“In addition to smartphones and Internet of Things (IoT) devices, another major driver of mobile wireless connectivity is transportation applications, including rail, truck, and asset tracking. These applications place a unique set of important requirements on system antennas, such as vibration, shock, extreme temperatures, rain, humidity, and the need to operate over a wide frequency band or even multiple frequency bands while providing stable performance.
By Bill Schweber
In addition to smartphones and Internet of Things (IoT) devices, another major driver of mobile wireless connectivity is transportation applications, including rail, truck, and asset tracking. These applications place a unique set of important requirements on system antennas, such as vibration, shock, extreme temperatures, rain, humidity, and the need to operate over a wide frequency band or even multiple frequency bands while providing stable performance.
While it is possible to design and manufacture suitable antennas, in almost all challenging applications, a properly designed, well-manufactured, fully-characterized off-the-shelf standard device is the most logical approach. Doing so reduces costs and development time while increasing the level of confidence in the final design.
This article will explore issues related to the design of transport antennas. It then introduces two of TE Connectivity’s multi-band antennas designed to be mounted on housing surfaces, including basic “boxes” and potentially exposed mobile devices.
Antennas are important transducers between Electronic circuits and free-space electromagnetic (EM) fields, and therefore tend to be the most exposed element of a design. However, it must be in a form factor that is compatible with the overall system design, yet provides the required electrical and RF performance under harsh environmental conditions.
For freight systems, especially high-speed passenger rail, the antenna must also be easily integrated into an aerodynamic enclosure with minimal wind resistance and protection from harsh environments (Figure 1). Similar restrictions apply in the case of asset tracking, where the antenna must be exposed to receive Global Navigation Satellite System (GNSS) signals.
Figure 1: Today, mobile high-speed installations such as trains face severe challenges due to wind resistance and harsh environments, as mobile connectivity is required that can utilize a variety of standards and frequency bands. (Image source: TE Connectivity)
The ideal antenna is a careful combination of application-specific characteristics including the desired radiation pattern, proper impedance matching, low voltage standing wave ratio (VSWR), mechanical integrity, case suitability, and ease of electrical connection. In many cases, it is also necessary to enhance the signal path and maximize the front-end signal-to-noise ratio (SNR) by using an active antenna with an integrated low-noise amplifier (LNA).
As with all components, almost all antenna designs and installations require consideration of some extremely critical parameters, as well as others that are critical in specific situations. As far as antennas are concerned, radiation patterns and performance in specific frequency bands are key considerations.
Antenna Implementation Principles
For transportation and asset tracking applications, where antenna orientation is a challenge due to random changes in orientation, it is important to have a consistent omnidirectional pattern in both top and side looking directions throughout the designated frequency band.
For example, the TE Connectivity 1-2309605-1 M2M MiMo LTE dual antenna is designed for use in the 698 to 960 megahertz (MHz) and 1710 to 3800 MHz frequency bands for 2G, 3G, 4G, cellular, GSM, and LTE applications (Figure 2). A single antenna can effectively meet this set of standards because it is independent of the specific signal format conveyed or supported standards; its design is primarily determined by frequency, bandwidth, and power.
Figure 2: The TE Connectivity 1-2309605-1 is a single module product that includes two separate antennas, one for the 698 to 960MHz frequency band and the other for the 1710 to 3800 MHz frequency band. (Image source: TE Connectivity)
Note that a “dual” antenna is not the same as a “dual-band” antenna. Dual antennas, such as 1-2309605-1, have two independent antennas in a single housing, each with its own feed; dual-band antennas are single antennas with one feed, designed to support two (or more) ) frequency band.
Below is the low frequency antenna of 1-2309605-1. The radiation pattern in the top and side view directions is uniform throughout the entire bandwidth, from the low frequency end around 700 MHz to the high frequency end around 900 MHz (Figure 3) .
Figure 3: Side (left) and top (right) gain plots (up, middle, and down, respectively) of 1-2309605-1 at 700, 800, and 900 MHz show a fairly uniform radiation pattern. (Image source: TE Connectivity)
At 700 MHz (the low end of the band), the gain in decibels (dBi) relative to an omnidirectional antenna—a standard measure of antenna directivity—is only 1.5 dBi, indicating a fairly uniform radiation pattern. This uniformity and uniformity helps achieve consistent performance regardless of antenna orientation. In addition, the radiation pattern at the high end of 900 MHz is also fairly uniform, with a gain of only 4.5 dBi.
Another important antenna parameter is VSWR, which is formally defined as the ratio of the highest voltage to the lowest voltage, or the ratio of transmitted and reflected voltage standing waves on a lossless transmission line. Ideally, VSWR is 1:1. While this is often difficult to achieve, operating at low single-digit VSWR is generally acceptable.
1-2309605-1 M2M MiMo LTE dual antenna can handle up to 20 watts of transmit power, with a maximum VSWR of 3:1 at one end when measured with 3 meters (m) of RG174 cable, and close to 1.5 over most of its operating frequency band :1 (Fig. 4). Generally, this is low enough for many target applications.
Figure 4: The VSWR (vertical axis) of the 1-2309605-1 M2M MiMo LTE dual antenna measured with a 3-meter RG174 cable shows low values (x-axis) over the entire active frequency range. (Image source: TE Connectivity)
In Figure 4, green is low frequency element #1, red is high frequency element #2, black is element #1 and #2 in free space, and blue is element #1 on a 400 x 400 millimeter (mm) ground plane and #2.
To cover multiple frequency bands, two or more separate antennas can be placed together. But this leads to several potential problems. First, an obvious problem is the need for panels or other surface space and mounting pieces, along with the associated installation costs. Second, electromagnetic interactions between the antennas can affect their mode and performance; this limits how the antennas can be placed relative to each other. This interaction is measured by antenna isolation, which is defined as the degree to which one antenna receives radiation from another.
The solution to this challenge is to use a single antenna unit, combining multiple antennas within a single housing or enclosure. Mechanically, this reduces overall size, simplifies installation and routing of antenna cables, and provides a streamlined appearance.
Electrically, this means that the isolation between the antennas can be measured and specified in advance, minimizing unexpected or unforeseen interaction problems. For the 1-2309605-1 M2M MiMo LTE dual antenna, the isolation is at least 15 dB and gradually increases towards the center of the two frequency bands served by the unit (Figure 5).
Figure 5: Isolation (y-axis, dB) between the two antennas within the 2309605-1 M2M MiMo LTE Dual Antenna Module is 15 dB or more, measured as a function of frequency (x-axis, MHz). (Image source: TE Connectivity)
Active receiving antenna function
In addition to the two frequency bands covered by the 1-2309605-1 dual antenna, many applications, such as asset tracking, require the reception of signals from GPS (US), Galileo (Europe) and Beidou (China) GNSS systems to obtain position or time information. To simplify this task and avoid the need for another external discrete antenna, TE provides 1-2309646-1. This product adds a third antenna in addition to the two antennas of the dual antenna unit, and is only used to receive GNSS signals between 1562 and 1612 MHz.
However, the need to receive GNSS signals presents another challenge for system designers, namely the basic requirements for transmit and receive functions. When used for transmission, the antenna and its feeder are in a determined state. They take a known, controlled, and well-defined signal from the transmitter power amplifier (PA) and radiate it out. There is little to worry about the signal’s internal noise, in-band interference, or out-of-band signals between the PA and the antenna.
Due to the principle of reciprocity that applies to all antennas, the physical antenna used for transmission can also be used for reception. However, the operating conditions for reception are quite different from those for transmission. Since the antenna is trying to capture the signal in the presence of unknown in-frequency or even out-of-band interference and noise, the desired received signal will have many random characteristics and lack certainty.
In addition, the received signal strength is low (between a few microvolts to a few millivolts) and the SNR is also low. For GNSS signals, the received signal power is typically between -127 and -25 dB, and the SNR is typically between 10 and 20 dB relative to one milliwatt (dBm). This weak signal is attenuated due to cable losses between the antenna and the receiver front end, and the SNR is degraded by thermal and other noise that is inevitable in the transmission cable.
For these reasons, the 1-2309646-1 incorporates LNA functionality into its third receive-only GNSS antenna. The LNA provides 42 dB of gain to the GNSS signal, which greatly increases the strength of the received signal. To simplify use, the LNA is powered by a proven stacking technique (3 to 5 volts DC, no more than 20 milliamps (mA)) over the coaxial cable that carries the amplified RF signal.
DC power is supplied through the cable between the receiver unit and the LNB (Figure 6). The DC power to the LNA (V1) is blocked by small series capacitors (C1 and C2), preventing it from reaching the radio head unit (front end). These capacitors allow the amplified RF signal from the antenna (ANT1) to pass to the radio head unit (OUT). At the same time, the amplified RF signal is blocked by series inductors (chokes) L1 and L2 and cannot return to the power supply V1. In this way, the DC power supply to the LNA and the amplified RF from the LNA to the radio head unit can share the same interconnecting coaxial cable.
Figure 6: The DC supply of the antenna LNA can be superimposed on the cable carrying the antenna/LNA output by clever placement of inductors and capacitors to separate and isolate the DC and RF signals at both ends. (Image credit: Electronics Stack Exchange)
establish a physical connection
Any antenna or antenna element assembly needs to be connected and disconnected from the radio front end it serves in a reliable, convenient, and electrically and mechanically safe manner. Additionally, the entire antenna assembly needs to be protected from the environment and easy to install with minimal impact on the mounting surface.
To achieve these goals, the dual-band 1-2309605-1 and tri-band 1-2309646-1 are each equipped with a 3-meter RG-174 coaxial cable terminated with a standard SMA plug (Figure 7) . Therefore, connecting or disconnecting one or more antennas is simple and can be easily done not only during system assembly in the factory, but also in the field as an add-on component.
Figure 7: Each antenna within 1-2309605-1 and 1-2309646-1 has its own RG-174 coaxial cable terminated with SMA plugs to simplify installation, connection, testing and removal if required. (Image source: TE Connectivity)
Additionally, multi-antenna modules can be easily secured to system surfaces by using a single internal 18mm mounting post, plus acrylic pads on the bottom edge of the antenna housing. The antenna installs quickly, with no exposed mounts to rust, loosen or mis-tighten.
The housings of these antennas are optimized for mobile, high-speed motion applications. The streamlined unit is only 45 mm wide and 150 mm long, with rounded edges (similar to the ‘shark fins’ on a car roof) to minimise drag coefficient and wind resistance. In addition, the shell is made of UV-stabilized material, which ensures that exposure to sunlight does not weaken the shell’s performance over time.
Mobile, high-speed, multi-band wireless connectivity for transportation applications requires antenna assemblies capable of meeting stringent electrical, environmental, and mechanical objectives. TE Connectivity’s dual- and triple-antenna modules offer low-band, high-band, and optional GNSS-band antennas, the latter with an internal LNA. These products feature individual coaxial cables and connectors for each antenna and feature a simple surface or panel mount layout for easy installation and critical environmental durability.