Coil design and air box in HFSS

Hello, I want to design coils in HFSS. A transmitting coil and a recieving coil for wireless power transfer and see the radiation pattern. My frequency is 450 KHz. I am doing this in ansoft, so I have to have an airbox right? But lamda for my case is 666m. lamda/4 is 666/4m which should be the air box size then. But my coil is only about 20 mm diameter. So can I violate the airbox condition for my simulation. Since the airbox length is very large. What can I do to solve my problem. Please advise.

I read somewhere in the discusson of antennas by some person that "Air box size has large effect on return loss, but small effect on radiation pattern". I need to check the radiation pattern and there is a hetrogeneous human body tissue model also involved. Will the size affect my simulation results considerably. Please help.

I would ask Ansys for advice if you don't get any good response here.

But my gut feeling is that a much smaller airbox will have no appreciate effect on S11 in your case. These antennas are very inefficient, so a large fraction of the power absorbed by the antenna will not be radiated. So the reflections from the airbox will be less probabematic than for "normal" antennas.

You could try an optimetics analysis and see the effect of varying the airbox size. I would suggest if you made it 10x the largest dimension of your antenna, that would be sufficient. But you can try this.

I have no idea how much effect the size of the airbox would have on calculating radiation pattern. Usually it is small as you note, but I've no idea if the algorithms would break down when the wavelength is very much larger than the size of the surface you integrate on.

It is possible that perhaps using the FE-BI technique would help here

http://www.ansys.com/staticassets/AN...GM_UK_2011.pdf

though I have never used it myself. It's used when you have large regions where you don't care what the fields are. So you use a finite element mesh around the antenna, but don't extend that mesh out a long way.

This is an interesting problem. If you have an HF-IE license, I would investigate the use of FE-BI. But if not, I have no idea how to proceed. But I really doubt you need an aribox much larger than 10x your antenna for the effect on S11 to be negligable.

DeboraHarry is right :) as your coil is a poor radiator, the radiation boundary can (and should) be closer. The lambda/4 condition is for antennas that are considered good radiators. The easy way to check this is the following:

1) Simulate your coil with a given airbox.
2) plot magE in a plane that cuts through the antenna and the whole aribox.
3) If the fields have decreased by ~3 orders of magnitude at the airbox faces, then your airbox is good to go!
4) If the fields are greater than this, there may be sufficient energy reflected by the radiation boundary to decrease accuracy as the reflected fields will couple back into the port causing a worse return loss that is real

DeboraHarrys suggestion of a parameterized airbox is also a great idea using the 'region' object to create the airbox!

Have Fun!

hi

I am working on similar topic as you are, my work is design of four coil resonator for wireless power transmission. but i am confused how to provide the excitation for the helical structure, I have a source coil-1 turn, transmitter coil 4-turns, receiver coil 4-turns and a load coil 1-turn.
operating frequency is 13.56MHz, coil diameter = 20cm, thickness 5mm, pitch is 10mm.

could u please help where to assign the ports and what type lumped or wave port, (4 ports for the system since 4 coils) how to calculate the coupling coefficient

I have given the size of the air box 4 times that of the coil size for assigning the radiation boundary.

thanks in advance