;{\text{mm}}$$
    (9)

    L5.9 = 18.44 mm corresponds to the λ/2 resonant as in Eq. (10),

    $$\:f5=\frac{c}{2{L}_{5.9}\sqrt{{\epsilon\:}_{reff}}\:}$$
    (10)

    where f5 is close to the simulated frequency of 5.9 GHz.

Antenna design process

Figures 2 and 3 show the antenna design stages, along with the impedance characteristics (in both sub-1 GHz and 5.9 GHz bands) of the antenna design process. The antenna has rotational symmetry and can be deployed on any vehicle surface. Since the antenna has a non-defected full ground plane, it produces a directional far-field pattern.

Fig. 2
Fig. 2
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Design phases of the proposed automotive antenna: (a) Antenna-1, (b) Antenna-2, (c) Antenna-3, (d) Antenna-4.

The antenna design begins with a conventional square patch with a full ground plane. The conventional square patch (antenna-1) is modified by introducing two fan-shaped slots to provide dual resonance, as displayed in Fig. 2a. The diagonal quadrants of the antenna design are mirror images of one another, and they are referred to as the first (Q1) and third (Q3) quadrants of the modified square patch antenna-1. The Q1 and Q3 quadrants of the modified square patch antenna are selected with a rotational symmetry, whereas the second (Q2) and fourth (Q4) quadrants restrict the direction of current flow by loading a pair of rectangular rings, as exhibited in Fig. 2b, assisting the lower frequency resonance in antenna-2.

Fig. 3
Fig. 3
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Impedance characteristics of the design phases of the proposed automotive antenna at (a) lower frequencies, (b) higher frequencies.

In antenna-3, two loops are introduced in quadrants Q1 and Q3 (in Fig. 2c), which increases the electrical length of the modified square patch antenna and providing multiple resonances in the sub-1 GHz band. Furthermore, two loop edge length slots are coupled with loops in both the Q1 and Q3 quadrants, resulting in a bandwidth improvement in the sub-1 GHz frequency band, while a quadrant length slot is introduced to support in bandwidth enhancement. The higher frequency band is achieved by increasing the current flow in the corners of Q2 and Q4. In the final stage, the 5.9 GHz bandwidth is obtained by increasing the stub length in both the quadrants Q1 and Q3 (antenna-4), as illustrated in Fig. 2d.

Results and discussion

The antenna prototype is developed on Rogers RT Duroid 5880 material, as shown in Fig. 4a, and its impedance characteristics are measured using a vector network analyzer (VNA), as shown in Fig. 4b.

Fig. 4
Fig. 4
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Measurement setup of the proposed automotive antenna: (a) fabricated prototype, (a) near-field measurements using vna, (c) far-field measurements in an anechoic chamber.

Figure 5 depicts a comparison plot of the simulated and measured reflection coefficients of the proposed automotive antenna. The proposed antenna design operates over a wide frequency range, including the sub-1 GHz (700 MHz, 800 MHz, 850 MHz, and 900 MHz) and 5.9 GHz bands. The difference in measured results can be attributed to cable and free space losses, but the resonance ranges remain unchanged.

Fig. 5
Fig. 5
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Impedance characteristics (simulated and measured) of the proposed automotive antenna.

Figures 6a and b depict a comparison between the simulated gain values and measurement data in the sub-1 GHz and 5.9 GHz bands. The gain of the proposed automotive antenna is measured with a standard reference horn antenna. Both the horn antenna and the proposed automotive antenna are placed in an anechoic chamber at a fixed distance. The use of an anechoic chamber (shown in Fig. 4c) reduces multipath reflections, thereby improving measurement accuracy. First, the received power is measured with the standard horn antenna, which has a known gain. Next, the reference antenna is replaced with the proposed automotive antenna, and the received power is measured under the same conditions. The gain values of the automotive antenna are 0.76 dBi, 0.94 dBi, 1.1 dBi, 1.4 dBi, and 8 dBi at 700 MHz, 800 MHz, 850 MHz, 900 MHz, and 5.9 GHz bands, respectively. Figure 6c shows the radiation efficiency of the automotive antenna, which is found to be greater than 70% across all resonating bands.

Fig. 6
Fig. 6Fig. 6
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Gain characteristics of the proposed automotive antenna at (a) lower frequencies, (b) higher frequencies, (c) efficiency.

The radiation pattern of the proposed automotive antenna is measured in an anechoic chamber, with a standard horn antenna serving as a transmitting antenna at a fixed distance from the test antenna. The automotive test antenna is mounted on a rotational platform and rotated 360˚ in both the horizontal and vertical planes, with a VNA recording the received power at each angle. Figure 7 depicts a comparison of the simulated radiation patterns with measurement data at 700 MHz, 800 MHz, 850 MHz, 900 MHz, and 5.9 GHz frequencies. The full ground plane used in the proposed antenna design produces the directional radiation pattern as all of the beams are reflected and directed perpendicularly.

Fig. 7
Fig. 7
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Simulated and measured radiation patterns (co-pol and cross-pol) of the proposed antenna: (a) 700 MHz, (b) 800 MHz, (c) 850 MHz, (d) 900 MHz, (e) 5.9 GHz (thick line: measured, dashed line: simulated).

On-vehicle analysis

The on-vehicle analysis is performed to determine the flexibility of the suggested antenna in real-world surroundings18,19,20. The designed antenna is placed next to the shark-fin in the car model, and to avoid discrepancies in the far-field results produced by the on-board antenna, the source is placed at 1 cm from the roof of the vehicle. It is evident that the full ground plane of the proposed automotive antenna helps to suppress back radiations, directing more energy toward the front, as illustrated in Fig. 8. Also, the antenna produces a more consistent radiation pattern, which is critical for automotive applications that require consistent performance.

Figure 9 shows the reflection coefficients of the suggested automotive antenna when it is placed on the car phantom in the simulation tool. The reflection coefficients are found to be consistent with those simulated in free space. This demonstrates that the automotive antenna is stable and reliable at frequencies of 700 MHz, 800 MHz, 850 MHz, 900 MHz, and 5.9 GHz.

Fig. 8
Fig. 8
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Onboard far-field analysis of the resonant frequencies: (a) 700 MHz, (b) 800 MHz, (c) 850 MHz, (d) 900 MHz, (e) 5.9 GHz.

Fig. 9
Fig. 9
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On-vehicle analysis of the proposed vehicular antenna.

Table 2 Performance comparison with previous antenna literatures.
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Table 2 compares the proposed antenna to the vehicular antennas reported in the literature. The key characteristics of the proposed automotive antenna are as follows:

  • The suggested multiband antenna supports multiple automotive frequencies, making it compatible with a variety of global vehicular communication standards. It also eliminates the need for multiple antennas, which simplifies design and minimizes costs.

  • Since the proposed multiband antenna operates in the sub-1 GHz (700 MHz, 800 MHz, 850 MHz, and 900 MHz) band, it experiences reduced attenuation, allowing for larger distance communication. Sub-1 GHz frequencies penetrate farther than higher frequency ranges.

  • Sub-1 GHz bands are less congested than higher frequency bands, resulting in lesser interference and more reliable transmission. Also, devices using sub-1 GHz antennas often consume less power, extending battery life, therefore the suggested antenna can be employed in IoT-based vehicle sensor applications.

  • Furthermore, the suggested antenna operates in the 5.9 GHz band, which is internationally allocated for ITS, making it future-proof for self-driving and connected cars. Also, the 5.9 GHz spectrum complies with industry standards such as IEEE 802.11p, DSRC, and 3GPP C-V2X (LTE-V2X and 5G NR-V2X).

  • In vehicle environments, reflections from metal surfaces can produce multipath fading. The suggested antenna with a complete ground plane stabilizes impedance matching and minimizes undesired radiation lobes, hence minimizing signal loss.

  • A full grounded antenna design is less susceptible to mounting variations, resulting in constant performance regardless of vehicle location. The suggested antenna has an uniform radiation pattern, which produces consistent connectivity in all directions.

Conclusion

A new low-profile multiband antenna for automotive communication is proposed, capable of operating in the lower sub-1 GHz (less than 1 GHz) and 5.9 GHz DSRC bands. The antenna is comprised of a full ground plane and a square patch with fan-shaped slots loaded in the Q2 and Q4 quadrants, as well as two loops introduced in the Q1 and Q3 quadrants to increase the electrical length of the modified square patch antenna and provide multiple resonances in the sub-1 GHz band. The full ground plane in the proposed antenna reduces cross-polarized radiation, improving signal purity, and provides structural support to the antenna, making it more robust and stable in a variety of environmental conditions. The on-vehicle analysis of the antenna is also performed, and the results are well suited to its practical applicability.