fix impedance calculation for lossy transmission lines

[dmarek-flex]

Realized that the new methods I implemented for calculating impedance would not work for lossy transmission lines because flux is returned as a real number. I also improved a bit of the sign handling when calculating impedance.

Will add section to flexcompute/tidy3d-notebooks#297 comparing the calculations for a lossy transmission line.

Here is a plot of the different methods for computing impedance:

[github-actions]

Diff Coverage

Diff: origin/develop...HEAD, staged and unstaged changes

• tidy3d/components/data/monitor_data.py (100%)
• tidy3d/plugins/microwave/impedance_calculator.py (96.0%): Missing lines 80

Summary

• Total: 35 lines
• Missing: 1 line
• Coverage: 97%

tidy3d/plugins/microwave/impedance_calculator.py

  76         # We explicitly take the real part, in case Bloch BCs were used in the simulation.
  77         flux_sign = 1.0
  78         # Determine flux sign
  79         if isinstance(em_field.monitor, ModeSolverMonitor):
! 80             flux_sign = 1 if em_field.monitor.direction == "+" else -1
  81         if isinstance(em_field.monitor, ModeMonitor):
  82             flux_sign = 1 if em_field.monitor.store_fields_direction == "+" else -1
  83 
  84         if self.voltage_integral is None:

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