A Compact and Wideband Dashboard Antenna for Vehicular LTE/5G Wireless Communications

Abstract

A wideband, low-profile, 3D automotive antenna for Long-Term Evolution (LTE) and 5G applications is presented in this paper. Different from other cellular antennas typically placed under the shark-fin cover or inside a car’s plastic spoiler, the proposed antenna is designed to be integrated inside the vehicle’s dashboard. The 35.5 × 40 × 45 mm³ antenna is compact, lightweight and robust. At the same time, this antenna is capable of operating from 670 up to 5000 MHz, covering the entire LTE/5G band (overall fractional bandwidth of 198%). A shunt stub was introduced between the monopole and ground plane to achieve a low LTE band and provide mechanical robustness for the proposed structure. Simulated performance in terms of reflection coefficient, radiation pattern and realized gain is described, showing a good agreement with the measurement results. Specifically, the antenna has a gain higher than −1 dBi at the low-frequency band (i.e., below 1 GHz) and higher than 3 dBi at the upper-frequency band (i.e., above 1.7 GHz). As per requirements, the ground plane size and layout can be properly chosen to fit the antenna into the available volume as well as to optimize the antenna’s performance.

1. Introduction

In recent decades, the number of mobile and wireless applications has rapidly increased and some specific applications, such as cellular communications, have experienced a significant evolution. For example, the demand for wideband cellular antennas for Long-Term Evolution (LTE) and 5G communication systems is continuously increasing in smartphones and vehicular applications. In particular, with the aim of making a vehicle as “smart” and autonomous as possible, in the automotive industry the number of wireless services is significantly increasing and this has consequently led to a higher number of embedded radiating elements. Even though the size of a vehicle is expected to allow for the easy integration of a multitude of antennas, there are specific integration constraints in aesthetic, mechanical and electromagnetic domains which must be taken into account in antenna and system design. It is well known that one of the best positions for antennas is above the vehicle roof, since the radiating elements are not affected by surrounding mechanical components and a better radiation toward the cellular base stations is guaranteed. Antennas are typically integrated together under plastic covers, referred to as shark fins. However, the increase of wireless functionalities in the automotive industry has resulted in complex electronic systems, networks and architectures [1]. Thus, the integration of antennas is challenging in cases where multiple antennas are present because antenna compactness and isolation requirements are difficult to meet. The size of an antenna is limited by the available volume under the cover and by the presence of other antennas placed in a close proximity. Typical car roof modules include antennas for cellular communications, Vehicle-to-Everything (V2X), Bluetooth, Wireless Local Area Network (WLAN), Global Navigation Satellite Services (GNSSs) and satellite radio services such as Satellite Digital Audio Radio Systems (SDARSs). Nevertheless, multiple antennas are used to improve the throughput of LTE or 5G cellular systems by implementing Multiple-Input, Multiple-Output (MIMO) wireless technology [2].

Initially, 2D antennas printed on substrates were developed as car roof antennas [3]. These were typically patch or monopole antennas [4,5,6,7,8,9]. An antenna integrated into a conventional roof-top antenna module for GSM and UMTS services was presented in [4], while a compact broadband printed monopole inspired by a Vivaldi antenna for various services was reported in [5].

Other wideband printed antennas were reported to cover various cellular services [6,7,8,9,10,11,12,13]. These were typically wideband monopoles [7], slots [8], Planar Inverted-F Antennas (PIFAs) [9,10,11] and polygonal monopoles [12]. However, since these antennas are planar, there are fewer degrees of freedom in designs and they are not able to exploit the entire available volume, thus providing relatively low gain and efficiency, especially at low frequencies. On the other hand, at high frequencies, it is relatively simple to fulfil the requirements in terms of realized gain and efficiency, since the antenna becomes electrically large. Hence, to guarantee the coverage of low-frequency bands, the radiating element has to be miniaturized, though this results in performance degradation in terms of radiation efficiency and gain [6,9]. A multi-band, planar, wideband, monopole, inkjet-printed antenna suitable for LTE was realized using additive manufacturing techniques [13]. The gain of this antenna was quite good even at low frequencies, but its size makes the antenna unsuitable to be fitted under a shark-fin cover.

In order to obtain a better performance in a limited space, 3D-printed antennas have been developed [14,15,16,17,18,19,20,21,22,23,24,25,26] to efficiently utilize the small available volume of devices. In [14], an antenna was 3D-printed on the surface of the mounting compartment of an automotive roof-top antenna using Molded Interconnect Device (MID) technology, while a compact, dual-polarized, unidirectional, wideband antenna using 3D printing technology was reported in [15]. However, the operating frequency of such an antenna is limited to the 1.7–2.9 GHz band. A low-profile, wideband, monopolar antenna integrated for vehicles and helmets with an operating range of 800–2300 MHz was developed in [16]. A 3D Nefer antenna operating in a range from 698 MHz to 960 MHz and from 1470 MHz to 2700 MHz has been presented in [17]. Few other 3D monopoles or PIFAs have been reported as car-roof antennas [18,19,20,21,22,23,24,25,26,27]. A MIMO antenna system covering the entire frequency range of LTE, 5G, WLAN and V2X between 700 MHz and 6 GHz was reported in [26], but the size of the antenna is large and it is not suitable to be used as a shark-fin antenna.

To accommodate all the radiating elements while preserving the satisfactory performance of antennas, new locations on vehicles have been investigated [27] (Figure 1). For example, cellular antennas have also been integrated into windshields or rear windows [28] as well as into new spaces carved out of the car’s roof [29,30]. The latter represents an interesting evolution of shark-fin covers, since the cavity created on the car roof can be large enough to accommodate both the radiating elements and electronic circuitry of multiple wireless services. However, due to mechanical constraints, the radiating elements must be low-profile (typically, the height must be lower than 2 cm).

In this paper, a 3D compact and wideband antenna operating from 670 MHz up to 5 GHz is proposed to be fitted under a vehicle’s dashboard where more space can be obtained with respect to the shark-fin cover. In order to guarantee a satisfactory communication link with the radio base station, the radiating element is placed just underneath the dashboard plastic cover and has a typical thickness of a few millimeters. Consequently, most of the mechanical parts and devices are placed under the antenna’s small ground plane and they do not significantly affect antenna performance. The antenna layout and measured performance in terms of reflection coefficient, gain and radiation pattern are described.

The paper is organized as follows. In Section 2, the design concept is described and simulated and measured performances are shown in terms of reflection coefficient, radiation pattern and average realized gain. Moreover, a comparison of the proposed design with earlier reported structures is presented. Finally, in Section 3, conclusions are drawn.

2. Antenna Layout

2.1. Design Steps of the Compact and Wideband Antenna Obtained from a Thin Metal Sheet

The proposed antenna was designed to be integrated under the vehicle’s dashboard. In such a position, more space can be obtained for easy integration of the antenna. Indeed, the height of the shark-fin cover on the roof is subjected to international regulations, and for aesthetic reasons the shark-fin cover should be as small as possible. A typical volume available under the shark-fin cover is 53 mm × 50 mm × 40 mm (height × length × width). To exploit the entire internal volume, a 3D antenna can be realized by cutting and folding a metal sheet instead of printed 2D antennas being used, but coexistence with other radiating elements may still be difficult.

In order to explain the operating principle of the proposed dashboard antenna, three steps have been considered (Figure 2). Initially, a three-branch monopole-like antenna was designed (Figure 2a) that provides three main resonances at 750 MHz, 1750 MHz and 2500 MHz. The antenna has its own ground plane with a size of 145 × 45 mm². The size of the PCB represents a trade-off between electromagnetic antenna performance (a monopole antenna needs a large ground plane to avoid feeding unbalance) and mechanical integration (the available space inside the dashboard is limited by the presence of other components).

In Figure 3, the reflection coefficients of each antenna design step are plotted. As shown in Figure 2a, the monopole can be divided in four sections (I to IV). The second antenna design step was obtained by folding the two side sections, i.e., III and IV (Figure 2b). Consequently, the two resonances at 1750 MHz and 2500 MHz were shifted toward higher frequencies, while other new resonances were generated thanks to inductive and capacitive couplings between the sections. Then, to reduce the overall height and make the antenna more compact and low-profile, the straight monopole was halved and section II was folded from the vertical to the horizontal. This led to a wider −10 dB bandwidth of the antenna from around 805 MHz to 4000 MHz, as shown in Figure 2c. The antenna is fed at the bottom of the monopole element. An optimization of the monopole dimensions and shape was performed, and the final layout is shown in Figure 4, together with its final dimensions. The dimensions of the proposed antenna (first configuration) are listed in

Table 1. Dimensions of the proposed antenna (first configuration).

Parameter	Value (mm)	Parameter	Value (mm)
H	45	LG	143.5
L1	34.6	W	40
L2	35.3	WG	47
L3	28.1

. All geometries are designed and simulated with the full wave electromagnetic software CST Microwave studio [31]. The simulated reflection coefficient of the final layout is plotted in Figure 3.

2.2. Simulated and Experimental Results

In Figure 5, a picture of the fabricated antenna prototype is shown. The proposed antenna has been measured using a Vector Network Analyzer (Keysight P937A). The measured reflection coefficient is plotted in Figure 6 and compared with that obtained with the full-wave analysis. The measured and simulated reflection coefficients were in good agreement. A small discrepancy in results is due to imperfections in the fabrication process.

Although the structure is realized with a thin (0.2 mm) metal sheet, it might be difficult to achieve the mechanical stability of the 35.5 × 40 × 45 mm³ structure. Indeed, antennas on a vehicle may be subjected to huge vibrations during vehicle movement, so that soldered parts of bulky and heavy antennas may be broken. Moreover, the integration with other radiating elements may be difficult with such geometry. For these reasons, the antenna weight was further reduced by reducing the metal sheet. The new layout is shown in Figure 7.

To better explain its operating principle, the antenna is divided into two parts, denoted as sections I and II in Figure 7. The lengths of both sections are different to obtain two separate resonances and cover the low LTE band (frequency below 1 GHz). Section II is responsible for a lower resonance frequency (around 775 MHz) compared to that of Section I (around 845 MHz). The difference in length of both sections is 32 mm, which corresponds to a shift of 70 MHz in resonance. Further increase in the length of section II led to a degradation of the input impedance matching to around 1850 GHz, which is a desired band.

Thus, to cover the LTE700 band, a small stub of with a length of 27 mm was introduced into the structure. The short-circuited stub is connected to the central part of the monopole antenna, as depicted in Figure 8a. A short and thin stub connected to the ground represents a shunt inductor. Together with the capacitive nature of electrically small and folded monopoles, it introduces a further resonance at low frequencies. Specifically, a resonance close to 700 MHz is obtained, and a good input impedance matching is obtained from 670 to 775 MHz. It is worth noting that the presence of the stub has two advantages. Firstly, a resonance at low-LTE band frequencies can be obtained without increasing overall antenna size. Secondly, the stub provides mechanical stability to the proposed structure, which is even lighter than the previous one (thin metal strips with a width of 5 mm were used). The dimensions of the proposed LTE/5G antenna (second configuration) are listed in

Table 2. Dimensions of the proposed antenna (second configuration).

Parameter	Value (mm)	Parameter	Value (mm)
H	35	L5	12
L	40	L6	12.5
L1	35.8	L7	30
L2	29.1	t	5
L3	35	W	40
L4	27

. A photograph of the fabricated antenna prototype is shown in Figure 8b. Simulated and measured reflection coefficients of the antenna are plotted in Figure 9 considering two different cases, namely, with and without the stub.

From Figure 9b, it can be observed that the antenna with the stub has a lower resonance and that it covers LTE700 band with a good reflection coefficient (S₁₁ < −10 dB). The discrepancy in the measured and simulated results is due to manufacturing imperfections. After validating the antenna resonance, the radiation characteristics of the antenna have been measured in an anechoic chamber using a standard double-ridged guide horn antenna (ETS-Lindgren’s model 3115) as a transmitter and the proposed antenna as a receiver. Normalized 2D radiation patterns in three different planes (xz-, yz- and xy-plane) are plotted in Figure 10. Peak realized gain is measured at various frequencies and compared to that obtained with full-wave analysis (Figure 11), resulting in a good agreement. It is worth noting that there is an almost 2 dB enhancement of gain at low frequency, while negligible variation can be observed in the remaining frequency band by introducing a shunt stub.

Comparison of the proposed work with earlier reported structures is made and listed in

Table 3. Comparison of the performance of the reported reconfigurable structures.

	Operating Frequency Bands (MHz)	Total Efficiency	Peak Gain in Operating Bands (dBi)	Total Size (mm3)	Technique Used	Antenna Type
[6]	698–960, 1700–2700, 5100–6000	>10% (698–960 MHz), >80% (1700–2700 MHz), >70% (5100–6000 MHz)	Not given	37.6 × 40 × 0.8	Shorted monopole	Printed
[9]	2000–2740, 4062–8000	>85% (2000–2740 MHz), >80% (4062–8000 MHz)	>2.7 (2000–2740 MHz) >2.6 * (4062–8000 MHz)	50 × 10 × 0.8	Monopole	Printed
[19]	576–2985	>85%	>2.0	Diameter is 62.6 mm and height is 39 mm	Monopole	3D
[21]	850, 1575, 2400, 5900	>30.7%	Not given	59.5 × 44.3 × 21	Planar inverted-F	3D
[22]	698–960, 1427–2700	68%	2.8 (900 MHz), 4.8 (1700 MHz)	55 × 33 × 10	Monopole	3D
[25]	700–960, 1700–2700	Not given	−4.5 * (850 MHz), −0.3 * (1700 MHz)	Not given	Monopole	3D
[26]	700–6000	>−1.5 dB	Not given	70 × 70 × 29	Wideband Nefer monopole	3D
[27]	617–5000	>62%	Linear Average Gain > 0 dB	15 × 39.8 × 60	Monopole	3D
This work (second configuration)	617–5000	>62% (670–960 MHz), >75% (1410–1550 MHz) >82% (1710–5000 MHz)	>−0.5 (670–960 MHz), >1.4 (1410–1550 MHz), >3.2 (1710–5000 MHz),	40 × 40 × 35	Monopole with shunt stub	3D

* Estimated from the data given in the paper.

. Various wideband antennas were presented to cover different cellular services. Most of the entries in Table 2 are 3D antennas. In general, although printed antennas occupy less volume in a system, they suffer from poor efficiency at high frequencies due to substrate losses and they have fewer degrees of freedom for optimization. The antenna reported in [6] is a printed antenna. It supports various wireless services but it has poor radiation efficiency at the low-frequency band (698–960 MHz) due to the small electrical size. The antenna reported in [19] is a highly efficient antenna but it is too big in size and it is impossible to fit it under a shark-fin cover or inside a car’s dashboard. The antennas presented in [21,22] are size-compatible with the volume under the shark-fin cover but they cover only few services due to their multiband behavior. Two wideband antennas have been presented in [25] but each has a relatively low gain. Wideband antennas covering almost the full 617–5000 MHz band were realized in [26,27] but they cannot be fitted under shark-fin covers due to their large sizes. It is worth noting that the antenna in [27] is designed to cover the entire 617–5000 MHz band but with a maximum Voltage Standing Wave Ratio of 3.3 (return loss higher than 5.6 dB). The antenna proposed here is compact in size and can be easily integrated inside the car’s dashboard (the antenna volume is 40 mm × 40 mm × 35 mm). The proposed antenna is mechanically robust and cannot be easily damaged by vibrations during braking or vehicle movement.

3. Conclusions

A novel, wideband, low-profile, 3D automotive antenna for Long-Term Evolution (LTE) and 5G applications has been proposed in this paper. Since the number of wireless applications is rapidly increasing in vehicular and automotive systems, there is a need for new spaces for accommodating both the radiating elements and electronic circuitry. In this study, a wideband and compact 3D antenna for Long-Term Evolution (LTE) and 5G applications has been designed to be integrated under the vehicle’s dashboard. The radiating element is made from a metal sheet by properly cutting and folding it to guarantee satisfactory performance in terms of input impedance matching as well as gain. Input impedance matching at the low LTE frequency band (e.g., 617–670 MHz) can be obtained by means of a shunt stub, which also gives robustness to the structure, which is subjected to vibration during vehicle movement.