A compact multiband monopole antenna is proposed for vehicular roof top shark-fin applications. The proposed multiband antenna covers 5G sub-6 GHz and LTE bands starting at 617 MHz to 5000 MHz and the higher GNSS band from 1559 to 1606 MHz as well as the V2X band at 5900 MHz. The presented antenna is a three-dimensional monopole antenna with two branches to cover the required bands with compact size to fit inside a roof top shark-fin. The long antenna branch covers the lower cellular frequency band from 617 to 960 MHz, while the short branch covers the higher frequency band from 1559 to 6000 MHz. The presented antenna is mounted on a double-sided FR4 PCB and is feeded through a short cable. The proposed antenna covers multiple frequency bands with compact size (H  xLx  W) of 58 × 37 × 17 mm3. The antenna is simulated and optimized, and then, a prototype is fabricated, and its radiation characteristics are measured when mounted on one-meter ground plane and on a vehicle’s roof. The maximum measured linear average gain is 3 dBi at 1900 MHz, and the maximum measured efficiency is 88% at 787 MHz. The active GNSS antenna gain is measured using an LNA with good isolation. A good agreement is achieved between the simulated and measured results when compared in terms of voltage standing wave ratio (VSWR), radiation patterns, linear average gain (LAG), and antenna efficiency.

1. Introduction

Vehicular communication research and development has spiked recently due to the introduction of autonomous driving and intelligent vehicles, which requires many communication systems and protocols to be implemented [1]. To improve safety and efficiency in vehicles and the whole transportation system, vehicle-to-everything (V2X) is introduced, which mainly includes vehicle-to-vehicle (V2V), vehicle-to-pedestrian (V2P), vehicle-to-infrastructure (V2I), and vehicle-to-network (V2N). By exchanging messages between vehicles, infrastructure, and pedestrians, V2X can support many use cases such as forward collision warning, parking search, and optimizing a vehicle’s traffic [2]. Another revolutionary technology for vehicle autonomy is the 5G cellular communications, which provides a reliable and low latency communications as well as high data rates. 5G wireless network provides an essential role by providing a reliable and fast communication between vehicles and other objects which means it can be integrated with V2X and this integration called later as cellular V2X (C–V2X). Global Navigation Satellite System (GNSS) is the main positioning system for self-driving vehicles, but it is not accurate enough, so it is integrated with other systems such as light detection and ranging (LiDAR) and inertial navigation systems (INS) to increase the positioning accuracy and availability, especially in urban areas [3].

Placement of antennas on vehicles has large impact on the antenna’s performance, so most of the antennas are placed on the vehicle’s roof to avoid signal blockage by metal. Roof antennas are usually placed in one cast called a shark-fin due to its shape. Shark-fins have limited size due to the vehicle’s aerodynamic design, and recently, many antennas are placed inside these limited size shark-fins, which increase the coupling and shadowing effects between different antennas [4]. One solution to reduce the shadowing and coupling effects on antennas inside a shark-fin is to design antennas that cover multiple bands and applications to reduce the number of antennas as well as reducing the cost [5].

The goal of this paper is to design a compact multiband antenna for the automotive industry that covers 5G sub-6 GHz and LTE cellular frequencies, V2X, and GNSS frequency bands. Most of the automotive cellular antennas found in the literature are for LTE frequency bands starting at 698 MHz to 2690 MHz [615]. The most used LTE antenna types are monopoles and planar inverted-F antennas (PIFAs). For the 5G sub-6 GHz, there are fewer antennas in the literature such as the monopole antennas reported in [1618]. These monopoles antennas cover only 5G sub-6 GHz frequencies and do not cover more frequency bands, and also, they have larger size compared with the proposed antenna. Ultrawide band (UWB) antennas based on metamaterial and metasurfaces are suitable candidates for the 5G sub-6 GHz [1922]. A PIFA that covers 5G sub-6 GHz with V2X bands is reported in [23], but it is suitable for certain wide types of shark-fins due to its size. UWB antenna for 5G sub-6 GHz and V2X bands also reported in [24], but it is not suitable to be placed inside a shark-fin.

Usually, vehicles have separate antennas for cellular coverage and navigation due to the polarization difference between these two applications [25, 26]. A few antennas are reported to integrate both GPS and cellular antennas in one antenna element such as the L shaped monopole and the quasi-Yagi antennas, which combine GSM and GPS [27, 28]. These antennas have large size and narrow cellular bandwidth, which makes them not suitable for automotive applications. Stacked antennas with smaller size are also reported for GSM and the Global Positioning System (GPS) but also have narrow cellular bandwidth [29]. From this literature survey discussion, compact multiband antennas are required for vehicular applications.

In this work, a compact multiband monopole antenna is proposed to cover 5G sub-6 GHz, V2X, and GNSS. The proposed antenna is a three-dimensional metal monopole antenna with two branches to cover this wide bandwidth from 617 MHz to 5925 MHz. Although a monopole antenna is a linear polarized antenna, right-hand circular polarization (RHCP) is measured for GNSS and a GNSS LNA is connected to the antenna to show that the antenna RHCP active gain and radiation pattern shape is suitable for the automotive industry. The antenna design and theory are presented in Section 2, Section 3 discusses the antenna results, and Section 4 concludes this paper.

2. Antenna Design and Theory

The proposed monopole antenna geometry is shown in Figure 1, and Table 1 shows the detailed dimensions of the antenna. The proposed antenna is a folded three-dimensional monopole antenna with one main slot to divide the antenna into two parts, and these two parts will be called long branch and short branch in the rest of the paper for illustration purposes as shown in Figure 1. The long branch is designed to cover the lower band from 617 to 960 MHz, and this branch is miniaturized by folding the antenna in a horizontal and vertical directions with the slot goes through the two folded parts to lengthen the current path for the antenna to be a quarter wavelength long at 788 MHz, which is the center frequency of the low band. The long-branch folded arm is added to the antenna to reduce the antenna height to fit inside a shark-fin, which has limited height, and the effect of the folded arm will be studied more with the antenna current distribution and parametric sweeps for the folded arm width (W) and height (H4). The short branch is responsible for the higher band resonance, which is from 1559 to 5925 MHz to cover GNSS, high cellular, and V2X frequency bands. The short branch has a horizontal top loading portion with width W2, this horizontal portion is to make a gap between the short branch and the folded arm of the long branch, and this gap is important for the GNSS signal to not get blocked by the long-branch folded arm, which is also tapered and chamfered for the same purpose. The gap between the short and long branch is optimized to be as large as possible within the antenna dimensions.

The proposed antenna is mounted on an FR4 PCB and connected to a short cable with SMA connector as a feed. The monopole antenna has a compact size (H  ×  L  ×  W) of 58 × 37 × 17 mm3. Table 2 shows some general performance requirements that the proposed antenna must meet for 5G, LTE, GNSS, and V2X. The performance requirements are in terms of frequency bands with its corresponding voltage standing wave ratio (VSWR) and the required linear average gain (LAG) in certain elevation angles as well as the polarization. The antenna performance requirements shown in Table 2 are derived from the automotive original equipment manufacturers (OEMs) specifications at the pigtail level [23, 30].

3. Results and Discussions

The proposed monopole antenna was simulated and optimized using ANSYS HFSS. To investigate more on how the antenna covers the required frequency bands, the antenna current distribution is shown in Figure 2 for three different frequencies. For the low-band center frequency (788 MHz), the highest current intensity followed the slot on the long branch as shown by black lines in the front and back views in Figure 2. The current path length with the highest intensity is measured to be 97 mm, which is very close to the quarter wavelength at 788 MHz (95 mm), and this explains the role of the slot in the proposed antenna miniaturization. For the higher band, which starts at 1559 MHz, the current path length on the short branch can be calculated by C2+H2+W2, which is 50 mm, and this is almost equals to the quarter wavelength at 1559 MHz (48 mm). The short branch is responsible for the higher band resonance from 1559 to 5925 MHz.

To develop an equivalent circuit model for the proposed antenna, the antenna simulated VSWR with its geometry will be used. The circuit model is derived from the simulated VSWR using the techniques presented in [31]. Figure 3 shows the proposed monopole antenna equivalent lumped element circuit model, and it consists of three main parts to represent the antenna short branch, long branch, and slot. The slot separates the antenna low band and high band and acts as a band stop filter around 1220 MHz, and this represented by an LC resonator to ground and a capacitor C2 in series. The antenna long branch, which is responsible for the antenna’s low band resonance, is represented by an RLC parallel band pass filter with center frequency of 775 MHz. The short branch is modeled by two parallel RLC band pass filters as the short branch is responsible for the wide high band from 1559 to 5925 MHz, which requires two band pass filters to cover it. The antenna equivalent circuit model is simulated and tuned using Advanced Design System (ADS). Figure 4 shows the VSWR for the circuit model, and for the antenna HFSS simulation, which shows a good agreement, this agreement proves that the circuit model represents the antenna accurately.

Parametric studies on the antenna slot width and length and on the top two-folded long-branch portions were conducted to observe their effects on the antenna resonance. The parametric sweeps were conducted when all the other parameter values are the same as in Table 1. Figure 5 shows the effect of the slot width Ws on the antenna VSWR when Ws changed from 1 to 3 mm. Increasing slot width Ws increases the current path, and this results in lowering the resonance frequency for both the low band at 617 MHz and the high band at 1559 MHz when the Ws is 3 mm, and the VSWR at 617 MHz is 2.7 compared with 3.3 when the VSWR is 1 mm, while at 960 MHz, the opposite effect to 617 MHz is observed. At 1559 MHz, increasing Ws shifts the resonance frequency lower, which is the same effect as for 617 MHz. Changing the slot length Hs affects both the low band and the high band due to increasing the current path for both the long and short antenna branches as shown in Figure 6. To conclude on the effect of the slot on the antenna resonance, slot width and length affect antenna low band and high band, increasing the slot width or length shifts the resonance frequency lower at 617 MHz and at 1559 MHz, which is the same effect as increasing inductor L2 or decreasing C5 in the equivalent circuit model shown in Figure 3. On the other hand, decreasing the slot width or length shifts the resonance frequency higher.

Figure 7 shows the effect of the long-branch horizontal folded arm width (W) on the antenna VSWR. Changing W affects the low band at 617 and 960 MHz, increasing W shifts the low-band resonance frequency lower, while decreasing W shifts the low-band resonance frequency higher. Figure 8 shows the effect of the long-branch vertical folded arm height (H4) on the antenna VSWR. Changing H4 mainly affects the low band, increasing H4 shifts the low-band resonance frequency lower and decreasing H4 shifts the low-band resonance frequency higher, which is the same effect as changing W. When increasing H4 to be larger than 20 mm, it starts affecting the higher band at 1559 MHz due to the coupling between long and short branches as the distance between both branches will be small. Changing parameters W or H4 have the same effect of changing inductor L1 and capacitor C1 in the equivalent circuit model.

A prototype for the proposed antenna is fabricated and measured on a one-meter diameter rounded ground plane as well as on vehicle’s roof as shown in Figure 9. The antenna VSWR is measured on one-meter ground plane and then compared with the antenna-simulated VSWR as shown in Figure 10. Both simulated and measured VSWR met the antenna VSWR requirements listed in Table 2 for all frequency bands. A good agreement is also achieved between the measured and simulated VSWR as shown in Figure 10.

The fabricated antenna prototype radiation characteristics is measured with a MVG Satimo chamber on a ground plane and then on a vehicle. In the next subsections, the far field measurements are discussed for 5G sub-6 GHz, LTE, V2X, and GNSS.

3.1. 5G Sub-6 GHz and LTE Far-Field Measurements

It is important for the cellular antenna to have an omnidirectional coverage when mounted on a vehicle roof. This omnidirectional radiation is essential in the elevation angles from 75° to 87° (15° to 3° above horizon). Figure 11 shows a comparison of horizontal cuts of the proposed antenna vertically polarized gain between simulation on a ground plane, measurements on ground plane and on vehicle’s roof at 80° elevation angle at frequencies of 788, 2300, 3300, and 5000 MHz. For the low band at 788 MHz, the antenna’s radiation pattern is omnidirectional with a good agreement between simulation and measurements. At 2300 and 3300 MHz, the radiation started to be directive, especially in the measurements on both the vehicle and the ground plane. At 5000 MHz, the radiation pattern is more directive to the back and front of the antenna. The vehicle’s roof curvature affects the radiation pattern at 5000 MHz as some ripples and reflections started to appear.

In the automotive industry, the antenna gain is expressed in linear average gain (LAG), which can be calculated using the following steps: (1) measure the vertically polarized gain in linear units for 13 elevation angles from 75° to 87° at each of the 360° azimuth angles; (2) calculate the average azimuth gain for each of the 13 elevation angles; and (3) calculate the average gain in linear units from the 13 elevation angles to get one gain value for each frequency and then convert to logarithmic scale (dBi). Figure 12 shows the measured LAG on ground plane and a vehicle. For the low band, the LAG is above -1 dBi for both measurements on the ground plane and vehicle, while the middle band is above 1.5 dBi. After 4 GHz, the LAG is lower due to the reflections that appeared in the 5000 MHz radiation pattern. The proposed antenna LAG meets the LAG requirements listed in Table 2 for the ground plane and vehicle measurements. Figure 13 shows the antenna 5G and LTE measured efficiency on a ground plane and on a vehicle, and the efficiency followed the same curve trend as the LAG curve. For the low band, the efficiency is more than 50% and more than 60% for the high band for both measurement’s conditions.

3.2. V2X Far-Field Measurements

Omnidirectional coverage is also important for V2X in elevation angles from 80° to 96°. Figure 14 shows the antenna vertically polarized gain radiation pattern at 5900 MHz and 90° elevation (Horizon) when simulated on ground plane, measured on ground plane, and measured on a vehicle. The measured radiation pattern has a good omnidirectional coverage with maximum gain of 3 dBi and minimum gain of -7 dBi when measured on a vehicle and a maximum gain of 0 dBi and a minimum gain of -5 dBi when measured on a ground plane.

The antenna V2X LAG is calculated following the same steps written in the previous subsection except the V2X elevation angles, which are from 80° to 96°. Figure 15 shows the V2X measured LAG on ground plane and on a vehicle, and the measured LAG on a vehicle is lower than the measured LAG on a ground plane due to the roof curvature. The antenna meets the V2X LAG requirements listed in Table 2.

Figure 16 shows the V2X measured efficiency on a ground plane and a vehicle. The measured efficiency is above 70% for the whole V2X bandwidth.

3.3. GNSS Far-Field Measurements

Although the monopole antenna is linearly polarized, GNSS measurements were conducted, and the measurements are presented in right-hand circular polarization (RHCP). The targeted GNSS frequency bands are BEIDOU B1 (1559.04–1563.16 MHz), GPS L1 and GALILEO E1 (1574.42–1576.42 MHz), and GLONASS G1 (1597–1606 MHz). Figure 17 shows two vertical cuts (XZ and YZ planes) for the measured GNSS passive RHCP gain at 1575 MHz when mounted on a vehicle and on a ground plane. The two vertical cuts did not show any nulls. The antenna RHCP gain at zenith is -9 dBc and the gain increases until it reaches 0 dBc around 45°. Monopole antennas have low gain at zenith, and when converting the linear polarized gain into RHCP gain, another 3 dBi is subtracted and this is the reason its RHCP gain at zenith is −9 dBc.

To check the antenna GNSS coverage, horizontal cuts for four different elevation angles (10°, 20°, 30°, and 40°) measured on ground plane at 1575 MHz are shown in Figure 18. The proposed antenna shows good radiation pattern without major nulls in angles of interest; however, the antenna has better coverage to the front of the antenna (azimuth angles from 0° to 180°) than to the back of it (azimuth angles from 180° to 360°).

To measure the antenna active performance, low noise amplifier (LNA) is constructed using two LNA stages of signal processing LNA-00120 connected to a Qualcomm GNSS Saw Filter (B4327) to provide the required isolation as shown in Figure 19. Figure 20 shows the S21 of the constructed LNA, which has a gain of 28 dB in the GNSS frequency band and a good isolation in the other bands. The LNA has the same gain as an automotive grade GNSS LNA gain.

Figure 21 shows the measured GNSS active RHCP LAG on a vehicle at the center frequencies of BEIDOU, GPS, and GLONASS, which are 1561, 1575, and 1601 MHz, respectively. The LAG is calculated for each elevation angle by averaging azimuth angles. The active RHCP LAG is more than 20 dBc for most of the elevation angles considering using an LNA with normal gain value for automotive industry. This shows that the proposed antenna can receive GNSS signals with acceptable gain despite the antenna’s dominant linear polarization. Adding GNSS to the antenna makes the antenna design challenging as the antenna long branch can block the GNSS signal if not designed carefully, and this is the reason for the chamfering and tapering in the upper folded parts of the antenna long branch.

Table 3 shows a comparison between the proposed monopole antenna and other automotive antennas with wide bandwidth found in the literature. Most of the listed antennas covers the cellular bands. Two antennas cover cellular and V2X frequency bands [23, 24] with larger size than the proposed antenna, which covers the GNSS higher band too.

4. Conclusions

A compact monopole antenna is designed to cover multifrequency bands for the automotive industry. The multiband antenna covers 5G sub-6 GHz, LTE, GNSS higher frequency band, and V2X. The proposed antenna is a three-dimensional monopole antenna with two branches to cover the enlarged bandwidth required which starts from 617 MHz to 6 GHz. The antenna is simulated and optimized, and then, a prototype is fabricated and measured on one-meter ground plane and on a vehicle roof. The antenna measurements are expressed in terms of VSWR, radiation pattern, LAG, and efficiency in which the presented antenna showed good results that met the performance requirements. The proposed antenna has measured efficiency of 50–90% in the operating frequency and LAG of −2.2 to 3 dBi across the entire frequency band. A good agreement is achieved between the simulated and measured results. The antenna is optimized to enhance the GNSS performance, and the active RHCP LAG is measured, which was more than 20 dBi in most of elevation angles using normal 28-dB automotive GNSS LNA.

Data Availability

The data used to support the findings of this paper are included within the article.

Conflicts of Interest

The authors declared that they have no conflicts of interest regarding the publication of this paper.

Supplementary Materials

The raw data of the figures of the article are added as supplemental materials. (Supplementary Materials)