Integrated Module Antenna for Automotive UWB Application

Abstract

In this paper, an integrated module antenna for automotive UWB application is proposed. The target applications of the proposed antenna are for UWB localization and rear passenger detection. The purpose of this work is to design an antenna with a wide and narrow beamwidth that can be attached to the exterior/interior of a vehicle for simultaneous UWB localization and a rear passenger detection sensor with a single-module substrate in a limited space. For UWB localization, the wide beam coverage is required so the antenna receives the signal from any incident angle in horizontal plane. Meanwhile, the rear passenger detection sensor requires a relatively narrower beamwidth than the localization antenna for accurate detection within the vehicle. To integrate two different antennas into a single compact module substrate, modified ground stubs and parasitic radiators are applied. The size of the entire antenna structure is 35 mm × 65 mm × 1.156 mm. The proposed antenna designed on the multi-layered FR-4 substrate with a dielectric constant of 4.3. The bandwidth of the monopole is 6.14~8.24 GHz, and the patch array is 6.95~8.47 GHz. The isolation between the monopole and the patch array is less than −23 dBi in the target band. The performance of the proposed antenna is verified with simulation and measurement. In addition, a simulation of the proposed antenna with the real vehicle model is also conducted to verify the feasibility on actual vehicle. Based on this work, the proposed antenna can be applied to multi-function antenna for automotive application with low cost.

1. Introduction

Automotive antennas have been widely researched in various configurations and applications, such as AM/FM, GPS, DSRC, LTE, 5G and radar [1,2,3,4,5,6,7]. Recently, the scope has been expanded to automotive UWB, wherein various studies on V2X (vehicle-to-things) communication, smart key localization, radar, and passenger detection sensors are being conducted [8,9,10,11,12]. In particular, the wide coverage with wide beamwidth antenna is required for communication or localization application. In contrast, the antenna with relatively narrower beamwidth than communication is required for radar or passenger detection sensor applications. However, designing and utilizing individual antennas increase the complexity and cost of developing electronic components for vehicles, which is significant. To solve this problem, integrated antennas have been proposed to perform the various automotive applications. Due to the limited antenna design space of the vehicle, for instance, integrated antennas supporting two or more services (e.g., LTE, GPS, WLAN, WAVE, etc.) have been developed [13,14,15,16]. In [13], substrate-type antennas and metal-type antennas using a ground surface are integrated into the shark pin antenna to support three services at the same time. An L-sleeve L-monopole antenna is proposed to support the multi band for vehicular LTE [14]. The L-shaped structure is solution to realize the compact size of the antenna in the shark-fin while maintaining the broad bandwidth. In [15], two arm structures with different dimensions are used to provide resonances at low and high bands of the monopole antenna with compact size in the shark-fin with compact. Four individual antennas for LTE, FM, GPS, and diversity are integrated inside the shark-fin [16]. In particular, the LTE antenna is designed as a modified form of a loop to cover two bands. Furthermore, due to functional parts, such as antenna diversity, and pattern coverage, pattern-switching or reconfigurable antennas are being developed [17,18]. In [17], active dipole with two parasitic patches and passive dipoles are integrated into single substrate for broad bandwidth. In addition, PIN diode is applied to pattern reconfigurable purpose. An integrated Tri-band/UWB polarization diversity antenna for vehicular networks is proposed [18]. To obtain triple and ultrawideband characteristics, the UWB monopole and modified monopole structures are integrated into one substrate, and each antenna operates in individual modes using multiple PIN diodes. Considering the limited antenna design space of the vehicle, these antennas are solution of reducing the system’s complexity and cost.

In this paper, the integrated type antenna is proposed for automotive UWB applications. The proposed antenna is combined with UWB localization antenna and rear passenger detection sensor antenna in single UWB module substrate. For the UWB applications, the monopole and patch array is adopted for prototype antenna. Because the wide coverage is required that the antenna can receive the signal at any incident angle in horizontal plane of the vehicle for UWB localization. In contrast, the rear passenger detection sensor requires a relatively narrower beamwidth than the localization antenna for accurate detection. Then, the modified ground stubs and parasitic radiators are applied to the monopole and patch array. Because the combined antenna has a structure in which the monopole and patch array share the ground; due to limited space of the automotive UWB module, mutual influence can degrade the radiation characteristics. The modified stubs are utilized in order to control the phase difference of the current between the shared ground and the antenna. In addition, the parasitic radiators are applied to increase the aperture of the parch array without the increase of the antenna size. In evaluating the performance of the proposed antenna, simulation with CST Microwave studio [19] and measurement results are analyzed. Moreover, a simulation with a model of a real vehicle is conducted to verify the feasibility of the proposed antenna. Based on this work, the proposed antenna can be utilized as a universal device that simultaneously performs UWB localization outside the vehicle and rear passenger detection sensor inside the vehicle as a single module.

This paper is organized in the following order. First, we discuss considerations for antenna design, such as antenna design space with substrate, antenna covering case, and the actual mounting position on the vehicle. Second, the prototype antenna is designed with monopole and patch array. In addition, the modified ground stub and parasitic radiators are applied to compensate for the degradation of the antenna performance due to shared ground in the limited module size. Third, the optimized antenna is fabricated and measured. The antenna simulation with an actual vehicle model is conducted. Then, we verify the performance of the proposed antenna by comparing it with other findings in the literature.

2. Proposed Antenna Design

2.1. Considerations for Antenna Design

Before designing the antenna, the available space of the module substrate, antenna case, and the antenna’s actual implementation on the vehicle must be considered. For UWB localization, an antenna with wide coverage and low fluctuation in the horizontal plane is required. In addition, a sufficient bandwidth for the automotive UWB band is required. The target frequency range is 6.25~8.25 GHz, covering channels 5 and 9 of the automotive UWB band. However, an antenna with a relatively narrower beamwidth and greater gain is required for rear passenger detection sensor. The target frequency band is 7.75~8.25 GHz, covering channel 9 of the automotive UWB band.

The proposed antenna was designed in a compact single multi-layered substrate that is primarily used for UWB modules, as shown in Figure 1. The substrate comprised four layers of 4-copper and three layers of FR-4 dielectric (${\mathsf{\epsilon }}_{\mathrm{r}}=4.3, \tan \delta =0.025$). The thickness of the substrate was 1.156 mm. The area available for antenna design measured 35 × 20 mm².

The geometry of the antenna case is shown in Figure 2. The dielectric constant of the antenna case material is 2.8. The distance between the top of the case and the 1st layer of substrate is 7 mm, and the distance between its bottom and the 4th layer is 5 mm.

For the localization antenna, four antennas were mounted at the corners of the vehicle. On the front end, two antennas were mounted under the headlamp in the engine room, and the others were mounted behind the rear bumper of the rear part of the vehicle, as shown in Figure 3. Meanwhile, the rear passenger detection sensor antennas were mounted on the upper ceiling of the rear headrest in the vehicle, as shown in Figure 4. Considering the mounting axis of the antenna in the vehicle, a wide coverage in the yz-plane with low fluctuation was required for the localization antenna. For the rear passenger detection sensor antenna, a directive radiation pattern in xz-plane with 90$°$ beamwidth for sufficient coverage was required.

2.2. Design of the Prototype Antenna

This work aims to design the integrated antenna with different radiation properties in a compact UWB module substrate, taking into consideration the antenna case and actual vehicle implementation. For wide coverage and sufficient bandwidth for automotive UWB frequency band, the monopole antenna is selected to satisfy our purpose. To secure antenna design space, a quasi-circular rim-type monopole antenna which satisfies the UWB bandwidth with a miniaturized size is designed based on [20]. For rear passenger detection, however, an antenna with relatively narrower beam and greater gain than a monopole antenna is required. Therefore, the patch array is adopted due to the aforementioned requirements and available space. These antennas are designed on the different layers in order to efficiently utilize the available space of the limited module substrate.

The basic monopole is designed with a feedline on the 4th layer, and the ground is assigned to the 3rd layer. The patch array is designed on the 1st layer, and the ground is assigned to the 3rd layer that is to be shared with the ground of the monopole. The patch array is fed by the rectangular slots on the ground through the feeding line on the 4th layer. The designed prototype antenna is shown in Figure 5. The return loss, isolation, and radiation patterns of the antennas are shown in Figure 6 and Figure 7, respectively.

As shown in Figure 7, the radiation pattern of the monopole at 6.5 GHz had a peak in the +y direction and a narrow beamwidth and pattern fluctuation. Likewise, the radiation pattern at 8.0 GHz showed a similar trend. This was due to the influence of the shared ground plane of the patch array. The radiation pattern of the patch array showed more than 5 dBi gain, but the beamwidth was insufficient to meet the appropriate margin considering the actual mounting position of the module in the vehicle.

2.3. Pattern Improvements with Modified Ground Stubs and Parasitic Radiators

In order to compensate the influence of the shared ground of the patch array, the additional ground stubs are applied on the 3rd layer. The improvements of the radiation pattern by adding the stubs are provided in stepwise in

Table 1. Radiation pattern improvements with modified ground stubs.

Step 1 (Prototype)	Step 2	Step 3
Radiation Pattern of Monopole (yz-Plane)
Radiation Pattern of Patch Array (xz-Plane)

.

When stub 1 is applied in step 2, the phase difference of the current between the monopole and edge of the module ground close to stub 1 is changed from 160$°$ to 90$°$. At 6.5 GHz, the beamwidth of the monopole is expanded from 86.9$°$ to 124$°$, and at 8.0 GHz, it slightly decreased from 119$°$ to 115$°$. The entire shape of the radiation pattern is broadened and the fluctuation is reduced. In addition, stub 1 is applied to the patch array with 70° of phase difference. The beamwidth of the patch array is also expanded from 75.6$°$ to 85.1$°$ in the xz-plane. However, the beamwidths of the monopole and patch array are not sufficient for the localization and the detection sensor. In step 3, when stub 2 is applied, the phase difference of the current between the monopole and stub 2 is 180°. Because of this, the beamwidth of the monopole is expanded from 124$°$ to 146$°$ at 6.5 GHz. At 8.0 GHz, the beamwidth is slightly reduced from 115$°$ to 111$°$. Meanwhile, stub 2 has no discernible effect on the patch array, and the beamwidth is maintained at 85.5°.

Additional stubs are added in order to expand the beamwidth of the patch array and monopole even further. Step-by-step descriptions of the effects of enhancing the radiation pattern by adding additional stubs are provided in

Table 2. Radiation pattern improvements with additional modified ground stubs.

Step 3	Step 4	Step 5
Radiation Pattern of Monopole (yz-Plane)
Radiation Pattern of Patch Array (xz-Plane)

.

In the step 4, the current path on the stub 1 is extended when the stub 3 is applied. Then the phase difference of the current between the patch array and the stub 1 is changed from 70$°$ to 90$°$ and the beamwidth of the patch array is expanded from 85.5$°$ to 95.7$°$ in the xz-plane. However, the gain is reduced under the 5 dBi. Meanwhile, the beamwidths of the monopole at 6.5 GHz and 8.0 GHz are still maintained as the beamwidths of step 3. In step 5, the stub 4 with a phase difference of 180$°$ from the monopole is added, and the stub 5 is added so that the phase difference between the ground of the monopole and the stub 2 is 180$°$. Then beamwidth of the monopole at 6.5 GHz and 8.0 GHz are expanded from 146$°$ to 175$°$ and from 111$°$ to 119$°$ when the stub 4 and 5 is applied. However, patch array has no notable change and the beamwidth is maintained at 95.7$°$.

To compensate the gain reduction of the patch array, parasitic radiators are applied on the 1st layer. As shown in step 6 of

Table 3. Radiation pattern improvements with parasitic radiators.

Step 5	Step 6	Step 7
Radiation Pattern of Monopole (yz-Plane)
Radiation Pattern of Patch Array (xz-Plane)

, the parasitic radiator 1 is located between patches. The parasitic radiator 2 is located at the top of the patch array and is connected with ground through the metal via, as shown in step 7 of Table 3. The effects of improving the radiation pattern by adding the radiators are summarized in Table 3 step by step.

Due to the parasitic radiators, the gain of the patch array increased from 4.31 to 4.91 dBi, whereas the beamwidth in the xz-plane remained greater than 90$°$. The parasitic radiators were electrically separated from the patch array, so these did not increase the resonance length of the patch, and contributed to enhancement in gain by expanding the aperture of the patch array within the assigned space. Meanwhile, the beamwidth of the monopole had no notable change in yz-plane.

To increase the gain in the patch array, the 1st and 2nd layer spaces of the module ground were minimally utilized. On the 1st layer, the ground of the module was etched for an additional parasitic radiator. In addition, the ground of the 2nd layer was etched to optimize the performance of the patch array. The final configuration of the proposed antenna is shown in Figure 8. The design parameters for each layer are shown in Figure 9 and summarized in

Table 4. Design parameters of the proposed antenna.

Parameters	Values	Parameters	Values
$W_1$	17.5 mm	$L_1$	0.75 mm
$W_2$	17.25 mm	$L_2$	7.88 mm
$W_3$	17.5 mm	$L_3$	1.37 mm
$W_4$	16.55 mm	$L_4$	0.65 mm
$W_5$	16.8 mm	$L_5$	0.74 mm
$W_6$	17.5 mm	$L_6$	0.74 mm
$W_7$	0.74 mm	$L_7$	0.57 mm
$W_8$	15 mm	$L_8$	1 mm
$W_9$	2 mm	$L_9$	1 mm
$W_{10}$	2.5 mm	$L_{10}$	0.5 mm
$W_{11}$	3 mm	$L_{11}$	1 mm
$W_{12}$	1.5 mm	$L_{12}$	8 mm
$W_{13}$	0.4 mm	$L_{13}$	15.3 mm
$W_{14}$	5.35 mm	$L_{14}$	1.07 mm
$W_{15}$	2.5 mm	$L_{15}$	5 mm
$W_{16}$	0.5 mm	$L_{16}$	6 mm
$W_{17}$	0.4 mm	g	0.4 mm

. The return loss, isolation, and radiation pattern of the antenna is shown in Figure 10 and Figure 11.

The proposed antenna shows 176$°$ and 118$°$ beamwidth in yz-plane at 6.5 GHz and 8.0 GHz when the monopole excited. Additionally, the patch array shows 5.11 dBi gain and 91.2$°$ beamwdith in xz-plane.

2.4. Fabrication and Measurement

The proposed antenna was verified with a fabricated structure and measurements. The proposed antenna was fabricated to verify the individual operating characteristics of the monopole and patch array, as shown in Figure 12. The antenna measurements were taken. The measurement setup is shown Figure 13. The measured return loss and radiation patterns of the proposed antenna are shown in Figure 14 and summarized in

Table 5. Radiation pattern of the proposed antenna with case.

Monopole (6.5 GHz)	Monopole (8.0 GHz)	Patch Array (8.0 GHz)

.

The measurement of return loss is conducted by using Agilent’s MS4644B Vector Network Analyzer. The measured return loss shows that the bandwidth of the monopole and patch array are 6.27~8.21 GHz and 6.85~8.61 GHz. The radiation pattern is measured in the anechoic chamber. The measured results of the beamwidth of the monopole in the yz-plane at 6.5 GHz are 172$°$, and 117$°$ at 8.0 GHz. The patch array has 5.22 dBi gain and 90.9$°$ beamwidth in the xz-plane. The measured results of the proposed antenna are well agreed with the simulations.

2.5. Vehicle Model Simulation

The measurements of the radiation pattern of the antenna mounted on the actual vehicle were replaced by a simulation, due to facility limitations and time difficulties. For the simulation, the SUV cad of the Hum3D was used as the vehicle model [21]. The simulation model of the vehicle had the same dimensions as that of the actual vehicle. For localization, the vehicle model was separated into front and rear parts. For simulation of the front part, the antenna modules with case was mounted under the headlamp in the engine room. In addition, the antenna was also mounted on the corner of the rear bumper for rear part simulation. The components of the vehicle model for the front part included a simplified engine room, battery, wheel with suspension head, front bumper, fender, and dashboard, as shown in Figure 15. The rear part of the vehicle model simulation consists of the rear bumper, wheel, fender, tail gate, and rear windshield, as shown in Figure 16. For the rear passenger detection sensor, the antenna module with case was mounted on the upper ceiling of the rear headrest in the vehicle. The components of the vehicle model for the rear passenger detection sensor included rear seats, interior frame, top ceiling with roof, trunk space, and rear bumper, as shown in Figure 17. The simulation was conducted with CST Studio Suite Full EM simulation software [19]. The radiation patterns of the monopole and patch array with actual mounting on the vehicle model are shown in

Table 6. Radiation pattern of the proposed antenna with vehicle model.

Monopole (6.5 GHz)—Front Part
Left side	Right side
Monopole (6.5 GHz)—Rear Part
Left side	Right side
Monopole (8.0 GHz)—Front Part
Left side	Right side
Monopole (8.0 GHz)—Rear Part
Left side	Right side
Patch Array (8.0 GHz)
Left side	Right side

.

The radiation patterns of the monopole with the vehicle model show that sufficient coverage for UWB localization in the horizontal plane. Furthermore, the patch array shows the appropriate radiation pattern for the rear passenger detection sensor in the vehicle.

3. Conclusions

This paper proposes an integrated module antenna with a monopole and patch array for automotive UWB applications. The proposed antenna is designed into single substrate. The monopole is designed with modified ground stubs for UWB localization requiring wide coverage in the horizontal plane. Moreover, the patch array is designed with additional ground stubs and parasitic radiators for rear passenger detection sensor in the vehicle requiring relatively narrower beamwidth and greater gain than the monopole. The modified ground stubs and parasitic radiators are applied to compensate the degradation of the antenna performances due to limited antenna design space of the module and coexisting ground layer. The size of the entire antenna structure is 35 mm × 65 mm × 1.156 mm. The proposed antenna designed on the multi-layered FR-4 substrate with a dielectric constant of 4.3. The bandwidth of the monopole is 6.14~8.24 GHz, and the patch array is 6.95~8.47 GHz. The isolation between the monopole and the patch array is less than −23 dBi in the target band. The proposed antenna is verified with its simulation and measurement. Furthermore, the simulations of the antenna with vehicle model are also conducted to verity its feasibility on actual vehicle. In order to compare the proposed antenna with the reference papers, the properties of each antenna are summarized in the

Table 7. Comparison of antenna properties to validate the proposed antenna.

Ref.	Antenna Size	Bandwidth	Radiation Pattern	Number of Supporting Band	Complexity
[13]	52 × 37 mm2	2.1~2.7 GHz /5.6~6.15 GHz	Omni only	2	Low
[14]	66 × 52 mm2	0.698~2.7 GHz	Omni only	3	Low
[15]	60 × 38.5 mm2	0.61~0.96 GHz /1.71~2.69 GHz /3.3~5 GHz	Omni only	3	Low
[16]	50 × 25 mm2	/0.698~0.96 GHz /1.71~2.69 GHz	Omni only	2	Low
[17]	50 × 35 mm2	3.3~4.22 GHz	Directional only (3-state beam)	1	Medium
[18]	30 × 30 mm2	3~12 GHz (mode 1) /2.42~2.7 GHz (mode 2) /3.47~4.05 GHz (mode 2 /5.73~5.84 GHz (mode 2)	Omni only (2-state beam)	4	High
This work	37.5 × 20 mm2	6.14~8.24 GHz (mode 1) /6.95~8.47 GHz (mode 2)	Omni and directional (dual beam)	3	low

.

As shown in Table 7, for an antenna designed on a single substrate, the proposed antenna has the advantage of covering three bands, with compact size, low complexity, and two types of patterns. Future directions of research would be to expand the operating bandwidth of the rear passenger detection antenna (patch array), which would allow the 6.5 GHz band to be used, such as the localization antenna (monopole antenna). Based on this work, the proposed antenna could be applied as a multi-function antenna for automotive UWB applications with low costs.