Antenna technology Mobile HF performance tests for complex antennas in vehicles
Performance tests of automotive antennas are a complex measurement task. Networked vehicle functions, broadband services and higher frequencies push conventional RF measurement technology to its limits.
In the early 1970s, a chrome-plated telescopic antenna mounted prominently on the left front fender of an Opel Rekord C was sufficient. Their only task was to provide the car radio on board with a more or less interference-free reception of one of the local radio stations via VHF between 87.5 and 108.0 MHz. But these times are - almost nostalgia - probably finally passé.
The history of automotive antennas at Hirschmann (now TE Connectivity) began in 1939 when the company presented its first automotive antenna in Berlin. As a result, the supplier of telescopic antennas enjoyed an excellent reputation in the industry, not least due to its Auta 6000 series. Compared to this, antennas of current vehicle generations have to perform significantly better.
The developers are focusing on modules that support more broadband services than VHF and combined transmission/reception units. In addition, much higher frequencies come into play to generate as a large bandwidth as possible for faster transmission of ever-larger amounts of data. In connection with antennas for mobile use and the new mobile radio standard 5G, frequency bands up to 6 GHz are currently relevant. TE Connectivity develops the entire portfolio of antennas for the automotive industry at the Neckartenzlingen site. The Swiss technology group offers several solutions for each service and adapts them individually to the requirements of its customers. The result is an antenna tailored to each vehicle model.
Complex vehicle functions require better antennas
The increasing networking of vehicle functions, offboard applications and innovations in infotainment, Internet connectivity and mobile telephony are leading to a drastic increase in the complexity of current architectures. And as one of their key components, they are based on powerful antennas. More precisely, on different types of antennas with very specific functions. These include services such as AM, FM and DAB radio, mobile radio, WLAN and Bluetooth for coupling individual devices on board as well as GNSS (Global Navigation Satellite System) and Car2X communication.
The latter uses radio links for data transfer between vehicles and between vehicles and their transport infrastructure for the safe, efficient and intelligent mobility of tomorrow. New times have long since dawned for the quality assurance of modern automotive antennas. This is because deficits in terms of reliability or reception or transmission quality today have far more serious consequences than simply the interruption of a captivating broadcast.
With the complexity of current automotive antennas, the demands on their mandatory real performance tests are also growing. All modules must function reliably in their real installation situation on the road. Therefore, after comprehensive simulations and stationary laboratory tests, for example in EMC measuring chambers and in radomes, a protected antenna dome, there is no way around final mobile tests on the road. Last year, the supplier's development engineers became aware of a new development from Narda Safety Test Solutions regarding the specific requirements for their mobile applications: the SignalShark. This is a handheld signal analyzer for real-time.
Test description for antenna patterns
In the documentation of a measurement, the resulting circle diagram provides TE engineers with valuable information on how well an antenna receives a signal depending on the direction of irradiation in the real installation situation. It shows the antenna gain (G), the instantaneous value of the field strength measured at the antenna output in [dBµV].
In these mobile performance tests, the so-called antenna pattern (pattern) is measured. The pattern gives the developers valuable information about the antenna gain (G). This indicates how well the antenna of a particular model of the vehicle receives a signal depending on the direction of beam input, or in other words, what power actually arrives from the emitted signal. In practice, a vehicle equipped with a variety of test antennas will be driven on a circuit within a wide, reflection-free area.
At the same time, a transmitter (signal generator) emits a signal with a defined power from a distance of approximately 100 m alternately with vertical and horizontal polarization in the direction of the test vehicle. Parallel to this, an electronic gyro sensor and an additional compass record the angle at which the vehicle is illuminated. This gives the TE engineers a pie chart showing the instantaneous value of the received field strength [in dBµV (decibel microvolts) or dBm (decibel milliwatts)] at the antenna output over the corresponding angle of incidence.
Measurements in the open air on a test site present measurement technicians with different challenges than in laboratories. The complete test setup including all connecting elements must be suitable for the situation in the test vehicle and the adverse conditions of a test drive. With a view to precise measurement results, it is imperative that the entire system measures robustly and error-free even when accelerations, decelerations and centrifugal forces are acting and everything in the vehicle wobbles and vibrates. And such tests do not only take place on luxury limousines in dignified surroundings. Tractors also want to be measured today. A combine harvester, for example, is not transported to Neckartenzlingen "just like that". Such an impressive vehicle has to be measured on-site at the manufacturer.
Original and current HF measurement technology
The RF measurement solution used to date meant a large, elaborately wired construction consisting of a measurement receiver rated up to 3 GHz and a separate RF switch, an additional laptop and an external gyro sensor. The safe recording of reliable measurement results with the required reliability was regularly associated with a high technical effort for the TE engineers. When changing vehicles within a test series, for example, all the measuring equipment that had to be transported as a whole had to be dismantled and reassembled each time. The construction had to be professionally wired for the individual measurements on the test site with a lot of time and effort and, if possible, stably accommodated in the test vehicle. Until then, the complex procedure involved in mobile antenna testing was regarded as comparatively complicated and costly in terms of handling and, accordingly, slow and cost-intensive.
Compared to this, the main achievements of the Narda SignalShark, which was used for the first time at TE, were quickly obvious. Literally. As a first consequence, the entire handling of the mobile antenna tests has been significantly streamlined since then. With all the positive consequences. The handheld analyzer, which is equally suitable for mobile and stationary use, has reduced the number of devices and thus the overall cabling effort to a minimum. Its intelligence - apart from the electronic compass - is in the form of a powerful computer integrated in the device. In addition, its four switchable RF inputs eliminate the need for a cumbersome external RF switch. In this way, measurement technicians effectively avoid potential sources of error from the past. Follow-up measurements of further antenna modules of other vehicle models, for example, no longer require the time-consuming and costly replugging of all connecting elements. In addition, significantly fewer connectors are exposed to mechanical loads during operation. The measurements become more robust and less error-prone.
Investigating the interference radiation of a vehicle
With the mobile equipment of the past, frequency ranges beyond 3 GHz were not feasible. Due to the developments and trends in automotive antennas, TE had to expand the frequency range. The SignalShark detects and analyzes, classifies and localizes RF signals between 8 kHz and 8 GHz. It thus opens up new, current and future applications in automotive engineering for the company. Due to its versatility, TE employees use the SignalShark as a spectrum analyzer beyond the pure antenna application. For example, they can easily detect the interference radiation situation of a specific vehicle model.
In an appropriate mode, the useful signal can be analyzed or potential interferers can be detected. This and the maximum mobility of the handheld enable them, for example, to quickly and reliably evaluate modified installation situations from a radiation point of view directly at the customer's site and immediately propose concrete solutions. The best antenna is of no use if it is installed directly next to an auxiliary spotlight whose switching power supply massively disturbs the reception. Useful hints for a suitable installation location for a certain antenna module improve the service performance of TE engineers.
Occasionally capture signals without gaps
With its real-time bandwidth (RTBW) of up to 40 MHz, the SignalShark allows measurement engineers to perform fast measurements. Significantly faster than with conventional spectrum analyzers. This means that the receiver is able to capture even short, sporadically occurring signals in real-time within this 40 MHz without missing a single event. This is ensured by a POI (Probability of Intercept) of 100% for signals with a signal duration greater than 3.125 µs. The RTBW is particularly advantageous in automotive engineering in that today many power supplies in motor vehicles are designed as switching power supplies, and switched processes emit extremely short times and many faults.
With the SignalShark measuring instrument, TE employees can also easily enter their sensitive EMC chambers and carry out measurements and optimizations during operation. The reason for this is the good shielding of the SignalShark. This makes it immune to field strengths of up to 100 V/m. Thus, it is possible to work in the environment of strong field sources without any problems. And good shielding offers radiation protection in both directions so that the reverse conclusion is also permissible: Those who are protected against external field strengths also protect their direct surroundings from their own. The SignalShark can operate as an extremely quiet computer. Because in EMC chambers in which the emissions of test equipment are examined, everything that emits itself is prohibited.
Measuring device controls itself
With the SignalShark, the computer is integrated as a Windows 10 computer. Not only the complete control of the device is disclosed and remote control commands are described, but the measuring device can also control itself remotely. TE offers the possibility to implement own measuring programs easily by the pre-installed programming language Python. The device can be adapted to the developer's tasks. The external compass is supplied with power and read directly from the SignalShark via the USB bus. The reading of the compass and the combination of the compass values with the measured values of the spectrum analysis are done on the SignalShark itself.
This article was first published in German by Elektronik Praxis.