Pulse Electronics Mobile Division produces compact antennas for mobile communications and networking. Mobile antennas need to function in complex and mechanically limited environments, and so most antennas used today are specially designed and customer-specific.
The antenna is one of the first electromechanical components considered in a new product concept design. In the past, most of the R&D work was done in the laboratory, with the engineers constructing and testing different antenna designs for customers’ products. While this is still a good approach for single antenna systems, the introduction of LTE diversity schemes and other radio systems such as Wi-Fi and GPS to current smartphones make reliable prototype evaluation very challenging.
Antenna prototypes typically include the device ground, PCBs, batteries, covers and any other large parts. Obviously, early prototypes seldom include any active transceivers, and so each antenna must be driven from an external coaxial cable. A typical LTE smartphone, with its main and diversity antennas, GPS and GLONASS systems and 2.4 GHz and 5 GHz WLAN capabilities, can need 7 or 8 cables to measure all the components at once. These cables would occupy too much of the volume of the prototypes, and severely distort the evaluation results. With electromagnetic simulation, the performance of a complex device can be calculated without worrying about these cable effects....
An example of an antenna product designed using only CST MICROWAVE STUDIO® (CST MWS) is shown in Figure 1.
Figure 1 LTE antenna audio-module, from simulation to mass production.
Extensive Simulation Capability
To improve the accuracy of designs and reduce products’ time-to-market, Pulse Electronics has developed an extensive antenna simulation capability with CST MWS. Pulse has been working with CST software for over a decade, beginning with the very first releases of CST MWS.
The rapid increase in computation capacity over recent years has allowed the use of simulation in everyday design work. Thanks to the support for GPU acceleration in CST MWS, antenna design and simulation can be done with detailed, accurate CAD models which can then be put directly into mass production. Because CST Studio Suite is the industry standard for antenna and electromagnetic simulations, over the development cycle, RF designers can share their models and results easily and efficiently with other engineers using CST Studio Suite project files.
Pulse has been in antenna manufacturing for many years, and over this time they have tested and verified the electrical properties of hundreds of materials used in today’s consumer electronics. This knowledge, combined with the extensive material database available in CST MWS, makes it possible to build very accurate simulation models. Simulation also means product performance can be estimated without having to build any prototypes, so the antenna workflow can include manufacturing-process-related design rules as early as possible.
Benefits of Simulation over Measurement
The over-the-air performance of mobile devices is traditionally evaluated by measuring free-space far-field radiation patterns and efficiency in an anechoic chamber. This is still a valid method for comparing different designs, since free-space measurement repeatability is usually very good, even between chambers.
However, the current industry trend is towards carrying out performance evaluations using phantoms to measure the effect of different phone positions, such as with the antenna next to the head, or held between the head and the hand. By comparing the radiation patterns and antenna efficiency from different phantom studies, the engineer can determine the best design for the product.
With a typical cable-fed prototype, the variation between measurements in the same chamber can exceed 1 dB due to difficulties in positioning the prototype. Carrying out simulations with CST MWS, instead of just using experimental measurements, means that this variation can be eliminated.
As shown in Figure 2 the simulated and measured phantom absorption loss correlate well within the given manufacturing tolerances of the phantoms. Thanks to the simple import functions in CST MWS, the phantom’s 3D, mesh and material properties can be copied from one project to another easily, keeping the exact same co-ordinate system. This allows the comparison of competing antenna designs for products in a matter of just hours, instead of the days it takes to build and measure reliable prototypes.
Figure 2a Simulated (solid) and measured (dashed) phantom loss, calculated from radiation efficiency, for the antenna in the lower GSM bands.
Figure 2b Simulated (solid) and measured (dashed) phantom loss for the same device in the higher GSM bands.
Cable-free ECC Evaluation
The envelope correlation coefficient (ECC) is a value used to compare the far-field radiation patterns of LTE system antennas. For lower frequency bands, the ECC is one of the key design parameters. On these frequency bands, the antenna radiation pattern is mainly defined by the antenna element’s coupling to the ground plane.
With cable-fed models, the coaxial cable coat is connected to the antenna ground plane, making the cables part of the ground plane structure. This change the ground plane’s electrical length and can affect the coupling between the radiator and the ground plane, and therefore cable placement can also have a significant effect on the far-field radiation pattern.
The graph in Figure 3 shows how different cable placements can change the ECC, using simulation to compare these with a model of the antenna in use without any cables. The presence of cables significantly decreases the measured ECC, leading to wildly optimistic results in cases where the antenna has a strong coupling to the ground plane. In the worst case, such measurements can lead to incorrect antenna placement when constructing the device. This means that in most cases, cable-fed prototypes cannot be used for reliable ECC performance evaluation. The only way to avoid these problems before building functional devices is to use simulation.
Figure 3 Simulated ECC for an antenna with different coax cable placements on ground plane, compared to real ground plane shape without cables.
EasyTtransfer to 3D mass Production
The modern smartphone market is dominated by large touch-screen displays. The twin demands of sleek, distinctive industrial design and large high-capacity batteries mean that the space left for antennas is very limited. To take full advantage of all the space available, antenna manufacturing technologies have moved from 2.5D radiators to full 3D, using either printed or plated conductors on a dielectric carrier.
CST MWS is well-suited to the 3D antenna manufacturing workflow. Because of its powerful import/export tools, the antenna radiator pattern created in CST MWS can be transferred into the manufacturing process with very little additional CAD work. Typically, just one prototyping round is needed, finetuning the design to take into account manufacturing considerations before starting mass production.