Radio Frequency Identification Systems (RFID) are nowadays widely used for a variety of applications in the area of authentication, ticketing, access control, supply management, parking, payment, vending,etc. The example presented here shows how CST MICROWAVE STUDIO® (CST MWS) and F-solver was used by UPM Raflatac, Finland (http://www.upmraflatac.com) to design an RFID UHF antenna for paper industry paper reel tracking purposes.
A paper reel core provides many challenges for the antenna design process as the radiation characteristics of the tag antenna will be influenced by the range of different dielectric materials used. Industry requirement was to be able to identify the paper roll despite its orientation around its winding axis. In order to evaluate the different effects of the enviroment to the antenna characteristics, such as frequency detuning, narrowing of operational bandwidth, and attenuation of signal strength, it was necessary to be able to use the sophisticated modeling and simulation tools that CST MWS provides. The most challenging aspect of this application was the handling of the bending of RFID antenna tag around the reel core structure to evaluate the real life radiation pattern of the antenna. CST MWS bending functionality made this part of the structure construction very straightforward, as shown in Figure 2....
To tackle the design problems, a Bowtie antenna topology was chosen to maximise the operating bandwidth, RCS, and material sensitivity. Similarly, an IC with a good enough sensitivity and memory capacity to fulfil the industry need was required. CST MWS optimization and postprocessing capabilities were a great help, as the farfield radiation values could be computed automatically during simulation runs and used as optimization goals. The simulation model was based on real-life dimensions of the paper reel, making the electrical size of the structure very large. This posed a challenge for the tetrahedral meshing accuracy and efficiency due to the very small geometric details of the antenna, since the memory requirements of the simulation were considerable. The advanced modeling features of CST MWS were also needed, as the radiation pattern simulation accuracy was greatly improved by bending the antenna layout around the reel core.
Figure 3 above shows the S11 parameter result from CST DESIGN STUDIO™ simulation with the IC added. The resonance frequency and return loss values match the design goals very well. Below in the Figure 4 the surface current distribution is shown. The surface current distribution shows strong peaks at the edges of the antenna near the discrete port. Due to this, it was necessary to have a relatively fine mesh around these areas to obtain accurate solution for the surface currents.
The radiation pattern, shown below in the Figures 5 and 6, has a very uniform behavior, which was also one of the design goals. The direction of the farfield main lobe is 106 degrees, and the width of the - 3dB beam 193 degrees, as is seen in the polar plot. The 3D radiation pattern shows the main and side lobe maximums around the winding axis. Radiation pattern minimum is directed to 180 degrees, which is through the core twice and once through the paper.