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A Bird-Cage Coil at 127.74 MHz

A Bird-Cage Coil at 127.74 MHz. The bird-cage geometry: 8-rungs Each rung split in 2 with (potential) tuning capacitors in between. Each flap represents a simple circuit or port element such as a 50 Ohm feed or a tuning capacitor.

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A Bird-Cage Coil at 127.74 MHz

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  1. A Bird-Cage Coil at 127.74 MHz

  2. The bird-cage geometry: 8-rungs Each rung split in 2 with (potential) tuning capacitors in between

  3. Each flap represents a simple circuit or port element such as a 50 Ohm feed or a tuning capacitor. (Standard finite element modeling approach for RF and Microwave components)

  4. The bird-cage geometry as seen in the COMSOL user interface.

  5. A section of surrounding air is added by enclosing the bird-cage geometry in a sphere.

  6. The only material property is air (permittivity=1, conductivity=0, and permeability=1)

  7. The sphere has a Scattering boundary conditions = non-reflecting. A refined model would, in addition, add a Perfectly Matched Layer for more precise absorption.

  8. The copper stripes are modeled as perfect electric conductors. This is the simplest possible model and neglects losses but includes field discontinuity. Several more advanced models are possible in COMSOL: - transition boundary conditions (simple field jump discontinuity) - slit boundary conditions (double field jump)

  9. Input Port #1, 5000 V and 50 Ohm

  10. Input Port #2, 5000 V and 50 Ohm Phase shift: 90 degrees Can also enter phase in radians, without giving units.

  11. Tuning Elements Here capacitances (impedances/reactances) at 2.2 pF Modeled as so called Port Elements

  12. Frequency input This parameter (and any other) can be swept

  13. Finite element mesh: 1.4M Degrees of Freedom (dofs) 230k elements Second-order vector finite elements Solution time: ~15 minutes on a PC with 4 cores (64 bit OS) ~ 24 minutes on a PC with 2 cores (64 bit OS) Memory used: ~3.5 GB

  14. Cross-section of the mesh

  15. Mesh on copper stripes

  16. Log10 plot of |B| Arrows: B-field, normalized

  17. Z_in_1=50 Ohm pure resistive Z_in_2=50 Ohm pure resistive Feed at 1 2 is listening S-parameter: S11 (non-db scale)

  18. Z_in_1=50 Ohm pure resistive Z_in_2=50 Ohm pure resistive Feed at 1 2 is listening S-parameter: S21 (db scale)

  19. Z_in_1=50-jX(2.2pF) Ohm Z_in_2=50-jX(2.2pF) Ohm Feed at 1 2 is listening S-parameter: S11 (db scale)

  20. Z_in_1=50-jX(2.2pF) Ohm Z_in_2=50-jX(2.2pF) Ohm Feed at 1 2 is listening S-parameter: S21 (db scale)

  21. Log10 plot of surface current density

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