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vor 8 Jahren

7-2015

  • Text
  • Technik
  • Radio
  • Oszillatoren
  • Quarze
  • Emv
  • Messtechnik
  • Bauelemente
  • Antenna
  • Amplifier
  • Wireless
  • Components
  • Frequenz
  • Mikrowellentechnik
Fachzeitschrift für Hochfrequenz- und Mikrowellentechnik

RF & Wireless (Figure

RF & Wireless (Figure 9), so that the antenna pattern with the proper loads at the SMD terminals is included. After simulating the schematic and EM structure, the 3D antenna pattern can be visualized as shown in Figure 10. The drop in radiation in the PCB plane isn’t real – this is an artifact from the Method-of-Moments (MoM) simulation method used by AXIEM, which simulates with substrates of infinite size. These infinite substrates cause a drop in simulated gain at the horizon (theta=90°). The true pattern shape was continuous at the horizon, with no such drop. The antenna peak gain was approximately -14 dBi with a very nice omnidirectional antenna shape. Besides the 3D pattern that gave a good overview of the radiation characteristics, the “classical” antenna parameters in 2D polar format were also plotted. Figure 11 shows the result for the gain (total over all polarizations) at theta=80°, or 10° above the PCB plane. A reasonable question is how much the finite PCB size influences the results obtained with an infinite PCB model. Analyst Simulation The PCB edge was relatively near the antenna where the relative permittivity changed from 4.5 (FR4) to 1.0 (air). It was expected that the measured antenna resonance frequency would be slightly above the resonance that was simulated with AXIEM (infinite dielectrics), because the antenna in reality would see slightly lower effective permittivity. With the Analyst 3D FEM solver, the finite PCB size could be included in the simulation. Analyst solves a more general problem, so that it takes more time and usually more memory. While the designer wanted to avoid heavy 3D simulations for planar antennas and could have used the AXIEM solution, which is much faster with adequate underlying assumptions, he went ahead and used Analyst for the PCB antenna comparison. Figure 14. The material definition was mapped in the PCB location and assigned to drawing layer “BoardRoute.” Figure 15. A finite size FR4 dielectric for the PCB in the Analyst 3D model. The EM stackup was modified for the finite substrate calculation: the infinite FR4 dielectric was changed to air and FR4 was only selectively inserted by drawing a PCB rectangle that was properly mapped, as shown in Figure 12. A material definition named “FinitePCB” was created with 1 mm thickness and material FR4 (Figure 13). It was then mapped in the PCB location and assigned to the drawing layer “BoardRoute” (Figure 14). The resulting finite PCB model is shown in Figure 15. The simulation boundaries (sidewalls as well as top/bottom) were placed at a minimum distance of ~ 1/4 wavelength. To calculate antenna patterns in Analyst 3D FEM, it is imperative to set the boundaries to Per- fectly Matched Layer (PML). PML increases the problem size, and thus the memory consumption and simulation time, but it is by all means better for antennas because it more accurately represents free space radiation. Because Analyst doesn’t support the internal port that was used for AXIEM, it must be changed to a differential port with separate (+) and (-) pins (Figure 16). After making that change, the EM model was simulated. The schematic for connecting the SMD was similar to the AXIEM schematic shown earlier in this application note in Figure 5. After running the simulation, the S11 results for AXIEM (infinite PCB) and Analyst (finite PCB) were compared, as shown in Figure 17. The result was as expected: with the finite PCB size, the effective dielectric constant was slightly smaller and the antenna resonance went up. However, the effect wasn’t too bad and was easily compensated for by tweaking the series capacitor value. This tweak was pure circuit simulation, so it wasn’t necessary to repeat the time-consuming EM simulation. What about the antenna pattern? Earlier it was noted that the drop in the gain at the horizon isn’t real, but is actually caused by the AXIEM infinite substrate. Figure 18 shows what the designer obtained from the finite pattern simulation in Analyst and how the patterns compared. Left is the infinite substrate (AXIEM) and right is the finite substrate (Analyst). The plots were done 58 hf-praxis 7/2015

RF & Wireless Figure 16. Analyst doesn’t support the internal port used in AXIEM, so it must be changed to a differential port with separate (+) and (-) pins. Figure 17. S11 results for AXIEM (infinite PCB) and Analyst (finite PCB) are compared. Figure 18. Comparison of the infinite substrate model in AXIEM (left) and the finite substrate model in Analyst (right). Figure 19. Comparison of the AXIEM (blue) and Analyst (magenta) 2D antenna patterns. with linear scale (not dB) for better comparison. Finally, the designer compared the 2D patterns. The antenna pattern and gain calculated by AXIEM with the infinite substrate was similar to the Analyst 3D FEM results, except for data at the horizon (Figure 19). Knowing that this drop at the horizon isn’t real for this antenna type, it was simply ignored and the analysis confirmed that AXIEM could reliably be used for efficient and fast design of the PCB antennas. Conclusion Radiation properties of PCB antennas modeled either with AXIEM or Analyst EM simulators can be obtained from a cosimulation with a driving schematic.This co-simulation takes into account losses in the circuit components, as well as the radiation efficiency of the antenna itself. Likewise, the radiation pattern is influenced by the discrete components and correctly taken into account in the pattern calculation. Comparison of the 3D Analyst model of a finite-sized PCB and the AXIEM model of an infinite PCB reveals that the influence of the finite PCB size has insignificant influence on the radiation pattern of the antenna. It does have a slight impact on the resonant frequency, but that is easily corrected by fine-tuning the matching components. This justifies the use of the much faster AXIEM for the analysis of planar antennas. ◄ Note of thanks: AWR Group, NI would like to thank Dr. Ing. Volker Mühlhaus, Dr. Mühlhaus Consulting & Software GmbH, for his contributions to this application note. www.muehlhaus.com The 868 MHz antenna design in this example was created by consultant Lutz Konstroffer (www.rfconsult.com) and is used with permission. hf-praxis 7/2015 59

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