Spiral inductor simulation in HFSS (Near field, Low Frequency ~1MHz)- Airbox?

[rieuwa]

Hi all,

Thanks to this forum, I've been able to get started in HFSS and obtained some working simulations of a copper planar spiral inductor on polyimide.

I am simulating it in order to extract its inductance, series resistance and parasitic capacitance, for a near-field coupling application at frequencies ~1MHz. I have built inductors almost identical to my HFSS design and as such have a baseline value for what I believe these values should be (~8uH, Rs=1.5Ohms), with these values closely matching 'theoretical' calculations. I would really like to be able to corroborate these in simulation.

My question relates to the choice of airbox size- having been through many of the previous posts on the topic, the usual recommendation seems to be lamda/10 to lamda/4. Of course at freqs of 1MHz, this would result in an enormous airbox compared to my inductor. I have tried a wide range of sizes, and either the simulation never finishes (Execution Failure on Server usually) or the values are way off from measured ones.

However, if I run the simulation with an airbox which is just very slightly larger than the antenna size, the simulated values become very close to the measured ones.

Would appreciate any insights/suggestions from the community on my approach.

Many thanks!

rieuwa

My HFSS design: View attachment SpiralInductor15turnsNearField.zip

 

[volker_muehlhaus]

However, if I run the simulation with an airbox which is just very slightly larger than the antenna size, the simulated values become very close to the measured ones.

Makes perfect sense.

These low frequency "antenna" tags have negligible far field radiation. They work with inductive coupling in the near field only, and that's indeed different from normal antennas. For "normal" antennas with significant far field radiation, the recommended distance to the absorbing boundary depends on wavelength. For your inductor, fields decay quickly, as you can visualize by plotting the H field. For that reason, the airbox can be really small, regardless of the large wavelength.

 

[DeboraHarry]

For "normal" antennas, the distance to the airbox should be at least lambda/4, so lambda/10 would be bad. The only exception appears to be in directions where you know the radiation will be small. In this case, you do have an exception, as the radiation is small from your structure.

Deborah