# How come lower frequency has deeper skin depth but less SAR?

1. ## How come lower frequency has deeper skin depth but less SAR?

I would imagine that with deeper skin depth or penetration, you would have more SAR (Specific absorption rate) than with less skin depth. How come that is not true?

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2. ## Re: How come lower frequency has deeper skin depth but less SAR?

wat i recon is most of the electromagnetic freq current density has falls to 1/e (about 0.37) in general

In a good conductor, skin depth varies as the inverse square root of the conductivity. This means that better conductors have a reduced skin depth. Thus less SAR.

3. ## Re: How come lower frequency has deeper skin depth but less SAR?

Actually good conductors would have more SAR due to equation of
SAR = σ/2ρ * |E|²

I think the answer may be conductivity of dielectrics increases with frequency hence increase in SAR and decrease in skin depth.

What do you mean by "most of the electromagnetic freq current density has falls to 1/e (about 0.37) in general". Please elaborate.

Thanks.

4. ## Re: How come lower frequency has deeper skin depth but less SAR?

Skin depth is small at large frequencies....i.e the current density will flow mostly within the thickness equal to the skin depth on the conductor. Below this depth the current density drops to 1/e. Within the skin depth the SAR values are high.

5. ## Re: How come lower frequency has deeper skin depth but less SAR?

lucy123, thanks for reply. I am talking about that total Local SAR in body is less with lower frequency, not just within the skin depth area.

I am trying to have a method for measuring power absorbed in a fat/muscle body and finding Total Local SAR integrated over whole body decreases with frequency, although waves should penetrate deeper with lower frequencies. I think this may be due what I mentioned above "conductivity of dielectrics increases with frequency".

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6. ## Re: How come lower frequency has deeper skin depth but less SAR?

Two points can basically answer the question:
- high conductance objects are mainly reflecting electromagnetic waves as long as δ << λ. Only a small part of the power is absorbed
- low conductance objetcs are completely penetrated by the electromagnetic waves and thus as more effective absorbers.

You should be abled to reproduce these effects with your simulator.

7. ## Re: How come lower frequency has deeper skin depth but less SAR?

oh yes than you are correct. SAR values depend on conductivity. Changes in the dielectric properties does not affect the electric field though.
Body tissue are considered as lossy dielectrics and the conductivity of dielectrics increases with increasing frequency whilst relative permittivity decreases with increasing frequency.

You are using experimental measurements or computational simulation?

8. ## Re: How come lower frequency has deeper skin depth but less SAR?

Fvm,
At high frequency (where dielectrics have higher conductivity), simulator shows more Total Local SAR than at lower frequency (where dielectrics have lower conductivity). Also, HFSS has a calculation called Volume Loss Density, which calculates how much power is lost in the fat/muscle object, and it's higher for higher frequencies.

I am beginning to think that better penetration does not necessarily mean better absorbance. Lower frequencies would penetrate better as seen in skin depth equation, but would have lower SAR as seen in SAR equation because SAR is proportional to conductivity and dielectrics have lower conductivity at lower frequencies.

---------- Post added 31-03-11 at 00:03 ---------- Previous post was 30-03-11 at 23:58 ----------

Originally Posted by lucy123
You are using experimental measurements or computational simulation?
computational simulation HFSS.

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9. ## Re: How come lower frequency has deeper skin depth but less SAR?

sorry to ask but did you validate your model with existent phantoms? are you sure it is not a simulation problem?

10. ## Re: How come lower frequency has deeper skin depth but less SAR?

no. I didn't. I am planning to validate with FEKO. Would that be ok?

11. ## Re: How come lower frequency has deeper skin depth but less SAR?

I expect, that the HFSS simulations are at least qualitatively correct, they correspond to an absorbance behaviour that can be observed in real life.

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