Power transfer between two coils
Posted 2025/05/27 0:29 JST Electromagnetics, RF & Microwave Engineering 9 Replies
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Hello,
I am trying to quantify the power transfer between two coils. I added lumped ports to both of these coils, and only turned wave excitation on for one of them. I am trying to get the S21 parameter, but it is always negative, and has a magnitude greater than 1. Does anyone know why I am getting these results, or how I could go about finding my power transfer efficiency?
Thanks
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I wouldn't use a perfectly conducting boundary for the sphere. Use a scattering boundary. I also suggest you mesh the space locally-around your coils more finely.
Finally, I noticed that it computes emw.S21dB = -26.623 dB. This is a number expressed in dB. Its magnitude = 10^(-26.623/10) = .002176 . Needless to say, that is less than 1.
-------------------Scientific Applications & Research Associates (SARA) Inc.
www.comsol.com/partners-consultants/certified-consultants/sara
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Thanks for the reply.
I am now wondering why my efficiency is so low, being only about 0.2%, when I am expecting a number in the high 90s. I suspect something about my simulation is not set up correctly. Do you have any idea why this might be happening?
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Well, that looks a lot like an air core transformer with minimal flux linkage, to me.
-------------------Scientific Applications & Research Associates (SARA) Inc.
www.comsol.com/partners-consultants/certified-consultants/sara
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Yes the flux linkage seems to be small. Additionally you are using 50 Ohm ports and the coils probably have very different and complex impedance. This also affects the S21 result. You should match the coils properly.
-------------------Edgar J. Kaiser
emPhys Physical Technology
www.emphys.com
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Hi, thanks again for the replies.
It is not clear to me how I should go about impedance matching the coils. I can print out the impedance from the global evaluation, giving me 0.61221+144.57i for the first lumped port, and -50 for the second lumped port. I am not sure what to do with this information to accurately impedance match the coils.
Any advice would be greatly appreciated
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You use exactly this complex value for the reference impedance of the first port. You must excite the second port to get the respective impedance for it.
In any real world application you must add a matching network to transform the coil impedances to a suitable real impedance such as 50 Ohm without reactive component.
Keep in mind that the coil impedances are frequency dependent.
-------------------Edgar J. Kaiser
emPhys Physical Technology
www.emphys.com
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When I change the impedance of one lumped port, it changes the impedance of the other, so I'm not sure how I can know what one should be without knowing what the other should be, I've attached the results of sweeping through the impedance of one of the lumped ports.
Also, I wanted to ask what the physical meaning of giving an impedance to the lumped port is? Is this adding an impedance in series to the rest of my coil? I am quite confused.
Thanks again
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Sure, the two coils interact and will mutually influence their impedance. Regarding the meaning of impedance I recommend some textbook study. Get yourself a book with a focus on electrodynamics and RF networks. This stuff is fundamental to what your model is about and it is a little beyond the scope of this forum.
-------------------Edgar J. Kaiser
emPhys Physical Technology
www.emphys.com
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Hi, I have tried something else now, and I am getting conflicting results.
I instead printed out the powers of the two lumped ports, and take the ratio of those to get the power transfer efficiency. I am getting a negative power on the second lumped port. Is that because it is receiving power as opposed to outputting it?
Also my S21 value is ~-40 dB, and still indicates that very little of the signal from port 1 is reaching port 2. This conflicts with the powers that I am seeing, which says ~90% of the power is being transferred. Am I mistaken in my interpretation of one of these values?
I have attached an updated model, with an image of the numbers I got from them.
Any input or advice is greatly appreciated Thanks
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