Loss tangent DECREASES with frequency???

[Adun]

hi as loss tangent can be expressed as
tanδ=σ/(ωε)

so it decreases with frequency.

but i also heard that loss tangent will increase over frequency. e.g. page 16 of Cam Nguyen's "Analysis Methods for RF, microwave, and millimetr-wave planar transmission line structures"

so am I missing something here?

 

[adel_48]

IF your conductivity is constant with frequency, then the loss tangent will decrease with frequency. It depends on the material characteristics

 

[Adun]

/(Thanks for the reply!

I have another doubt. When σ increases, that means the medium is more "conductive" right? That means "less lossy". Then how come loss in the medium will increase according to equation tanδ=σ/(ωε)

 

[adel_48]

Hi,

that is a nice question, but you can think of it this way

For perfect dielectric (no losses) tand = 0
For perfect conductor (no losses, if you like to call it this way) tand = inf.

The losses of the medium depends on where the current will flow. For lossy dielectrics, the current (if you consider it conduction current) flows in the bulk, so you will have some losses = sigma * E . For perfect conductor, no current flows in the bulk, that is why you do not see losses in perfect conductor. For normal metals, the current flows mostly in the surface, and a small fraction in the volume, which causes losses (this is what is called the surface resitance of metals)

regards,
Adel

 

[Adun]

Thanks for the reply. It helps me a lot!

I have one more question: for lossless medium, σ=inf, then what can we say about the current? Does it flow everywhere in the medium or on the surface or what?
Thanks!

 

[qrs90]

As the conductivity goes to infinity, you get only surface currents -- no current flows inside the actual medium. When you use PEC approximations to solve emag problems you are allowing for Js on the surface -- these surface currents are only realizable on PEC surfaces.