1. HFSS needs airbox to model free space radiation. A radiation boundary is used to emulate free space by truncating infinite free space to finite calculation domain. This minimize reflections from outer surfaces and ensures maximum absorption. This is very similar to an anechoic chamber.

2. For this, the distance from the radiator must be a minimum of λ/4 (or λ/2) for strong radiating surfaces. Here λ represents the wavelength for the minimum frequency used.

3. A distance of λ/4 from the radiating surface is taken for the reason that similar impedance mismatches can be cancelled at quarter wavelengths. This ensures that there are no reflected waves and thus the radiation boundary absorbs all the radiation.

4. Maximum absorption of energy occurs when the fields are perpendicular to the boundary.

Also, please refer to PML (Perfectly Matched Layer) and FEBI (Finite Element-Boundary Integral) boundary conditions too for more clarity.

Its resonance and max power (EM energy) transfer concept/principle

The nature of magnetic waves traveling in the air will come back to its orignal source, if that radius of circulation is too large, HFSS will not bother simulating it, in order to simulate it, you need to expand your radiation volume. Also the dependency is merely an error but ass long as you use a radiation field such that it is greater than half of the wavelength used, (padding from all sides), you should get a somewhat accurate result. 

Regards, 

AO

Hello @Sreenath .R,

In the example that you have attached, you have calculated 1-mag(S(FloquetPort1:2,FloquetPort1:1))^2-mag(S(FloquetPort1:2,FloquetPort1:2))^2

Is that the expression for absorptivity?

If yes, please tell which parameters represent S11, S12 and S21. Since you have used only one floquet port, how have you calculated the transmission coefficient?

I am new to designing and simulation of metamaterial absorbers, kindly help.

Thanks

Hello @Priyanka,

The expression given in the example i.e., 1-mag(S(FloquetPort1:2,FloquetPort1:1))^2-mag(S(FloquetPort1:2,FloquetPort1:2))^2 is a wrong formula to calculate. In my previous simulations, I used this formula. But I came to know recently that this formula is wrong.

Yes, as you mentioned, it is not possible to calculate transmission coefficient with out using 2 floquet ports.

Let's consider mag(S(FloquetPort1:2,FloquetPort1:1))^2 term. Here you can observe 1:2, 1 represents floquet port 1. 2 represents second mode. Often people consider this term as S21, but it is not.

Expression for S21 is mag(S(FloquetPort2:1,FloquetPort1:1), This only possible with the second floquet port.

Thank you so much for the clarification. Can you please send some simulated example for extraction of material parameters of the absorber? I tried but could not achieve the expected results.

Thanks

Sreenath .R ,Hello Sir,

Can i use the below given formula for finding out absorptivity?

1-sqr(mag(S(FloquetPort1:1,FloquetPort1:1)))-sqr(mag(S(FloquetPort2:1,FloquetPort1:1)))

Rahul Manohar O Yes, you can use this formula

could someone tell me how to do floquet port defining to find s11 and s21, as I am beginner to this hfss, and also mention best version of the software?

Thank you