RF coil decoupling

Altair Forum User

Hello,

 

I've been modelling a two coil (built from copper tape) RF transmit array with a phantom for MRI tuned to 63.87 MHz, but I've been having difficulty decoupling the two coils via overlapping. The best I've achieved is S12 < -10 dB and I'm not sure why I cannot get any better while the individual coils are tuned (Picture attached of S-parameters). I'm now trying to add more mesh elements and see how that affects the results.

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Altair Forum User

Hi @bcoles,

 

I didn't look at your model in detail but can give a few general comments:

1. At your design frequency, the free space wavelength is about 4.7m.  The coils in your model have dimensions <20cm, which makes them electrically quite small.  In these cases, the default mesh sizes might not be sufficient and you'll need to do a mesh convergence study.  Maybe have a look at error estimates and adaptive mesh refinement.  A final note on meshes is that ports are typically quite sensitive; make extra sure that those are fine enough.
2. When you are looking at S-parameters (here e.g. S12), you are looking at 'how much of the power given to port 1 is reaching port 2'.  This implies that some of the power can get diverted elsewhere.  With a well-matched input port (as in your case) we know that power lost due to reflections is quite small.  However, you still need to consider the power that is dissipated by material losses, power absorbed by other ports or power that gets radiated away.

Basically, once you know that you can trust your mesh, try to find all of the power sinks in your model.  This won't necessarily improve S12 but at least you'll know where the power is going.

 

Regards,

Andries

Altair Forum User

Hello,

 

Indeed it is not a trivial problem to decouple coils as you are trying. Some tips that might help further:

1) to improve the simulation time - or at least in the first step - try modeling the phantom as a homogenous half space

2) you may need to increase the mesh density of the coils in the area where they overlap where the coupling occurs

3) try running an optimization to minimize the S12 while moving the relative position of the 2 coils wrt one another and if possible change some of geometry parameters of the coils.

 

Hopefully these steps should further reduce the coupling between the 2 coils.