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vor 7 Jahren

4-2017

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Fachzeitschrift für Hochfrequenz- und Mikrowellentechnik

RF & Wireless Antennas

RF & Wireless Antennas Synthesis of Robust UHF RFID Antennas on Dielectric Substrates Figure 1: UHF RFID tag and environment Figure 2: Setting dielectric values in band control AntSyn, a new antenna synthesis tool within the NI AWR software portfolio, has been enhanced to rapidly explore the design space more efficiently, supporting the simultaneous optimization of RFID antennas on a wide variety of dielectric substrates as specified by the user. This article will discuss the methods used for optimization and describe two examples of RFID antennas created using this technology. UHF RFID Tag Antenna Design Challenges and electromagnetic (EM) tools generally offer limited ability to explore the design space beyond modest tweaking of the antenna’s geometry through parameterization. Furthermore, limited design space optimization is particularly restrictive when making the antenna environmentally robust. Antenna synthesis has proven to be very effective at creating antennas for a wide variety of applications and has now been applied to this challenging problem. An important challenge in the design and integration of UHF RFID tag antennas in the real world is the difficulty of making them environmentally immune to the mounting platform, particularly if they will be installed over a dielectric, since the underlying dielectric properties are likely to be highly variable. A single tag design may need to be installed, for instance, on cardboard, drywall, plastic, fiberglass, wood, or other dielectrics [1], as illustrated in Figure 1. Placing a tag on different dielectrics will shift its resonant frequency. If it is sufficiently wideband, the tag will still have good performance as its resonant frequency shifts. Reducing the antenna footprint is another challenge for designers. Tags of λ/3 or less can be used in many more situations and cost much less. Therefore, it is desirable that tags be as electrically small as possible, however, smaller antennas necessarily have smaller bandwidth than larger antennas [2]. While Derek S Linden Derek.Linden@ni.com Jennifer Rayno Jennifer.Rayno@ni.com AWR Group, NI Ultra-high frequency (UHF) radio-frequency identification (RFID) tag antennas need to be inexpensive, efficient, and robust for the installation environment (immune to change in its electrical behavior due to its proximity to the mounting platform). Designing and optimizing such antennas by hand is a timeconsuming and difficult process, Figure 3: Example of original planar XYmesh antenna with dielectric on ground plane 82 hf-praxis 4/2017

RF & Wireless Figure 4: New antenna type with a single-layer antenna on a dielectric (green rectangle) that simulates its installation environment. The red dot indicates the chip position Figure 5: Specifications used to synthesize RFID antenna across four different substrates. The impedance used for each band was to provide a conjugate-match to a chip with 16 – j148 Ω impedance [4] a very large tag antenna would easily be able to be wideband and thus very robust, it would only be able to be installed on relatively large objects and it would be higher in cost than a small tag. While tags of λ/3 or less can be used in many more situations and cost much less, when a tag is small compared to wavelength, bandwidth narrows and thus it is much more sensitive and difficult to design. In addition, while antennas are usually designed to match a real (typically 50 Ω) standard line impedance, the RFID chips themselves are generally not 50 Ω devices and have reactive impedances. A typical value for a chip impedance might be 16 - j150 Ω [3]. To minimize reflection losses, it is desirable to design the antenna impedance to be a conjugate match of the RFID chip’s complex impedance directly so a matching network will not be necessary. Direct antenna-to-chip matching will significantly decrease the cost and complexity, and improve the overall reliability. However, this non-standard impedance matching makes the design challenge even more complex. Various approaches have been used in prior research to meet Figure 6: Design 1, where the red dot indicates chip location. The antenna’s dimensions are 94.4 x 23.7 mm, while the substrate overall is 162.2 x 91.5 x 2 mm these challenges [3 - 6]. The most commonly used technique is making an antenna broadband to enable performance to be maintained over a set of substrates, which will shift the resonant frequency. In [4], a combination of equations and simulations are used to manually optimize existing commercial antennas to have good performance over a wide range of materials. Broadband performance is achieved in [5] by combining a small inductive coil with a planar dipoletype antenna, whereas [6] uses a complex design with multiple arrays of planar inverted-F antennas (PIFAs). Another method is making the antenna easily tunable to the specific dielectric by manually trimming in preset locations [3]. All of these approaches have used standard human-in-the-loop engineering methods. This article discusses a new method of designing RFID antennas over a wide range of substrates using automated synthesis. A New Design Approach In this new design approach, AntSyn was used to create new RFID antennas on a variety of substrates. AntSyn uses evolutionary algorithms (EAs), a programmatic method that leverages EM simulations to efficiently explore the design space and automatically locate high-performance design options. Antenna synthesis with AntSyn is proving to be highly effective at generating antenna structures with excellent performance and has already been used to create many successful fielded antennas, including several that have been used on spacecraft [7]. AntSyn allows the user to enter RF and form-factor specifications, such as bands, patterns, efficiency, geometry constraints, and more. It has a library of design templates and uses fullwave 3D simulation [8, 9] to obtain performance information on candidate designs. Advanced optimization algorithms are used hf-praxis 4/2017 83

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