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Guide to select the antenna of a vehicle

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Mark Patrick, Mouser Electronics

Modern vehicles offer a fantastic set of features, such as safety, comfort or infotainment, and a large number of these depend on wireless connectivity. Pre-certified wireless modules with antenna are very popular, but do not have a viable option for some applications. In this article, we'll talk about what options are available when engineers need to add an antenna to the design plan. As an aid to the reader, we are going to include a section on the basic principles of antenna design, in order to explain the meaning of some terms necessary when selecting a model. We will talk about antennas that are mounted on the outside of the vehicle (for example, for SAAC, GPS or V2X connectivity) and those that are installed inside a UCE or infotainment system for Wi-Fi and Bluetooth connectivity inside the vehicle.

Connectivity in the modern car

Car antennas have changed greatly from the simple telescopic models of the 2s. Back then, before infotainment systems, the antenna was only used for radio. Today, antennas are used for many more things. For example, the infotainment system includes a radio, navigation system and wireless connectivity for mobile phones. Some functions of Advanced Driving Assistance Systems (SAAC) also require wireless connectivity to other cloud services for routing and, soon, for advanced vehicle-to-vehicle (V2V/C2C) and vehicle functions. to infrastructure (V2X/CXNUMXX). Within the vehicle itself, wireless communication is used to provide Wi-Fi to passengers and to communicate between various functions, such as blind spot indicators in the mirrors.

We live in the age of the connected car, and something as basic as the antenna plays a fundamental role in providing reliable and robust communications for in-vehicle and external services.

To select the right antenna, it is essential that we have a good understanding of the needs of the application, and the initial parameter is the operating frequency. Currently, the car radio tuner does not usually have medium wave (530 to 1700 kHz) or long wave (150 to 250 kHz); the most popular option is very high frequency (VHF) modulated (FM, 80 to 108 MHz). However, higher quality digital radio, such as UK Digital Audio Broadcasting (DAB), uses the 175 to 240 MHz spectrum.

Apart from radio transmission, the main frequency ranges are for Wi-Fi and Bluetooth (2,4 Ghz), GNSS (1,1 to 1,6 Ghz) and cellular (700, 850 and 1700-2100 Mhz). ). Also, 5G networks add the 6 Ghz frequency to this list. Most V2X/V2V applications use 5,9 GHz-centric mobile communication methods.

Types of antennas

In general terms, we can talk about external and internal type antennas.

External antennas are usually mounted on the rear of the vehicle's roof, in order to minimize electromagnetic interference from the engine. The electrical characteristics of the antenna, which we will discuss in the next section, are important, but it is also essential to protect the antenna against the ingress of moisture, dust and other contaminants. The degree of protection against the entry of elements is defined by international IP regulations. The antenna is exposed to all elements of the weather, so it must function reliably despite rain, frost or strong winds. Many external antennas are encased in a shark fin-shaped plastic casing for aesthetic and environmental purposes. The shell is aerodynamic and made of a material that can be painted in the same color as the vehicle. Other external antennas have a disk-like device that is usually magnetically attached to the chassis of the car. Many external antennas are actually several antennas in one box, but connected independently to the different systems: GNSS, cellular/5G and V2X/V2C.

In the field of internal antennas there is a bit more variety, and they are usually mounted on a printed circuit board or recorded on a track of it. Some internal antennas are designed to be mounted outside of the system to which they are attached and often have a self-adhesive mounting panel and coaxial cable for the integrated system.

Basic principles of the operation of an antenna

When selecting an antenna for your application, you must consider some important parameters. The performance of an antenna is made up of different attributes that ensure the most efficient possible transfer of radio frequency energy from the antenna's transmitter to the outside. The output of the transmitter is sent over a transmission line, such as a coaxial cable, to the antenna. Similarly, the antenna must also effectively pick up the signal and transmit it to the receiver. The basic concept of an antenna is that of a cable of a certain length. That length depends on the frequency at which it must operate. The antenna is resonant when its length is equal to the wavelength (measured in meters) of the received or transmitted signal. For example, a frequency of 2,4 Ghz has a wavelength of 12,5 cm.

In most cases, the effectiveness of an antenna is limited to a narrow band of frequencies. Antennas are made in many ways, and the simplest is a dipole. Typically, dipoles are described according to their operating wavelength; that is, full-wave or half-wave dipole.

Here is a brief explanation of the most important parameters you may encounter to help you select an antenna and interpret data sheets:

Return Loss: this term refers to the degree of adaptation of the antenna with the output section of the transmitter and the transmission line. It is expressed in dB and indicates the part of the output that is reflected from the antenna and the transmission line due to the mismatch. Most antennas have an impedance of 50 ohms, so they need to be matched to the transmission line and the final stage of the transmitter. The higher the return loss in dB, the lower the reflected power. A perfect adaptation would mean an almost infinite return loss. Another way to measure return loss is through Voltage Standing Wave Ratio (VSWR), which indicates the relationship between the power at the transmitter output and the amount of reflected power. A perfect fit is expressed as VSWR equal to 1 (or 1:1). The VSWR or return loss parameter is found on an antenna's data sheet, for a specific frequency or a small range of operating frequencies.

A VSWR equal to 2 indicates a return loss of 10 dB, parameters that are used as the ideal standard for design.

Datasheets often include graphs of return loss performance and VSWR vs. frequency. In image 1, you can see the VSWR graph of a PulseLarsen antenna, with the optimal range of operating frequencies from 650 to 880 Mhz for mobile communications or ISM.

antennas
Figure 1: Plot of VSWR on a PulseLarsen ICEFIN series low profile antenna (Source: PulseLarsen).

Radiation pattern and antenna gain: Image 2 shows the characteristics of the radiation pattern of a PulseLarsen ICEFIN antenna. By design, some antennas are more effective than others at emitting radio frequency energy in some directions. An antenna has gain when the amount of power emitted in a given direction is greater than the power input to the antenna. An omnidirectional radiation pattern is considered ideal for most practical applications, and plane characteristics (vertical/elevation or horizontal) are essential in some cases. Note that antenna characteristics are just as important to receiver performance as they are to transmitter performance.

types of antennas
Image 2: Radiation characteristics of a PulseLarsen ICEFIN antenna (source: PulseLarsen).

Impedance matching: as stated before, most antennas have an impedance of 50 ohms. The antenna must match the output impedance of the transmitter and the transmission line; It will probably be a coaxial cable with the proper impedance for external antennas, but it will need to be matched to the antenna's impedance if the cable needs to be added. For PCB antennas, it may be necessary to create a matching network (usually with coils and capacitors) between the transceiver IC and the onboard or surface mounted antenna. The impedance changes with frequency. The gain loss/VSWR characteristics of an antenna are measured with special equipment called a vector network analyzer (VNA). VNAs are sold as a stand-alone instrument or as part of a high-end RF Spectrum Analyzer.

A VNA typically includes many test functions with graphical displays, such as VSWR, return loss, and impedance. The antenna impedance is usually displayed as a Smith chart (see image 3).

different types of antennas
Image 3: Smith chart with the impedance of a Taoglas MA310 antenna for GNSS (source: Taoglas).

In short, the center line (in red) of the chart is a resistive impedance; above, it becomes inductive and, below, it becomes capacitive. The right side is an open circuit and the left is a short circuit. The green curve shows the change of the antenna impedance as a function of frequency. The 1.0 position on the red line shows the perfect match of the antenna impedance.

Examples of antennas

The Taoglas Raptor 3 (image 4) is an example of an externally mounted, IP67 rated antenna with a shark fin housing. It is a good choice for cars and commercial vehicles, combining multiple antennas in one housing: GNSS, 4G/5G mobile MIMO, dual-band Wi-Fi, active AM/FM antenna, and TETRA (radio trunking).

examples of antennas
Image 4: Taoglas Raptor 3 antenna, externally mounted and with shark fin housing (source: Taoglas).

In GNSS applications, the integrated, internally mounted Taoglas GPDF357 patch antenna is compatible with all GPS and Galileo bands, and offers excellent omnidirectional and gain capability. In addition, it is a compact model (see image 5) ideal for different high-precision positioning applications in the field of citizen security and smart agriculture.

vehicle antenna
Image 5: The Taoglas GPDF357 GNSS antenna (source: Taoglas).

Another multiband antenna is the 4-in-1 Taoglas MA354 model. This compact, low-profile, IP65-rated antenna has an integrated magnetic mount, so it can be installed on the vehicle quickly and without drilling holes. It contains five internal antennas to have dual band Wi-Fi, 4G/5G with 3G/2G compatibility and GNSS.

vehicle antenna
Image 6: Taoglas MA354 compact antenna; magnetic and external mounting (source: Taoglas).

Another IP67 rated, externally mounted, multiband antenna is the PulseLarsen IceFin series model (see image 7). This series offers antennas in a frequency range from 698 to 6000 MHz.

luxury antenna for vehicles
Image 7: PulseLarsen IceFin multiband antenna (Source: PulseLarsen).

PulseLarsen also offers different antenna kits with different models (integrated, internal and external) for prototyping and development. The kits are available in three versions: Instrumentation, Medical and Scientific (ISM) kits, LoRa and Internet of Things (IoT) kits. The IoT antenna kit is one example, and is made up of twenty-six different antennas for IoT applications; They have mobile connectivity, GNSS and dual-band Wi-Fi. There are different types of antenna, such as ceramic, helical, on a printed circuit board, stick or blade type.

Reliable connectivity requires the right antenna

In this article, we've explained what you need to know to help you select an antenna. We have talked about some of the most common types of antenna and we have given some examples. To help you understand the data sheets, we have briefly described some of the most important antenna parameters to know in order to select the best model for your application.