How to measure radiated power through surface?

I used HFSS Field Calculator to measure radiated power through a surface (Boundary between muscle and vacuum or substrate).
The result is the radiated power < 0. I think it is because reflected power (at surface) > transmitted power (at surface). So the total power < 0. Right?
How to measure transmitted power through a surface? (not include reflected power)
Many thanks

I do not know about simulations but electromagnetic wave does go through a planar or other surface and its power can be MEASURED by a power meter connected to a suitable antenna or a pickup probe.
One such case is the depth of penetration which is a function of frequency and material conductivity, dielectrics behave according to Snells Law. The power passing through a boundary is never <0.
If the incident power is 1, then the leaked power is always between 1 and 0. The effect is important in screens and enclosures, and is measured as power ratio in dB. Measuring leakages < 90 dB is rather difficult, best done by a spectrum analyzer for a CW signal.

Thank jiripolivka for your reply. I really appreciate your quick answer.
At present, I used HFSS Simulation for this.
Pointing Vector has two directions: backward and outward from surface. And Total Power through the surface is the integral of Pointing Vector over this surface. When backward Pointing Vector overcomes outward Pointing Vector, this integral will be negative (<0). So the total Power through a surface <0. Right?
And How HFSS calculate Power through a surface? How to calculate only outward Power through a surface?
Many thanks

The correct name is Poynting vector. Your HFSS makes funny calculations: if the backward vector has a higher power than forward vector, it means that inside of your surface there is a source.
I would rather recommend you to forget about HFSS and learn the basics, in order to understand the physics behind. Otherwise you make wrong conclusions like just now.

Thank jiripolivka very much for spending time to answer my question.
Let me describe my work. I have an antenna inside a box (insulator) and muscle out side this box (muscle surrounds antenna and insulator). And I want to calculate radiated power through surfaces between insulation box and muscle. And The result is a negative power. I saw the Poynting Vector and see that there is backward Poynting Vector at these surfaces. So I think that reflected Power higher than transmitted power makes a negative result. Is this right?
Is there any way to calculate only transmitted (outward) power through these surfaces? Because when I use HFSS Field Calculator, It made integral both backward and outward Poynting vector through these surfaces?
Many thanks

I think that your HFSS calculator is wrong. "Negative" power means that it is generated somewhere.
In your situation, define the boundary between the "insulation box" and muscle, best by the complex permittivities. Then Snell's Law applies for the power coming from one medium to another, and the boundary always introduces a power loss. Incidence angle is also important.
Technically if the boundary is planar or curved, it passes the power in one direction while some of it will be reflected back. If there is an antenna in the insulation box, you will see that some power gets into the muscle , some reflected back. Antenna radiation pattern will be changed by the boundary and media permittivities. I would prefer to use a muscle phantom to make experiments as calculations are never giving real results.

I think the reflected power must be lower than the reflected power, because it is usually a part of the transmitted power.
So probably something else goes wrong.